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U.S. Department

of Transportation

**National Highway**

**Traffic Safety**

**Administration**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

**DOT HS 810 794 July 2007**

**NHTSA Technical Report**

**Statistical Analysis of the Effectiveness of**

**Electronic Stability Control (ESC) Systems --**

**Final Report**

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

This document is available to the public at the Docket Management System
of the U.S. Department of Transportation.

The United States Government does not endorse products or manufacturers.
Trade or manufacturers' names appear only because they are considered
essential to the object of this report.

**Technical Report Documentation Page**

+-----------------------+-----------+-----------+-----------+-----------+
| 1\. Report No.        | 2\.       |           | 3\.       |           |
|                       | G         |           | Re        |           |
| DOT HS 810 794        | overnment |           | cipient's |           |
|                       | Accession |           | Catalog   |           |
|                       | No.       |           | No.       |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 4\. Title and         |           |           | 5\.       |           |
| Subtitle              |           |           | Report    |           |
|                       |           |           | Date      |           |
| STATISTICAL ANALYSIS  |           |           |           |           |
| OF THE EFFECTIVENESS  |           |           | July 2007 |           |
| OF                    |           |           |           |           |
|                       |           |           |           |           |
| ELECTRONIC STABILITY  |           |           |           |           |
| CONTROL (ESC) SYSTEMS |           |           |           |           |
| -- FINAL REPORT       |           |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
|                       |           |           | 6\.       |           |
|                       |           |           | P         |           |
|                       |           |           | erforming |           |
|                       |           |           | Org       |           |
|                       |           |           | anization |           |
|                       |           |           | Code      |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 7\. Author(s)         |           |           | 8\.       |           |
|                       |           |           | P         |           |
| Jennifer N. Dang      |           |           | erforming |           |
|                       |           |           | Org       |           |
|                       |           |           | anization |           |
|                       |           |           | Report    |           |
|                       |           |           | No.       |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 9\. Performing        |           |           | 10\. Work |           |
| Organization Name and |           |           | Unit No.  |           |
| Address               |           |           | (TRAIS)   |           |
|                       |           |           |           |           |
| Evaluation Division;  |           |           |           |           |
| National Center for   |           |           |           |           |
| Statistics and        |           |           |           |           |
| Analysis              |           |           |           |           |
|                       |           |           |           |           |
| National Highway      |           |           |           |           |
| Traffic Safety        |           |           |           |           |
| Administration        |           |           |           |           |
|                       |           |           |           |           |
| Washington, DC 20590  |           |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
|                       |           |           | 11\.      |           |
|                       |           |           | Contract  |           |
|                       |           |           | or Grant  |           |
|                       |           |           | No.       |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 12\. Sponsoring       |           |           | 13\. Type |           |
| Agency Name and       |           |           | of Report |           |
| Address               |           |           | and       |           |
|                       |           |           | Period    |           |
| Department of         |           |           | Covered   |           |
| Transportation        |           |           |           |           |
|                       |           |           | NHTSA     |           |
| National Highway      |           |           | Technical |           |
| Traffic Safety        |           |           | Report    |           |
| Administration        |           |           |           |           |
|                       |           |           |           |           |
| Washington, DC 20590  |           |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
|                       |           |           | 14\.      |           |
|                       |           |           | S         |           |
|                       |           |           | ponsoring |           |
|                       |           |           | Agency    |           |
|                       |           |           | Code      |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 15\. Supplementary    |           |           |           |           |
| Notes                 |           |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 16\. Abstract         |           |           |           |           |
|                       |           |           |           |           |
| Electronic Stability  |           |           |           |           |
| Control (ESC) is a    |           |           |           |           |
| safety technology     |           |           |           |           |
| designed to enhance a |           |           |           |           |
| vehicle's stability   |           |           |           |           |
| and control in all    |           |           |           |           |
| driving situations.   |           |           |           |           |
| ESC first became      |           |           |           |           |
| available in the      |           |           |           |           |
| United States in      |           |           |           |           |
| 1997. Statistical     |           |           |           |           |
| analyses of 1997-2004 |           |           |           |           |
| crash data from the   |           |           |           |           |
| Fatality Analysis     |           |           |           |           |
| Reporting System      |           |           |           |           |
| (FARS) and 1997-2003  |           |           |           |           |
| crash data from the   |           |           |           |           |
| State data files      |           |           |           |           |
| estimate reductions   |           |           |           |           |
| with ESC for various  |           |           |           |           |
| types of crash        |           |           |           |           |
| involvements.         |           |           |           |           |
|                       |           |           |           |           |
| -   ESC reduced       |           |           |           |           |
|     **fatal           |           |           |           |           |
|     run-off-road**    |           |           |           |           |
|     crashes by 36     |           |           |           |           |
|     percent for       |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 70 percent    |           |           |           |           |
|     for light trucks  |           |           |           |           |
|     and vans (LTVs).  |           |           |           |           |
|     The reductions    |           |           |           |           |
|     are statistically |           |           |           |           |
|     significant.      |           |           |           |           |
|                       |           |           |           |           |
| -   **Police-reported |           |           |           |           |
|     run-off-road**    |           |           |           |           |
|     involvements were |           |           |           |           |
|     decreased by 45   |           |           |           |           |
|     percent in        |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 72 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|     decreases are     |           |           |           |           |
|     statistically     |           |           |           |           |
|     significant.      |           |           |           |           |
|                       |           |           |           |           |
| -   **Fatal           |           |           |           |           |
|     single-vehicle**  |           |           |           |           |
|     crashes that did  |           |           |           |           |
|     not involve       |           |           |           |           |
|     pedestrians,      |           |           |           |           |
|     bicycles, and     |           |           |           |           |
|     animals decreased |           |           |           |           |
|     (due to ESC) by   |           |           |           |           |
|     36 percent in     |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 63 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|                       |           |           |           |           |
| > decreases are       |           |           |           |           |
| > statistically       |           |           |           |           |
| > significant.        |           |           |           |           |
|                       |           |           |           |           |
| -   ESC reduced       |           |           |           |           |
|     **police-reported |           |           |           |           |
|     single-vehicle**  |           |           |           |           |
|     crashes           |           |           |           |           |
|     (excluding        |           |           |           |           |
|     pedestrian,       |           |           |           |           |
|     bicycle, animal   |           |           |           |           |
|     crashes) by 26    |           |           |           |           |
|     percent for       |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 48 percent    |           |           |           |           |
|     for LTVs. The     |           |           |           |           |
|                       |           |           |           |           |
| > reductions are      |           |           |           |           |
| > statistically       |           |           |           |           |
| > significant.        |           |           |           |           |
|                       |           |           |           |           |
| -   **Rollover**      |           |           |           |           |
|     involvements in   |           |           |           |           |
|     **fatal** crashes |           |           |           |           |
|     were decreased by |           |           |           |           |
|     70 percent in     |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 88 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|     decreases are     |           |           |           |           |
|     statistically     |           |           |           |           |
|     significant.      |           |           |           |           |
|                       |           |           |           |           |
| -   **Police-reported |           |           |           |           |
|     crashes involving |           |           |           |           |
|     rollovers** were  |           |           |           |           |
|     reduced by 64     |           |           |           |           |
|     percent in        |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 85 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|     reductions are    |           |           |           |           |
|     statistically     |           |           |           |           |
|     significant.      |           |           |           |           |
|                       |           |           |           |           |
| -   ESC reduced       |           |           |           |           |
|     **culpable fatal  |           |           |           |           |
|     multi-vehicle**   |           |           |           |           |
|     crashes by 19     |           |           |           |           |
|     percent for       |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 34 percent    |           |           |           |           |
|     for LTVs. Only    |           |           |           |           |
|     the reduction     |           |           |           |           |
|     involving LTVs is |           |           |           |           |
|     statistically     |           |           |           |           |
|                       |           |           |           |           |
| > significant.        |           |           |           |           |
|                       |           |           |           |           |
| -   **Culpable        |           |           |           |           |
|     involvements in   |           |           |           |           |
|     police-reported   |           |           |           |           |
|     multi-vehicle**   |           |           |           |           |
|     crashes were      |           |           |           |           |
|     decreased by 13   |           |           |           |           |
|     percent in        |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 16 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|     decreases are     |           |           |           |           |
|                       |           |           |           |           |
| statistically         |           |           |           |           |
| significant.          |           |           |           |           |
|                       |           |           |           |           |
| -   **Overall**, ESC  |           |           |           |           |
|     reduced all       |           |           |           |           |
|     **fatal** crashes |           |           |           |           |
|     by 14 percent for |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 28 percent    |           |           |           |           |
|     for LTVs. Only    |           |           |           |           |
|     the reduction in  |           |           |           |           |
|     LTVs is           |           |           |           |           |
|     statistically     |           |           |           |           |
|     significant.      |           |           |           |           |
|                       |           |           |           |           |
| -   **Overall,        |           |           |           |           |
|     police-reported   |           |           |           |           |
|     crash             |           |           |           |           |
|     involvements**    |           |           |           |           |
|     decreased by 8    |           |           |           |           |
|     percent in        |           |           |           |           |
|     passenger cars    |           |           |           |           |
|     and 10 percent in |           |           |           |           |
|     LTVs. The         |           |           |           |           |
|     decreases are     |           |           |           |           |
|     statistically     |           |           |           |           |
|     significant.      |           |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 17\. Key Words        |           | 18\.      |           |           |
|                       |           | Dis       |           |           |
| NHTSA; FARS; ESC;     |           | tribution |           |           |
| statistical analysis; |           | Statement |           |           |
| evaluation; benefits; |           |           |           |           |
| effectiveness;        |           | Document  |           |           |
| fatality reduction;   |           | is        |           |           |
| electronic stability  |           | available |           |           |
| control systems;      |           | to the    |           |           |
|                       |           | public at |           |           |
|                       |           | the       |           |           |
|                       |           | Docket    |           |           |
|                       |           | M         |           |           |
|                       |           | anagement |           |           |
|                       |           | System of |           |           |
|                       |           | the U.S.  |           |           |
|                       |           | D         |           |           |
|                       |           | epartment |           |           |
|                       |           | of        |           |           |
|                       |           | Transp    |           |           |
|                       |           | ortation, |           |           |
|                       |           | [http:    |           |           |
|                       |           | //dms.dot |           |           |
|                       |           | .gov](htt |           |           |
|                       |           | p://dms.d |           |           |
|                       |           | ot.gov/), |           |           |
|                       |           | Docket    |           |           |
|                       |           | Number    |           |           |
|                       |           | 28629.    |           |           |
+-----------------------+-----------+-----------+-----------+-----------+
| 19\. Security         | 20\.      |           | 21\. No.  | 22\.      |
| Classif. (Of this     | Security  |           | of Pages  | Price     |
| report)               | Classif.  |           |           |           |
|                       | (Of this  |           | 63        |           |
| Unclassified          | page)     |           |           |           |
|                       |           |           |           |           |
|                       | Unc       |           |           |           |
|                       | lassified |           |           |           |
+-----------------------+-----------+-----------+-----------+-----------+

**Form DOT F 1700.7 (8-72) Reproduction of completed page authorized**

**TABLE OF CONTENTS**

LIST OF ABBREVIATIONS
................................................................iv

ACKNOWLEDGEMENTS
...................................................................v

EXECUTIVE SUMMARY
................................................................... vi

BACKGROUND
...........................................................................\...\...1

ESC INFORMATION
..........................................................................5

CRASH DATA (STATE AND FARS)
..................................................... 11

ANALYSIS DATABASES
.................................................................. 12

VIN decode
......................................................................................12

Crash involvements
............................................................\...\...\...\...\...\...\...
13

2X2 CONTINGENCY TABLE ANALYSIS
.............................................\...18

Analysis of fatal crashes (FARS)
.............................................\...\...\...\...\...\...\...
20

Analysis of crash involvements (State Data)
................................................25

LOGISTIC REGRESSION ANALYSIS
..................................................... 46

COMPARISON OF 2-CHANNEL AND 4-CHANNEL ESC SYSTEMS...............48

CONCLUSIONS.................................................................................
52

**LIST OF ABBREVIATIONS**

ABS Antilock brake systems

ASE Asymptotic standard error

CATMOD Categorical models procedure in SAS

ESC Electronic Stability Control

FARS Fatality Analysis Reporting System

FMVSS Federal Motor Vehicle Safety Standard

GENMOD Generalized estimating models procedure in SAS

GM General Motors

IIHS Insurance Institute for Highway Safety

LR Likelihood-ratio

LTVs Light trucks and vans

MEANS MEANS (simple statistics) procedure in SAS

NCSA National Center for Statistics and Analysis

NHTSA National Highway Traffic Safety Administration

OMB Office of Management and Budget

SAS Statistical analysis software produced by SAS Institute, Inc.

SE Standard error

SUV Sport utility vehicle

TCS Traction control systems

VIN Vehicle Identification Number

**ACKNOWLEDGEMENTS**

I owe special thanks to the two researchers who peer-reviewed a draft of
the report:

1)  Dr. Charles M. Farmer, Director of Statistical Services, Insurance
    Institute for Highway Safety, Arlington, Virginia

2)  Professor Claes Tingvall, Dr Med Sc, Director of Traffic Safety at
    Swedish Road Administration

This study estimates the effectiveness of Electronic Stability Control
(ESC) in reducing crashes, specifically crashes where ESC is likely to
have made a difference in the vehicle's involvement, based on
statistical analyses of crash data. The National Highway Traffic Safety
Administration (NHTSA) published a draft of this report in support of a
proposed rulemaking to establish a new Federal Motor Vehicle Safety
Standard, FMVSS No. 126, which requires ESC systems on passenger cars,
multipurpose vehicles, trucks, and buses with a gross vehicle weight
rating of 10,000 pounds or less (See NHTSA Docket Number 25801-02 at
[http://dms.dot.gov](http://dms.dot.gov/)). Because of the potential
impacts of the proposed regulation, the report contains "highly
influential scientific information" as defined by the Office of
Management and Budget's (OMB) "Final Information Quality Bulletin for
Peer Review" (available at
[www.whitehouse.gov/omb/inforeg/peer2004/peerbulletin.pdf](http://www.whitehouse.gov/omb/inforeg/peer2004/peerbulletin.pdf)).
Therefore, the report had to be peer-reviewed in accordance with the
requirements of both Sections II and III of OMB's Bulletin.

The peer-review process differed from the type used by journals. The
effort by Dr. Farmer and Professor Tingvall was essentially assessment
of the scientific adequacy of the draft to identify weaknesses and help
NHTSA strengthen the report. These two reviewers were selected by NHTSA
staff. The publication of this report does not necessarily imply that
the reviewers supported it or concurred with the findings. You may
access their comments on the draft along with the revised report and the
entire review process in the NHTSA docket (Number 26415) at
[http://dms.dot.gov](http://dms.dot.gov/). We have tried to address all
of the comments in our revised report (but we did not send it back to
the reviewers for a second round of review). The text and footnotes of
the report single out some of the reviewers' comments that instigated
additions or revisions to the analyses.

**EXECUTIVE SUMMARY**

Electronic Stability Control (ESC) is a safety system designed to
recognize adverse driving conditions by 1) continuously measuring and
evaluating the speed, the steering wheel angle, the yaw rate, and the
lateral acceleration of a vehicle from various sensors and 2) using
those measured data to compare a driver's steering input with the
vehicle's actual motion. If an unstable situation is detected, then ESC
automatically intervenes to assist the driver and stabilize the vehicle
by applying the brakes to individual wheels as needed and possibly
reducing engine torque. This technology is expected to reduce the number
of crashes due to driver error and loss of control, because it has the
potential to anticipate situations leading up to some crashes before
they occur and the capability to automatically intervene to prevent
them. A major benefit should be the reduction of single-vehicle crashes
that involve losing control and running off the road.

In September, 2004, the National Highway Traffic Safety Administration
(NHTSA) issued an evaluation note on the *Preliminary Results Analyzing
the Effectiveness of Electronic Stability Control (ESC) Systems*. The
data suggested that ESC was highly effective in reducing single-vehicle
run-off-road crashes. The study was based on Fatality Analysis Reporting
System (FARS) data from calendar years 1997-2003 and crash data from
five States from calendar years 1997-2002. The data were limited to
mostly luxury vehicles because ESC first became available in 1997 in
luxury vehicles such as Mercedes-Benz and BMW.

NHTSA has now updated and modified its 2004 report, extending it to
model year 1997-2004 vehicles -- and to calendar year 2004 for the FARS
analysis and calendar year 2003 for the State data analysis.
Nevertheless, even as of 2004, a large proportion of the vehicles
equipped with ESC were still luxury vehicles. Moreover, only passenger
cars and SUVs had been equipped with ESC -- no pickup trucks or
minivans.

The FARS database included fatal crash involvements from calendar years
1997 to 2004. The State databases included crash cases from California
(2001-2003), Florida (1997-2003), Illinois (1997-2002), Kentucky
(1997-2002), Missouri (1997-2003), Pennsylvania (1997-2001, 2003), and
Wisconsin (1997-2003).

The basic analytical approach was to estimate the reduction of crash
involvements of the types that are most likely to have benefited from
ESC -- relative to a control group of other types of crashes where ESC
is unlikely to have made a difference in the vehicle's involvement.
Crash involvements in which a vehicle 1) was stopped, parked, backing
up, or entering/leaving a parking space prior to the crash, 2) traveled
at a speed less than 10 mph, 3) was struck in the rear by another
vehicle, or 4) was a non-culpable party in a multi-vehicle crash on a
dry road, were considered the **control group** (non-relevant
involvements) -- because ESC would in almost all cases not have
prevented the crash. The types of crash involvements where ESC would
likely or at least possibly have an effect are:

-   Single-vehicle crashes in which a vehicle ran off the road and then
    hit a fixed object and/or rolled over.

-   Involvements as a culpable party in a multi-vehicle crash on a dry
    or wet road.

-   Collisions with pedestrians, bicycles, or animals.

The principal findings and conclusions of the statistical analyses are
the following:

RUN-OFF-ROAD CRASHES

-   ESC reduced involvements in all types of single-vehicle run-off-road
    crashes the following percentages:

> Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

Fatal crash involvements 36 70

Police-reported crash involvements 45 72

-   All four of these reductions are statistically significant.

SINGLE-VEHICLE CRASH REDUCTION

-   ESC reduced all single-vehicle involvements (excluding pedestrian,
    bicycle, animal crashes) by the following percentages:

> Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

Fatal single-vehicle crash involvements 36 63

Police-reported single-vehicle crash involvements 26 48

-   All four reductions are statistically significant.

ROLLOVER CRASHES

-   ESC was especially effective in preventing single-vehicle
    first-event rollovers.

> Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

Fatal rollovers 70 88

Police-reported rollovers 64 85

-   All four of these reductions are statistically significant.

CULPABLE INVOLVEMENTS IN MULTI-VEHICLE CRASHES

-   ESC likely reduces involvements as a culpable party in multi-vehicle
    crashes.

Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

Fatal culpable multi-vehicle crash involvements 19 34

Police-reported culpable multi-vehicle crash involvements 13 16

-   Only the reduction in fatal crash involvements in passenger cars is
    [not]{.underline} statistically significant.

COLLISIONS WITH PEDESTRIANS -- BICYCLES -- ANIMALS

-   There are no consistently significant results in either direction
    for crashes that involve pedestrians, bicycles, or animals

> Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

Fatal pedestrian, bicycle, animal crashes -36 -6

Police-reported pedestrian, bicycle, animal crashes 26 -11

-   Only the reduction in police-reported crash involvements in
    passenger cars is statistically significant. We will continue to
    monitor the effect of ESC on this particular type of crash
    involvements in the future -- because we do not have enough data (at
    the moment) for any conclusions.

OVERALL CRASH REDUCTION

-   ESC reduced all crash involvements by the following percentages:

> Crash reduction by ESC (%)

[Cars]{.underline} [LTVs]{.underline}

All fatal crash involvements 14 28

All police-reported crash involvements 8 10

-   Only the reduction in fatal crash involvements in passenger cars is
    not statistically significant.

4-CHANNEL VERSUS 2-CHANNEL ESC SYSTEMS IN PASSENGER CARS

The passenger car sample includes certain make-models that had 2-channel
ESC systems and others that had 4-channel systems. Separate analyses
were performed to analyze the difference in effectiveness (if any)
between 2-channel and 4-channel systems. We found:

[Greater]{.underline} [fatal]{.underline} run-off-road reduction with
[2-channel systems]{.underline} -- but the larger observed fatality
reduction with 2-channel systems is not statistically significantly
different from the observed reduction with 4-channel systems. The
reductions were most certainly influenced by the small samples.

[Larger]{.underline} reduction with [4-channel systems]{.underline} when
police-reported crash involvements were included (as expected with
larger samples). The larger reduction with 4-channel systems in all
run-off-road involvements (mostly non-fatal crashes) is statistically
significant.

**BACKGROUND**

Automotive braking technologies have evolved from very simple systems
(i.e., block brakes) to more sophisticated systems (i.e., cable-operated
four-wheel brakes, hydraulic four-wheel brakes, drum brakes, disc brakes
with front-rear split, etc.). Today, drivers rely on much more
technologically-advanced systems to help them not only to decelerate and
accelerate but also to stabilize their vehicles while in motion, such
as:

Antilock Brake Systems (ABS) are the first of a series of three braking
technology developments. They are four-wheel systems that prevent wheel
lock-up by automatically modulating the brake pressure when the driver
makes an emergency stop.

Traction Control Systems (TCS) are the second technology. They deal
specifically with front-to-back loss of friction between the vehicle's
tires and the road surface during *acceleration*.

Electronic Stability Control (ESC) systems are another important
technology evolving from and incorporating the first two technologies --
ABS and TCS -- [with additional capabilities]{.underline}. They are
stability enhancement systems designed to improve vehicles' lateral
stability by electronically detecting and automatically assisting
drivers in dangerous situations (e.g., understeer and oversteer) and
under unfavorable conditions (e.g., rain, snow, sleet, ice). ESC systems
have sensors that monitor the speed, the steering wheel angle, the yaw
rate, and the lateral acceleration of the vehicle. Data from the sensors
are used to compare a driver's intended course with the vehicle's actual
movement to detect when a driver is about to lose control of a vehicle
and automatically intervene in split seconds by applying the brakes to
individual wheels and possibly reducing engine torque to provide
stability and help the driver stay on course. For example, if an ESC
system detects that the rear wheels have begun to slide to the right and
the vehicle is yawing counter-clockwise, it may momentarily brake the
right front wheel, imparting a clockwise torque to counteract the
excessive counterclockwise yaw and stabilize the vehicle. Depending on
the driving situation, these brake interventions may also be used to
slow down the vehicle to a speed more appropriate for the operating
conditions.

The reasons ESC appears to provide safety benefits are twofold: (1) it
can anticipate situations leading up to some loss-of-control crashes
before they occur, and (2) it has the capability to mitigate these
crashes via automatic intervention. Hence, the potential benefit should
be primarily a reduction of single-vehicle crashes that involve losing
control and running off the road. These crashes include rollovers and
collisions with fixed objects.

In a first published study[^1] analyzing the effectiveness of ESC on
three Toyota passenger car make-models, Aga and Okada reported in 2003,
a 36 percent reduction in single-vehicle crash rates (single-vehicle
crashes per 10,000 vehicles per year) when they compared make-model
vehicles with ESC and those without ESC. The study also showed a
reduction (28 percent) in the rates of head-on collisions in studied
vehicles that are equipped with ESC relative to those that are not
equipped with ESC.

In 2004, the National Highway Traffic Safety Administration (NHTSA)
initiated an evaluation to assess the effectiveness of ESC in reducing
single-vehicle crashes in various domestic and imported passenger cars
and Sport Utility Vehicles (SUVs). The preliminary results from that
study[^2] showed that ESC is highly effective in reducing single-vehicle
crashes. In fact, single-vehicle crashes were reduced by 35 percent in
passenger cars, and 67 percent in SUVs. Similarly, fatal single-vehicle
crashes were reduced by 30 percent in passenger cars, and 63 percent in
SUVs.

In a study published in 2004 by the Insurance Institute for Highway
Safety (IIHS)[^3], Farmer compared per vehicle crash involvement rates
of vehicles with model years that had ESC as standard equipment with
identical make-model year vehicles that did not. Farmer found that ESC
reduced single-vehicle crash involvement risk by approximately 41
percent and reduced single-vehicle [injury]{.underline} risk by the same
amount. Overall, crash involvement risk was reduced by 7 percent for all
crashes and 9 percent for injury crashes. For fatal crash involvements,
it was estimated that ESC reduced single-vehicle crash involvement risk
by 56 percent and overall fatal crash involvement risk by 34 percent.
The effect of ESC on multi-vehicle crashes was found to be minimal and
not statistically significant. Unlike the NHTSA evaluation, this study
combined the results for passenger cars and SUVs.

In another study published in 2004[^4], researchers in Sweden (Tingvall
et al.) reported a 32 percent crash reduction with ESC on wet roads and
a 38 percent reduction on roads covered with ice and snow. For all
crashes except rear-end impacts, the study found an overall 22 percent
reduction in crashes involving vehicles with ESC versus those without
ESC. In this study, rear-end crashes on dry surfaces, assumed to be
unaffected by the presence of ESC, were used as the control group. The
data set used in the analysis consists of Swedish police reported
crashes where at least one occupant was injured. In 2006, Tingvall et
al. conducted a follow-up study analyzing the effectiveness of ESC
specifically by injury severity.[^5] For serious and fatal
loss-of-control type crashes, it was estimated that ESC reduced those
types of crashes by 56 percent on wet roads and 49 percent on roads that
were covered with ice and snow.

The latest study from the University of Michigan Transportation Research
Institute[^6] confirmed the results of earlier studies worldwide -- ESC
is highly effective in preventing single-vehicle crashes, especially
rollovers in SUVs. Similar to the NHTSA evaluation, this study analyzed
the effectiveness of ESC separately for passenger cars and SUVs. The
study found that for single-vehicle crashes, ESC reduced the risk of a
fatal crash involvement by 31 percent for passenger cars and 50 percent
for SUVs. These analyses were repeated, for both passenger cars and
SUVs, after restricting the data to vehicles that were three years old
or newer at the time of the crash -- to control for the effect (if any)
of the age of the vehicle. The results showed that the vehicle age
factor did not compromise the significant reductions in the risk of
single-vehicle crash involvements associated with ESC-equipped vehicles.
The risks of fatal rollover involvements were also decreased for
passenger cars and SUVs -- 40 percent and 73 percent, respectively. For
the analysis of fatal crash involvements, multi-vehicle crash
involvements were used as the control group. The study also analyzed a
nationally representative probability sample of crashes that included
mostly non-fatal crashes. It found significant reductions in
run-off-road crashes for passenger cars and SUVs -- 55 percent and 70
percent, respectively. For the analysis of these mostly non-fatal
crashes, struck vehicles involved in rear-end crashes were used as the
control group.

The 2004 NHTSA study used crash data from five States from calendar
years 1997 to 2002 and Fatality Analysis Reporting System (FARS) data
from calendar years 1997 to 2003, because the study was limited to the
years when ESC was offered as standard equipment on certain vehicle
models. Mercedes-Benz and BMW were the first two manufacturers that
installed ESC as standard equipment in certain models in 1997 and in all
models by 2000 and 2001, respectively. Their vehicles constituted 61
percent of the passenger car sample used in the 2004 study. The
passenger car sample also includes some luxury GM cars, which
constituted 23 percent of the sample, and a few luxury cars from other
manufacturers. As for the sample used in the analysis for SUVs, Toyota
and Lexus models constituted 78 percent of that sample. In other words,
the data samples used in the 2004 study are limited to mostly imported
luxury vehicles, and thus, they are not well represented across the
entire fleet.

Sales of vehicles equipped with ESC are gradually increasing as shown in
Table 1. In 2003, merely 10 percent of that year's vehicle sales are
from vehicles that had ESC. By 2006, nearly [one-third]{.underline} of
the new vehicles are expected to have ESC. Thus, future analysis data
samples should consist of a more representative cross-section of the
fleet that will include not only non-luxury vehicles but also a wider
variety of manufacturers. The availability of such data is likely to
take at least a few more years. Until then, NHTSA will continue to
evaluate the effectiveness of ESC with limited analysis samples of crash
data from selected make-models of vehicles that had ESC and earlier
versions of similar make-models that did not. The make-models used in
this study consist of not only those used in the 2004 study but also
other make-models and the model years extended to one more year.

***[Table 1]{.underline}: Percent of the Vehicle Sales that are from
Vehicles Equipped with ESC***

By Vehicle Group and Model Year

  ------------------ ------------- ------------- -------------- --------------
  **Vehicle Group**  **Model Year  **Model Year  **Model Year   **Model Year
                     2003**        2004**        2005           2006
                                                 (expected)**   (expected)**

  **Domestic         3.5%          7.7%          8.2%           10.0%
  cars\***                                                      

  **Domestic         4.9%          10.2%         15.2%          26.6%
  trucks\***                                                    

  **Imported cars**  36.6%         45.5%         40.5%          54.3%

  **Imported         31.8%         48.4%         63.4%          73.5%
  trucks**                                                      

  **All new          **9.7%**      **15.9%**     **19.2%**      **28.6%**
  vehicles**                                                    
  ------------------ ------------- ------------- -------------- --------------

\* Includes transplants

With one more year of crash data recently available, NHTSA has embarked
on this update and modification of its 2004 analysis, extending it to
calendar year 2004 for the FARS analysis and calendar year 2003 for the
State data analysis. The primary objective of this study is to assess
the effectiveness of ESC in reducing crashes, specifically crashes where
ESC is likely to have made a difference in the vehicle's involvement,
while controlling for others in which ESC is unlikely to have been a
factor. The study consists of a series of analyses of crash data of
various domestic and imported passenger cars and LTVs (light trucks and
vans, including pickup trucks, SUVs, minivans, and full-size vans with
Gross Vehicle Weight Rating up to 10,000 pounds) from model years 1997
to 2004. As its principal analytic tool, the agency utilized **2x2
contingency tables** of crash data from specific make-models equipped
with ESC versus earlier versions of similar make-models not equipped
with ESC, using non-relevant crash involvements as a control group. As a
check, a logistic regression analysis of the effectiveness of ESC in
reducing relevant crash involvements (specifically single-vehicle
crashes which involve a vehicle running off the road, while controlling
for other non-relevant crash involvements) was also performed using FARS
data in order to adjust for the imbalances in various make-models of
vehicles with ESC versus those of similar make-models without ESC and to
account for the confounding effects from other external factors. We also
looked separately at crash reduction with 2-channel and 4-channel ESC
systems. The reason for doing this is that GM make-models used in the
analyses (except for the Corvette) all had 2-channel ESC systems,
whereas non-GM make-models all had 4-channel systems.

The next several sections discuss in detail: (1) the availability of ESC
(particularly, which make-model passenger cars and LTVs had ESC and
which did not); (2) the analysis databases (specifically, what and how
various data files were obtained and used in the analysis); and (3) the
methods of analyzing crash data and estimating the effectiveness of ESC
in reducing crashes -- followed by the results from a series of
statistical analyses of crash data. A summary discussion of the findings
concludes the report.

**ESC INFORMATION**

Due to the fact that (1) ESC was first introduced in the U.S. on certain
luxury vehicles in 1997 and (2) it was not until recently that ESC was
offered on non-luxury vehicles, information is not readily obtainable
for earlier model years. For example, ESC was not listed in Buying a
Safer Car[^7] until 2001, and Ward's Automotive Yearbook[^8] did not
begin reporting ESC information in its yearbook until 2004. The
yearly-published Buying a Safer Car booklet lists safety feature
information for the current year's production of vehicles, whereas
Ward's yearbook contains a list of factory-installed equipment for the
previous year's production of vehicles. For instance, Ward's Automotive
Yearbook 2001 contains a list of factory-installed equipment for
vehicles produced in calendar year 2000, whereas Buying a Safer Car 2001
lists safety information for vehicles produced in calendar year 2001.
Hence, information such as the availability of ESC in vehicles with
model years prior to 2000 had to be obtained elsewhere (e.g.,
www.edmunds.com, www.cars.com). Furthermore, since ESC is a fairly new
technology for the U.S. market, information on its availability can vary
from one source to another. In such cases, information had to be
verified with the manufacturers for accuracy.

For the purpose of analysis, make-models of passenger cars and LTVs that
are equipped with ESC as standard equipment were compared with earlier
versions of similar make-models. Thus, only vehicles in which 1) the
percentage of factory-installed equipment for "stability control"
systems increased from 0 to 100 percent (Wards' Automotive Yearbook) or
2) ESC is listed as standard in all vehicles in that model line for
particular model years and not available at all for previous model years
(Buying a Safer Car Booklet, etc.) were included in the analysis.
Vehicles with ESC as optional equipment were excluded from the analysis
because we could not determine (from the VIN) which vehicles had ESC and
which did not.[^9]

Tables 2 through 4 show ESC availability on selected make-model
passenger cars. In some cases, models that had factory-installed ESC as
a standard feature were compared with earlier versions of
[identical]{.underline} models because those models remained the same
(e.g. same chassis generation) during the pre- and post-ESC years -- as
shown in Table 2. In other words, for each vehicle listed in Table 2,
exactly the same model -- before and after ESC was introduced -- was
used for comparison. The conversion from not having ESC at all to having
ESC as standard equipment should occur in consecutive model years as
shown in most models except for the Chevrolet Corvette. ESC was
introduced as optional equipment on the Chevrolet Corvette for model
years 1998-2000. Those model years were eliminated from the study.

***[Table 2]{.underline}***: \"Identical\" Make-Model Comparison of
Vehicles (except Mercedes-Benz) In Model Years - With ESC and Without
ESC - for Passenger Cars

+------------------------------------+------------------+--------------+
| **Make Model**                     | **Model Years**  | **Model      |
|                                    |                  | Years with** |
|                                    | **with**         |              |
|                                    |                  | **ESC        |
|                                    | **No ESC**       | (Standard)** |
+------------------------------------+------------------+--------------+
| Acura 3.5RL                        | 1997-1999        | 2000-2004    |
+------------------------------------+------------------+--------------+
| Audi A6 (2.7L) Sedan               | 2000             | 2001-2004    |
+------------------------------------+------------------+--------------+
| Audi TT (1.8L) 180hp Coupe         | 2000             | 2001-2004    |
+------------------------------------+------------------+--------------+
| BMW 740I                           | 1997             | 1998-2001    |
+------------------------------------+------------------+--------------+
| BMW 740IL Sedan                    | 1997             | 1998-2001    |
+------------------------------------+------------------+--------------+
| Buick Park Avenue Ultra            | 1997-1999        | 2000-2004    |
+------------------------------------+------------------+--------------+
| Cadillac DeVille Concours          | 1997             | 1998-1999    |
+------------------------------------+------------------+--------------+
| Chevrolet Corvette                 | 1997             | 2001-2004    |
+------------------------------------+------------------+--------------+
| Lexus GS400                        | 1998             | 1999-2000    |
+------------------------------------+------------------+--------------+
| Lexus LS400                        | 1997-1998        | 1999-2000    |
+------------------------------------+------------------+--------------+
| Saab 9-5 Aero Sedan/Sport Wagon    | 2000-2001        | 2002-2004    |
+------------------------------------+------------------+--------------+

In other cases, [similar]{.underline} make-models (except for the
Mercedes-Benz vehicles which will be discussed separately) were used for
the comparison since the original models were redesigned, not available,
or not of the same chassis generation during the transitional years
(Table 3). For instance, the 2000 Audi A4 (1.8L), 2000 A4 (1.8L)
Quattro, and 2000 A4 (1.8L) Avant Quattro models were not of the same
chassis generation as their 2002-2004 models. ESC was offered as
optional feature on the model year 2001 for those vehicles, and thus,
that model year was not included in the study. The 2004 Audi A8 4.2L
Quattro was not available; thus, the 2004 A8 L was included in the
analysis instead. The 1997 and 1998-2004 Cadillac Seville SLS and STS
models were also on a different chassis, and so did the 1997 and
1998-2004 Lexus GS300, 1997-1999 and 2001-2003 Oldsmobile Aurora (4.0L),
and 1999 and 2002-2004 Volkswagen GTI VR6 models. As previously
mentioned and currently illustrated in Table 3, BMW installed ESC in
certain 5, 7, and 8 series models as early as 1997 and had made it
standard in all their models by 2001, but the installation did not
necessarily occur in the same year for all the sub-series of a
make-model. For example, the 3 series coupe and sedan models had ESC in
2000 whereas the 3 series convertible had it in 2001. Therefore,
comparison of vehicles with ESC and those without ESC was done at the
sub-series level. Furthermore, some manufacturers often changed their
sub-series models; thus, it is very difficult to make an exact
model-to-model comparison since the analysis databases consist of crash
data of various make-models ranging from model years 1997-2003 for the
State data analysis and 1997-2004 for the FARS data analysis. In such
cases, the best approach would be to compare similar sub-series models
as shown in Table 3. Since BMW's 3 and 5 series models were completely
redesigned and/or grouped into different sub-series at some point during
model years 1997-2004, different versions of the 3 and 5 series were
used depending on which model years had ESC and which did not. Similar
to BMW, General Motors changed their Pontiac Bonneville SSE model to
SSEi in 2000 and installed ESC in the new model as well. The reason that
model year 2000 was not included in the analysis for the BMW 3 series
convertible is because that model was not available in 2000. Model year
1999 was excluded for the BMW 525i and 530i sedan for the same reason.
The BMW 528i sedan was available in 1999; however, it did not include
ESC as standard feature -- only as an option. Hence, the 1999 BMW 528i,
525i, and 530i sedans were not included in the study. For the Volkswagen
Passat, the 2000 GLS and GLX models were compared with the 2004 GLX
model because the 2003 model year for both make-models had ESC as an
option. The 2004 GLS model had optional ESC as well. Likewise, the 2002
Volkswagen New Beetle (1.8L) was compared with the 2002-2004 Turbo S
models because the model years 2003-2004 of the former had optional ESC.

***[Table 3]{.underline}***: \"Similar\" Make-Model Comparison of
Vehicles (except Mercedes-Benz) In Model Years - With ESC and Without
ESC - for Passenger Cars

+---------------------+----------+-------------------------+----------+
| **Make Model**      | **Model  | **Make Model**          | **Model  |
|                     | Years    |                         | Years    |
|                     | with**   |                         | with**   |
|                     |          |                         |          |
|                     | **No     |                         | **ESC    |
|                     | ESC**    |                         | (Sta     |
|                     |          |                         | ndard)** |
+---------------------+----------+-------------------------+----------+
| Audi A4 (1.8L)      | 2000     | Audi A4 (1.8L) Sedan    | 2        |
| Sedan               |          |                         | 002-2004 |
+---------------------+----------+-------------------------+----------+
| Audi A4 (1.8L)      | 2000     | Audi A4 (1.8L) Quattro  | 2        |
| Quattro Sedan       |          | Sedan                   | 002-2004 |
+---------------------+----------+-------------------------+----------+
| Audi A4 (1.8L)      | 2000     | Audi A4 (1.8L) Avant    | 2        |
| Avant Quattro Wagon |          | Quattro Wagon           | 002-2004 |
+---------------------+----------+-------------------------+----------+
| Audi A8 (4.2L)      | 1        | Audi A8 (4.2L) Quattro  | 2        |
| Quattro Sedan       | 998-1999 | Sedan                   | 001-2003 |
|                     |          |                         |          |
|                     |          | Audi A8 L Sedan         | 2004     |
+---------------------+----------+-------------------------+----------+
| BMW                 | 1        | BMW                     | 2        |
| 3                   | 997-1999 | 323                     | 000-2004 |
| 18/323/325/328is/M3 |          | CI/325Ci/328Ci/330/M3CI |          |
| Coupe (6cyl)        |          | Coupe (6cyl)            |          |
+---------------------+----------+-------------------------+----------+
| BMW 323iC/328iC/M3  | 1        | BMW 325Ci/330Ci/M3Ci    | 2        |
| Convertible         | 997-1999 | Convertible             | 001-2004 |
+---------------------+----------+-------------------------+----------+
| BMW                 | 1        | BMW                     | 2        |
| 318/323i/328iSedan; | 997-1999 | 323/325i/328i/330i/Xi   | 000-2004 |
|                     |          | Sedan                   |          |
| BMW 318TI 2Dr.      |          |                         |          |
+---------------------+----------+-------------------------+----------+
| BMW 528i Sedan      | 1        | BMW 528i/525i/530i      | 2        |
|                     | 997-1998 | Sedan                   | 000-2004 |
+---------------------+----------+-------------------------+----------+
| BMW 540i (AT/MT)    | 1997     | BMW 540i (AT) Sedan     | 1998     |
| Sedan               |          |                         |          |
+---------------------+----------+-------------------------+----------+
| BMW 540i (MT) Sedan | 1998     | BMW 540i/545i (AT/MT)   | 1        |
|                     |          | Sedan                   | 999-2004 |
+---------------------+----------+-------------------------+----------+
| BMW Z3 (2.8L)       | 1        | BMW Z3                  | 2        |
|                     | 997-1999 | (2.3L,2.5L,2.8L,3.0L)   | 000-2002 |
+---------------------+----------+-------------------------+----------+
| Cadillac Seville    | 1        | Cadillac Seville SLS    | 1        |
| SLS                 | 997-1998 |                         | 999-2004 |
+---------------------+----------+-------------------------+----------+
| Cadillac Seville    | 1        | Cadillac Seville STS    | 1        |
| STS                 | 997-1998 |                         | 999-2004 |
+---------------------+----------+-------------------------+----------+
| Lexus GS300         | 1        | Lexus GS300             | 1        |
|                     | 997-1998 |                         | 999-2004 |
+---------------------+----------+-------------------------+----------+
| Oldsmobile Aurora   | 1        | Oldsmobile Aurora       | 2        |
| (4.0L)              | 997-1999 | (4.0L)                  | 001-2003 |
+---------------------+----------+-------------------------+----------+
| Pontiac Bonneville  | 1        | Pontiac Bonneville SSEi | 2        |
| SSE                 | 997-1999 |                         | 000-2004 |
+---------------------+----------+-------------------------+----------+
| Volkswagen GTI VR6  | 1999     | Volkswagen GTI VR6      | 2        |
|                     |          |                         | 002-2004 |
+---------------------+----------+-------------------------+----------+
| Volkswagen Passat   | 2002     | Volkswagen Passat GLX   | 2004     |
| GLS/GLX Sedan/Wagon |          |                         |          |
|                     |          | Sedan/Wagon             |          |
+---------------------+----------+-------------------------+----------+
| Volkswagen New      | 2002     | Volkswagen New Beetle   | 2        |
| Beetle (1.8L)       |          | Turbo S (1.8L)          | 002-2004 |
+---------------------+----------+-------------------------+----------+

Like BMW, Mercedes-Benz began installing ESC in certain sub-series of
the S class model in 1997 and had made it a standard feature in all
their make-models (except one) by 2000. For this reason, not all the
sub-series (even within the same make-model -- i.e. "class") had ESC in
the same model year. Furthermore, most sub-series were changed during
calendar years 1997-2004 -- either redesigned or replaced. Table 4 lists
all the available Mercedes-Benz models and sub-series from 1997 to 2004,
but only with the model years that had ESC as standard equipment and
those that did not. In Table 4, various sub-series within the
Mercedes-Benz C, E, and S class that are equipped with ESC were compared
with earlier versions of [similar]{.underline} sub-series, for reasons
already discussed in the previous section. Unlike other manufacturers,
quite a few Mercedes-Benz sub-series had ESC as optional equipment for
one, two, or even three years; hence, those model years were not
included in the analysis. For example, the C280, E300DT sedans and the
CL500C coupe all had ESC as an optional feature from 1998 to 1999.

***[Table 4]{.underline}***: Mercedes-Benz Make-Model Comparison of
Model Years With ESC and Without ESC - for Passenger Cars

+--------------------+----------+-------------------------+-----------+
| **Make Model**     | **No     | **Make Model**          | **ESC     |
|                    | ESC**    |                         | (St       |
|                    |          |                         | andard)** |
+--------------------+----------+-------------------------+-----------+
| C36AMG             | 1997     | C43 Sedan               | 1998-2000 |
|                    |          |                         |           |
| C220, C230 Sedan   | 1        | C230ML Sedan            | 2000      |
|                    | 997-1999 |                         |           |
| C280 Sedan         |          | C280 Sedan              | 2000      |
|                    | 1997     |                         |           |
|                    |          | C Sedan                 | 2001-2004 |
|                    |          |                         |           |
|                    |          | CL Coupe                | 2000-2004 |
|                    |          |                         |           |
|                    |          | CL600 Coupe             | 1998-1999 |
|                    |          |                         |           |
|                    |          | CLK55AMG                | 2001-2002 |
|                    |          | Coupe/Cabriolet         |           |
|                    |          |                         | 2000-2003 |
|                    |          | CLK320 Coupe/Cabriolet  |           |
|                    |          |                         | 2003-2004 |
|                    |          | CLK320/CLK500           |           |
|                    |          | Coupe/Cabriolet         | 1998-2003 |
|                    |          |                         |           |
|                    |          | CLK430 Coupe/Cabriolet  |           |
+--------------------+----------+-------------------------+-----------+
| E300D Sedan        | 1997     | E320 Sedan/Wagon        | 2000-2003 |
|                    |          |                         |           |
| E320 Sedan         | 1997     | E320W/E500 Sedan        | 2003-2004 |
|                    |          |                         |           |
|                    |          | E430/E55 AMG Sedan      | 1999-2002 |
+--------------------+----------+-------------------------+-----------+
| S320 SWB Sedan     | 1        | S430/S500/S600 Sedan    | 2000-2004 |
|                    | 997-1999 |                         |           |
| S320 LWB Sedan     |          | S600 Sedan              | 1997-1999 |
|                    | 1997     |                         |           |
| SL320 Roadster     |          | S600 Coupe              | 1997      |
|                    | 1997     |                         |           |
| SLK230 Kompressor  |          | SL500 Roadster          | 1999-2002 |
|                    | 1        |                         |           |
|                    | 998-2000 | SL500/SL55 Roadster     | 2003-2004 |
|                    |          |                         |           |
|                    |          | SL600 Roadster          | 1997-2002 |
|                    |          |                         |           |
|                    |          | SLK230/SLK320/SLK32AMG  | 2001-2004 |
+--------------------+----------+-------------------------+-----------+

Tables 5 through 7 show the availability of ESC by make-model for
selected LTVs. As shown in these tables, the make-models used in the
analysis consist of SUVs and passenger vans -- because most
manufacturers had not yet installed ESC in their pickup and minivan
models as of 2004 -- the most recent and available fatal crash data used
in this study. With the exception of the Infiniti, Land Rover, Toyota 4
Runner, Mercedes-Benz, and one Lexus model, none of the models used in
the analysis went through a redesign in the year that ESC became
standard equipment or even in the year immediately before or after it
(Table 5). Thus, we were able to compare several model years of
[identical]{.underline} make-models for the vehicles with ESC versus
those without ESC. Furthermore, these vehicles went from not having ESC
at all in one year to having ESC as standard equipment in the following
year.

***[Table 5]{.underline}: \"Identical\" Make-Model Comparison of LTVs In
Model Years***

With ESC and Without ESC

+----------------------------------------+------------+---------------+
| **Make Model**                         | **Model    | **Model       |
|                                        | Years**    | Years**       |
|                                        |            |               |
|                                        | **with**   | **with**      |
|                                        |            |               |
|                                        | **No ESC** | **ESC         |
|                                        |            | (Standard)**  |
+----------------------------------------+------------+---------------+
| Acura MDX                              | 2001-2002  | 2003-2004     |
+----------------------------------------+------------+---------------+
| Cadillac Escalade 2WD                  | 2002       | 2003-2004     |
+----------------------------------------+------------+---------------+
| Chevy Express 3500 Extended Wheelbase  | 2003       | 2004          |
+----------------------------------------+------------+---------------+
| GMC Yukon Denali AWD                   | 2001-2002  | 2003-2004     |
+----------------------------------------+------------+---------------+
| GMC Yukon Denali XL AWD                | 2001-2002  | 2003-2004     |
+----------------------------------------+------------+---------------+
| GMC Savana G3500 Extended Wheelbase    | 2003       | 2004          |
| RWD                                    |            |               |
+----------------------------------------+------------+---------------+
| Lexus LX470                            | 1999       | 2000-2004     |
+----------------------------------------+------------+---------------+
| Mitsubishi Montero Limited             | 2001-2002  | 2003-2004     |
+----------------------------------------+------------+---------------+
| Toyota Land Cruiser                    | 1999-2000  | 2001-2004     |
+----------------------------------------+------------+---------------+
| Toyota RAV4 4x2                        | 2003       | 2004          |
+----------------------------------------+------------+---------------+
| Toyota RAV4 4x4                        | 2003       | 2004          |
+----------------------------------------+------------+---------------+

As for the Infiniti vehicles listed in Table 6, since there are no
previous versions of the FX35 AWD, FX45AWD, and QX56 models -- which had
ESC starting in 2003 -- the model years 2002-2003 of the QX4 model were
used as comparison vehicles that did not have ESC. The 2001-2002 and
2003-2004 Land Rover Range Rover models were not of the same chassis
generation. The Toyota 4Runner had a design change in 2003 for both the
4x2 and 4x4 models. The static stability factor was higher on the
2003-04 4Runners. Hence, the later model-year vehicles (post 2002 model
year) are more stable than the earlier model-year vehicles. Thus, one
would expect the 2003-04 4Runners to be involved in fewer rollover
crashes than the earlier models, even if the newer models are
hypothetically not equipped with ESC. For that reason, the 2003-04 model
years were not included in the analysis.[^10] Lexus changed their RX300
model to RX330 in 2004, the new model was included in the analysis.

The Mercedes-Benz ML320 -- Mercedes' first ever U.S. SUV model -- came
on the market in 1998 but was not equipped with ESC. A year later,
Mercedes-Benz had made ESC standard equipment in all their SUV models.
We could have included only the ML320 model in the analysis -- since the
model was still available in 2004; however, we decided to include other
Mercedes SUV models in the analysis to increase our sample size.

***[Table 6]{.underline}: \"Similar\" Make-Model Comparison of LTVs In
Model Years***

With ESC and Without ESC

+-----------------+-------------+----------------------+--------------+
| **Make Model**  | **Model     | **Make Model**       | **Model      |
|                 | Years**     |                      | Years**      |
|                 |             |                      |              |
|                 | **with**    |                      | **with**     |
|                 |             |                      |              |
|                 | **No ESC**  |                      | **ESC        |
|                 |             |                      | (Standard)** |
+-----------------+-------------+----------------------+--------------+
| Infiniti QX4    | 2002-2003   | Infiniti FX35 AWD    | 2003-2004    |
| 2WD             |             |                      |              |
|                 | 2002-2003   | Infiniti FX45 AWD    | 2003-2004    |
| Infiniti QX4    |             |                      |              |
| 4WD             |             | Infiniti QX56 4x2    | 2004         |
|                 |             |                      |              |
|                 |             | Infiniti QX56 4x4    | 2004         |
+-----------------+-------------+----------------------+--------------+
| Land Rover      | 2001-2002   | Land Rover Range     | 2003-2004    |
| Range Rover     |             | Rover                |              |
+-----------------+-------------+----------------------+--------------+
| Lexus RX300     | 1999-2000   | Lexus RX300          | 2001-2003    |
|                 |             |                      |              |
|                 |             | Lexus RX330          | 2004         |
+-----------------+-------------+----------------------+--------------+
| Toyota 4Runner  | 1999-2000   | Toyota 4Runner 4x2   | 2001-2002    |
| 4x2             |             |                      |              |
+-----------------+-------------+----------------------+--------------+
| Toyota 4Runner  | 1999-2000   | Toyota 4Runner 4x4   | 2001-2002    |
| 4x4             |             |                      |              |
+-----------------+-------------+----------------------+--------------+

***[Table 7]{.underline}***: \"Similar\" Make-Model Comparison of
Mercedes-Benz LTVs In Model Years With ESC and Without ESC

+-------------+--------------+--------------+-------------------------+
| **Make      | **Model      | **Make       | **Model Years**         |
| Model**     | Years**      | Model**      |                         |
|             |              |              | **without**             |
|             | **with**     |              |                         |
|             |              |              | **ESC (Standard)**      |
|             | **No ESC**   |              |                         |
+-------------+--------------+--------------+-------------------------+
| ML320       | 1998         | ML320        | 1999-2004               |
|             |              |              |                         |
|             |              | ML350        | 2003-2004               |
|             |              |              |                         |
|             |              | ML430        | 1999, 2001-2002         |
|             |              |              |                         |
|             |              | ML500        | 2002-2004               |
|             |              |              |                         |
|             |              | ML55         | 2000,2004               |
+-------------+--------------+--------------+-------------------------+

**CRASH DATA (STATE AND FARS)**

Once specific make, model, and model year of vehicles -- equipped with
ESC and those not equipped with ESC -- were selected for passenger cars
and LTVs, those vehicles can then be identified in the crash data for
analysis. In general, a large sample of crash-involved cars from the
selected models is desirable to statistically estimate the effect of ESC
in reducing crashes. The best source of data available to NHTSA for this
analysis is the State data files maintained by the agency's National
Center for Statistics and Analysis (NCSA). NCSA currently receives crash
data from 28 States and maintains these data files for calendar years
1989 and onward. Since ESC is currently not required safety equipment,
its presence in crash-involved vehicles is not encoded as a separate
field in the police reports. However, we can identify the make, model,
and model year of each crash-involved vehicle from the State data files
by decoding the Vehicle Identification Number (VIN) -- if such
information is available.

Twenty-one States were excluded from the analysis for various reasons.
For instance, Alabama, Arkansas, Colorado, Delaware, Indiana, Minnesota,
Montana, South Carolina, Texas, and Virginia do not have VIN information
in their files. Georgia, Michigan, North Carolina, New York, and
Washington were excluded because NHTSA does not have their files beyond
calendar year 2000, a critical year in the analysis because the majority
of our passenger car sample consists of Mercedes-Benz and BMW vehicles
-- the first two manufacturers that offered ESC on all their models by
2000 and 2001, respectively. Thus, without the post 2000 data, our data
sample would consist of mostly crashes from non-ESC vehicles. Kansas,
New Mexico, and Ohio were not uniform in reporting VIN, having a low
percentage of VIN in some years and a high percentage in others. Utah
and Wyoming are States with small numbers of crashes. Maryland data in
nearly half of the reported cases do not indicate the contributing
circumstances for the vehicles involved in the crashes. We must have
this information to 1) determine which vehicle was responsible for the
crash and which was not and 2) classify each vehicle's involvement as
either the control group or response group (relevant) involvement.

As a result, data from seven populous States (California, Florida,
Illinois, Kentucky, Missouri, Pennsylvania, and Wisconsin) were used in
the analysis. These States (except California) consistently have a high
percentage of VIN information in their data files (Table 8). California
does not have VIN information in its data files. Thus, we were not able
to identify (from the VIN) specific make-models that had ESC and those
that did not, but we included California in the analysis because it is a
large State with large numbers of crash data. We know that Mercedes-Benz
first installed ESC in certain S class models and had made it standard
equipment in all their passenger car models by 2001. Mercedes-Benz did
have a few models that had ESC as optional equipment prior to 2001. As
previously mentioned, we did examine vehicles with ESC as optional
equipment under the assumptions that those vehicles did not have ESC,
and the initial results indicated that the additional data did not make
a big difference in the overall effectiveness. Thus, for the analysis of
crash data in California, we assumed that all Mercedes-Benz passenger
car models prior to 2000 did not have ESC at all. As for their SUV
models, Mercedes-Benz installed ESC in all their models from model years
1999 and onward. Thus, only the 1998 Mercedes-Benz SUV models did not
have ESC. As a result, in the California analysis, we were able to
analyze crashes that involved only Mercedes-Benz vehicles -- because we
were able to identify those vehicles using the variables MAKE, MOD_YR
(model year), and CHP_TYP (California Highway Patrol vehicle type -- to
differentiate cars from SUVs).

Although the number of vehicles involved in fatal crashes is relatively
small, ESC is believed to be highly effective in reducing fatal crashes.
If so, even a relatively small sample size will suffice for
statistically significant effects. Currently available crash data from
Fatality Analysis Reporting System (FARS) from calendar years 1997-2004
were used for the fatal crash analyses. FARS data files consistently
have high percentage of VIN information in their data files (97% or
better). Thus, the make, model, and model year of fatal crash-involved
vehicles can be decoded from the VIN information.

***[Table 8]{.underline}***: Percentage of VIN Information by State and
Calendar Year

  -------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------
  **State**      **Calendar   **Calendar   **Calendar   **Calendar   **Calendar   **Calendar   **Calendar
                 Year 1997**  Year 1998**  Year 1999**  Year 2000**  Year 2001**  Year 2002**  Year 2003**

  Florida        90%          90%          90%          89%          88%          90%          90%

  Illinois       86%          88%          87%          77%          80%          87%          \-

  Kentucky       95%          95%          92%          97%          97%          97%          \-

  Missouri       84%          87%          87%          88%          87%          93%          93%

  Pennsylvania   93%          94%          94%          94%          95%          \-           95%

  Wisconsin      91%          91%          90%          91%          90%          90%          90%
  -------------- ------------ ------------ ------------ ------------ ------------ ------------ ------------

Since NCSA typically receives crash data from the States two years after
or even longer in some States, our State data analyses consist of crash
data from calendar years 1997-2003 for Florida, 1997-2002 for Illinois,
1997-2002 for Kentucky, 1997-2001 and 2003 for Pennsylvania, and
1997-2003 for Wisconsin -- Table 9. Currently, the 2003 Illinois, 2003
Kentucky, and 2002 Pennsylvania crash files are not available. Only data
from calendar years 2001-2003 were included in the California analysis
because we could not distinguish (from the "vehicle type" variable in
the data files prior to 2001) between crashes involving passenger cars
and those involving SUVs. Post-2000 California data files differentiate
passenger car and SUVs crash involvements. As for the fatal crash
analyses, FARS data files are generally available one year after. Thus,
our FARS data analysis consists of fatal crash data from calendar years
1997-2004.

***[Table 9]{.underline}***: Calendar Years of Crash Data By State

  ------------------------------------- ---------------------------------
  **State**                             **Calendar Years**

  California                            2001-2003

  Florida                               1997-2003

  Illinois                              1997-2002

  Kentucky                              1997-2002

  Missouri                              1997-2003

  Pennsylvania                          1997-2001, 2003

  Wisconsin                             1997-2003
  ------------------------------------- ---------------------------------

**ANALYSIS DATABASES**

*A) VIN Decode*

The analysis databases are crash files initially obtained from the FARS
and State data files and then decoded from the VIN to include only the
selected make-models listed in Tables 2-7. In other words, in Florida,
Illinois, Kentucky, Missouri, Pennsylvania, and Wisconsin State data
files, vehicle make, model, and model year were decoded from the
variable VIN and matched with the VIN information obtained from the
Passenger Vehicle Identification Manual[^11] for the selected
make-models. To ensure that there are no discrepancies in reporting VIN
information of crash-involved vehicles, the decoded model year from the
variable VIN had to match with the variable MOD_YR (model year) taken
directly from the State data files. Since California does not have the
VIN variable, the selected Mercedes-Benz make-models were obtained by
using the variable MAKE in the data files. Like other States, California
does have the variable MOD_YR in its data files; thus, the model year of
crash-involved vehicles was obtained directly from that variable.

Like the State data files (except California), FARS data files include
both the variables VIN and MOD_YEAR (model year). Thus, we were able to
extract information that identified the vehicle make, model, and model
year from the VIN variable and established an analysis database
consisting of crash data of make-models listed in Tables 2-7. Also, the
model year taken from the VIN variable had to correspond with the
variable MOD_YEAR.

The resulting analysis databases -- whether they are FARS or State files
-- are vehicle-oriented files, with one record for each vehicle that was
involved in a crash. Since these files only contain the selected
make-models from Tables 2-7, only vehicles with model years 1997-2004
for the FARS files and model years 1997-2003 for the State files were
included in the study. One of the critical parameters used in the
databases is the parameter that identifies the presence or absence of
ESC. Each record in the databases will have a variable **ESC** with a
value of 1 for a vehicle that has ESC and a value of 0 for the one that
does not.

*B) Crash involvements*

Certain crash involvements in which a vehicle 1) was stopped, parked,
backing up, or entering/leaving a parking space prior to the crash, 2)
traveled at a speed less than 10 miles per hour (mph), 3) struck in the
rear by another vehicle, or 4) was a non-culpable party in a
multi-vehicle crash on a dry road, were considered the **control group**
(non-relevant involvements) -- because ESC would not have prevented the
crash. Since ESC has the ability to detect when a driver is about to
lose control of a vehicle and the capability to automatically intervene
to assist that driver and ultimately reduce the likelihood of a roadside
departure, it may provide considerable benefits to crashes that involve
a vehicle *running off the road* or *running out of the lane and hitting
other vehicles*. Most run-off-road crashes are single-vehicle
involvements, and these involvements include rollovers and collisions
with fixed objects. Other relevant crash involvements -- where ESC could
be a factor in the vehicle's involvement -- include collisions with one
or more vehicles due to the driver's error or the vehicle's
unsatisfactory performance (known as culpable involvements in
multi-vehicle crashes). These crashes are relevant because they may (but
not necessarily) have involved loss of control. Hence, these relevant
involvements were included in the analysis and considered the **response
group**. We also analyzed crashes that involved a pedestrian, bicycle,
or animal -- although the vehicles involved in these crashes were not
typically at fault. We would still be interested in knowing whether or
not ESC is effective in reducing these types of crashes.

An important concept in setting up a control- and response-group
experiment is that not every vehicle in the response group
[necessarily]{.underline} went out of control or stood to benefit from
ESC. For example, a careless driver could steer a vehicle off the road
or into the path of another vehicle with full directional control.
Moreover, these crash data generally do not specify if a vehicle "went
out of control" or not. Instead, it is the converse that is true: if a
vehicle [did]{.underline} go out of control and get into a crash, it
will be in a response group and [not]{.underline} in the control group.
The vehicles that could have benefited from ESC are a subset of the
response-group involvements and are not control-group involvements. As
mentioned previously, vehicles that most likely would [not]{.underline}
benefit from ESC are considered the control-group involvements -- and
while this group ideally should be as unaffected as possible by ESC, the
effectiveness estimates are quite dependent on the choice of the control
group. For example, if crashes that were affected by ESC were included
in the control-group involvements, the estimate of ESC effectiveness
would be lower. On the other hand, if crash involvements that were
unaffected by ESC were included in the response-group, the effectiveness
estimate would not be compromise as long as the set-up of a control- and
response-group experiment was correct and valid.

The next several sections discuss how these crash involvements were
obtained. The control-group involvements (non-relevant) will be
discussed first followed by the relevant involvements -- the response
group.

*Control group -- non-relevant crash involvements*

Ideally, only crash involvements where a vehicle was standing still
prior to the crash, moving less than 10 miles per hour, or backing up
should be considered the control group -- since we are almost certain
that these vehicles were moving too slowly to trigger ESC and make it a
factor in the vehicle's involvement. But such fatal involvements are few
in number -- when compared to the relevant involvements. Thus, a larger
sample of vehicles is needed for the control group. Other non-relevant
involvements where ESC is unlikely to provide any benefit include
situations where 1) a vehicle was struck in the rear in a rear-end
collision or 2) a vehicle was involved in a multi-vehicle crash on a dry
road but was not at fault. In both situations, the motion of this
vehicle is not what precipitated the crash, and this vehicle definitely
did not hit something else because it had gone out of control.

In FARS, vehicle "maneuver" and "travel speed" prior to the crash were
used to define one portion of the control-group involvements in which a
crash-involved vehicle was either at a complete stop or moving at a low
speed -- less then 10 miles per hour. In addition, the maneuvers
"backing up", "parking", or "leaving a parking space" indicate the
vehicle was moving slowly (most likely less than 10 mph) even if FARS
does not specify a travel speed. The other portion of the control group
includes vehicles that are considered a non-culpable party in a fatal
multi-vehicle crash on a dry road. To determine whether or not a vehicle
was at fault, related factors such as the driver's physical and mental
condition, attitude, and driving actions as well as the condition of the
vehicle and roadway (prior to the crash) were taken into consideration.
These factors are coded in FARS and State data files. Essentially, a
vehicle is in the control group if none of these factors were present.
Multi-vehicle crash involvements will be discussed separately in the
next section.

In theory, "not at fault" involvements on wet roads should be in the
control group. However, given that many involvements follow loss of
control, and given some uncertainty as to the accuracy of the "fault"
determination, those involvements were not assigned to the control
group.

Finally, in a front-to-rear collision (neither vehicle backing up), we
will always assign the rear-impacted vehicle to the control group and
the frontally impacting vehicle to the response group.

As for the States, every State has its own unique way of coding vehicle
"maneuver", "speed", and initial "point of impact", and not every State
includes these variables in their data files. Similar to FARS,
contributing circumstances relating to the driver, vehicle, and roadway
were used to determine which vehicle was responsible for the crash and
which was not. If a State has a variable that indicates whether or not
the vehicle's driver was charged with a moving violation, then we also
used that variable to classify the vehicle's involvement (culpable or
non-culpable) in a multi-vehicle crash. Again, not all variables are
included in every State, and even the same variables are not coded
exactly the same in each State. Thus, depending on what crash
information is available in each State, the control group used in each
of the State data analyses includes different types of non-relevant
crash involvements. But at least the definitions for each of these types
of involvements are made as similar as possible.

The definitions used in all the State data analyses for "stopped",
"parked", "backing", and "parking" maneuvers are self-explanatory and
were taken directly from the variable VEH_MAN1 in the State data files.
If a State also reports the speed of a vehicle (variable SPEED) prior to
the crash, then the crash-involved vehicle with a speed ranging from 0
to 10 mph would be included in the control group as well. Similar to the
control group used in the FARS analysis, the multi-vehicle crash
involvements had to occur on dry roads and be *non-relevant*
(non-culpable) -- in order for those involvements to be included in the
control group for the State data analyses. The next section will discuss
in detail the different types of multi-vehicle crash involvements used
in the FARS and State analyses.

*Classification of multi-vehicle crash involvements (control and
response group)*

Multi-vehicle crash involvements were classified as either culpable or
non-culpable based on what information is available in the data files.
In many crashes that involve two motor vehicles in transport, one
vehicle can be identified as responsible for the crash, and the other
one not. In some State files this is almost a hard-and-fast rule, with
one culpable and one non-culpable vehicle in nearly every crash, but in
other States there may be many crashes where none or sometimes even all
the vehicles are judged culpable. In multi-vehicle crashes other than
front-to-rear collisions, the action of the driver, the characteristic
of the vehicle, the condition of the roadway, or simply the traffic
violation charged were taken in account to determine which vehicle(s)
(if any) contributed to the crash and which did not. In general, the
vehicle's involvement -- where the driver in that vehicle was distracted
or under the influence of drugs and/or alcohol, drove carelessly or
aggressively, failed to yield right-of-way, performed an improper turn,
backing, passing, or lane change, followed too closely, disregarded a
traffic signal, sign, or other traffic control, or exceeded the stated
speed limit -- was considered the most likely to cause the crash. Other
contributing circumstances include the condition of the vehicle's tires
(i.e., worn, blowout, puncture, etc.) or the malfunction of the
vehicle's brake systems or the steering mechanism prior to the crash.
Although certain roadway conditions (i.e., wet, slush, snow, icy, sand,
dirt, oil, etc.) might have caused the driver to lose control of the
vehicle and run off the road or hit another vehicle, the striking
vehicle would still be considered the culpable party. Furthermore, if a
crash-involved vehicle in which the driver were charged with a traffic
violation, then that vehicle would be considered the offender. Other
contributing factors -- whether they are driver, vehicle, or roadway
contributing circumstances -- were not considered evidence of
culpability. As an exception to this approach, in a two-vehicle
front-to-rear collision, neither vehicle backing up, the collisions are
identified by MAN_COLL="rear-end" on the accident level and IMPACT1 (or
its equivalent) on the vehicle level: one vehicle has frontal damage and
the other, rear damage. The vehicle with frontal damage (known as
rear-end striking) will always be classified as the culpable party, and
the one with rear damage (known as rear-end struck) will be
non-culpable. The criteria for determining culpability in the FARS
analysis are similar to those used in the State analysis. In addition to
the ideal control-group involvements (i.e., vehicle was parked, stopped,
or traveled at very slow speed), the control-group also included
non-culpable involvements in multi-vehicle crashes on dry roads.

Up until now, we have discussed all control-group involvements in which
a crash-involved vehicle was 1) standing still, 2) moving less than 10
miles per hour, 3) backing up, 4) parking or leaving a parked position,
5) being struck in the rear by another vehicle, or 6) non-culpable in a
multi-vehicle crash on a dry road. We have also talked about certain
relevant involvements (i.e., response group) where 1) a crash-involved
vehicle was considered a culpable party in a multi-vehicle crash on any
roads (dry or wet) and 2) ESC could play a critical role in preventing
those crashes.

*Categories of single-vehicle crashes*

Now, let us discuss further other relevant involvements -- mainly
crashes that involve only one motor vehicle in transport --
single-vehicle crashes. In most single-vehicle involvements -- whether
they are fatals or non-fatals -- the case vehicle either rolled over or
hit a fixed object. Those incidents will most likely occur when the
vehicle [unintentionally]{.underline} leaves the travel lane (runs off
the road), which is an indication of a [driver's error]{.underline} due
to loss of steering and/or directional control of the vehicle, because
it is very unlikely for a vehicle (even for the high center-of-gravity
vehicle) to tip over or hit a fixed object while it is still in the
travel lane and under control. In other single-vehicle involvements
where the driver [intentionally]{.underline} steered the vehicle away
from the incoming object (i.e., pedestrian, bicycle, or animal) to avoid
hitting the object but ended up hitting it, the driver is typically [not
at fault]{.underline} in this case, but the involvement is still
considered relevant because ESC could perhaps play a role in the
vehicle's involvement.

Run-off-road crashes involve vehicles that travel out of the lane --
where they may or may not yaw out of control, leave the roadway, and
eventually roll over and/or hit one or more natural or artificial
objects along or off the roadside -- such as trees, guardrails,
embankments, etc. They can also occur on the median of a divided roadway
or on the other side of a non-divided roadway. Run-off-road crashes
typically involve a single motor vehicle in transport -- although a
moving vehicle hitting a parked vehicle could also be considered a
multi-vehicle involvement. For the purpose of analysis, those
involvements are treated as single-vehicle involvements.

We know from our preliminary study, as well as studies by other
researchers that ESC appears to provide significant benefits related to
single-vehicle crashes -- especially those that are likely to have
involved yawing: rollovers and side impacts with fixed objects.
"Rollover" generally involves a vehicle that went out of control,
started to yaw while it was still on the road, and eventually rotated 90
degrees or more, side-to-side or end-to-end, as a result of a tripping
mechanism when it left the roadway. "Side impact with a fixed object"
usually indicates a vehicle that left the roadway, possibly after yawing
out of control, and collided with a fixed object. We were able to define
these crash involvements using the "first harmful event" variable at the
accident level and the "initial point of impact" variable at the vehicle
level. These variables are coded in the FARS and State data files.

In the 2004 NHTSA's report, we studied ESC by looking at its
effectiveness in reducing single-vehicle crashes as a whole and
excluding crashes that involve a vehicle colliding with a pedestrian,
bicycle, or animal. In other words, the response group (used in the 2004
study) excluded all crash involvements with a pedestrian, bicycle, or
animal and included all other involvements with one motor vehicle in
transport as single-vehicle crashes. In this follow-up study, we not
only included pedestrian, bicycle, and animal crashes in our study but
also looked at other single-vehicle crashes separately. In other words,
we analyzed separately -- the effectiveness of ESC in reducing 1)
rollovers, 2) side impacts with fixed objects 3) other run-off-road
crashes, 4) pedestrian, bicycle, and animal crashes, and 5) other
single-vehicle crashes (such as collisions with parked cars, impacts
with thrown or falling objects, undercarriage scrapes, or first-event
fires). Rollovers, side impacts with fixed objects and other
run-off-roads are considered "all run-off-road" crashes, and they are a
subset of the single-vehicle involvements used in the 2004 study. "All
run-off-road" and "other single-vehicle" involvements used in this study
are what we have used in the 2004 study as "single-vehicle crashes" --
since we excluded pedestrian, bicycle, and animal crashes in that study.
However, a direct comparison of the relative effectiveness of ESC should
be made with caution since the control groups used in the two studies
are not the same -- as already discussed in the previous section. In
this study, we also included culpable involvements in multi-vehicle
crashes on all roads as part of the response group -- because ESC could
be a factor in the vehicle's involvement. We also calculated the effect
of ESC on all crash involvements, including the control-group
involvements as well as the relevant involvements.

**2X2 CONTINGENCY TABLE ANALYSIS**

This study consists of a series of analyses of crash data from currently
available State and FARS databases. Crash data from calendar years
1997-2003 from seven States as well as FARS data from calendar years
1997-2004 were used in the analysis. The analysis then compares specific
make-models of passenger cars and LTVs with ESC versus earlier versions
of similar make-models, using non-relevant crash involvements as a
control group, essentially creating 2x2 contingency tables of crash
involvements (control group and response group) of vehicles with ESC and
those without ESC. A general notation of a 2x2 contingency table is
shown below in Table 11.

***[Table 11]{.underline}***: General Notation for a 2x2 Contingency
Table

+---------------------+----------------+--------------+----------------+
| **Type of crash     | **Vehicles**   | **Vehicles   | **Totals**     |
| involvement**       |                | W*ith* ESC** |                |
|                     | ***Without*    |              |                |
|                     | ESC**          |              |                |
+---------------------+----------------+--------------+----------------+
| Control-group       | a              | b            | a + b          |
|                     |                |              |                |
| (non-relevant       |                |              |                |
| crashes)            |                |              |                |
+---------------------+----------------+--------------+----------------+
| Response group      | c              | d            | c + d          |
|                     |                |              |                |
| (relevant crashes)  |                |              |                |
+---------------------+----------------+--------------+----------------+
| Total               | a + c          | b + d        | a + b + c + d  |
|                     |                |              | = N            |
+---------------------+----------------+--------------+----------------+

To detect if ESC is effective, we considered and tested the null
hypothesis that the ratio of d to c is the same as the ratio of b to a.
To calculate the effectiveness of ESC, first, we compute the ratios of
relevant crash involvements to control-group involvements. Then, we
compute the percentage reductions in these ratios in vehicles with ESC
versus earlier versions of similar make-models without ESC. These
calculations are illustrated below by the formula:

$\text{Effectiveness}$ $(\text{\%})$ $=$
$\left\{ 1 - \lbrack(\frac{d}{c}\frac{)}{(}\frac{b}{a})\rbrack \right\} \times \text{100}$

To test for statistical significance of the effectiveness of ESC, we
used the chi-square statistic$(\chi^{2})$for the 2x2 table.

For a 2x2 contingency table, the degrees of freedom are always 1. For a
two-sided 95% confidence level (a conventionally accepted significance
level) and degrees of freedom equal to 1, a chi-square statistic has to
exceed 3.843 -- which indicates that the two distributions are not the
same -- to reject the null hypothesis and accept our hypothesis that the
ratio of relevant to control-group crashes is lower in the ESC-equipped
vehicles than in the comparison vehicles without ESC. We will also
accept as "statistically significant" a one-sided 95% confidence level,
when chi-square exceeds 2.71. NHTSA evaluations of safety equipment
customarily employ the more lenient one-sided test when there is a clear
expectation that the effect of the equipment will be in the "right"
direction (saving lives, preventing crashes) or, at worst, zero. It is
unlikely to be negative. Only in situations where there is no realistic
a priori expectation of an effect in either direction do we rely
exclusively on the more stringent two-sided test.

The next several sections will show 2x2 contingency tables of different
types of crash involvements (FARS and State files) for vehicles with ESC
versus those with no ESC. As previously discussed, the control group
includes crash involvements in which a vehicle was standing still prior
to the crash, moving less than 10 mph or backing up, struck in the rear,
or non-culpable party in a multi-vehicle crash on a dry road. The tables
will also show different types of relevant crash involvements such as
single-vehicle run-off-road, other single-vehicle collision, culpable
involvement in a multi-vehicle collision, and collision with a
pedestrian, bicycle, or animal. Run-off-road involvements such as side
impacts with fixed objects, rollovers, and other run-off-roads will also
be included in these tables. *All run-off-road involvements discussed in
this study are single- vehicle involvements*. In addition, the
effectiveness of ESC and the chi-square statistic of each type of
relevant involvement will be calculated and illustrated in these tables.
Lastly, the overall effectiveness of ESC on all crash involvements
(including the culpable, non-culpable, and control-group involvements)
will be shown in these tables -- to demonstrate the overall effect of
ESC since these relevant involvements consist of only a fraction of all
the involvements. Thus, the actual effectiveness will always be less
when considering all crash involvements.[^12] For instance, if the
effectiveness of ESC is estimated to be 30 percent for the relevant
involvements (i.e., all run-off-roads, other single-vehicle collisions,
collisions with pedestrians, bicycles, or animals, and all culpable
multi-vehicle collisions), which are 50 percent of all crash
involvements, then the effectiveness on all crash involvements is
reduced to

(0.30 x 0.50) x 100 = 15 percent

As previously mentioned, the control group includes not only crash
involvements in which a vehicle was standing still prior to the crash,
moving at a very low speed, or backing up, but also other involvements
where a vehicle was considered a non-culpable party in a multi-vehicle
crash on a dry road. For completeness, these tables will also show other
multi-vehicle involvements for vehicles with ESC and those without ESC
-- so that the sum of all the involvements (control group, all
run-off-road, other single-vehicle, pedestrian, bicycle, animal,
culpable involvements as well as other involvement in multi-vehicle
crashes) equals the "total" involvements, which are shown prior to the
last row in each of the tables. But the effectiveness and chi-square
statistic will not be calculated for other multi-vehicle involvements
because interpretation of the effectiveness estimates on those
involvements is not clear -- since these involvements also included the
non-relevant involvements in multi-vehicle crashes on non-dry roads. As
for the "total" involvements, the effectiveness calculation is also not
suitable because they also include the control-group involvements.

*[Fatal crashes in passenger cars]{.underline}*

Now let us first look at the effectiveness of ESC in fatal crashes in
passenger cars. As illustrated in Table 12, ESC reduced fatal
run-off-road crashes by 36 percent

\[1 -- {(154/217)/(183/166)}\]. The reduction is statistically
significant with a chi-square statistic of 8.62 for the 2x2 table
consisting of these four cells. It is also evident that fatal rollover
risk is substantially lower with ESC -- a statistically significant 70
percent reduction relative to the control group. Fatal side impacts with
fixed objects were significantly reduced as well -- as shown by the 49
percent effectiveness and a 5.89 chi-square statistic. The negative
effectiveness (-36 percent) relative to the control group in pedestrian,
bicycle, and animal crashes suggests possible increase, but the increase
is statistically non-significant. On a more positive note, culpable
involvements in multi-vehicle crashes were also decreased (19 percent),
but the reduction is not statistically significant. Overall, ESC is
still very beneficial (14 percent effectiveness) if all crash
involvements were considered -- which included culpable, non-culpable,
and the control-group involvements. To calculate the overall
effectiveness, the sum of all run-off-roads, other single-vehicle,
pedestrian, bicycle, and animal involvements, as well as culpable and
non-culpable involvements in multi-vehicle crashes must first be
calculated for vehicles with ESC and those with no ESC
\[(154+3+69+157+76=459) and (217+6+46+176+65=510), respectively\]. The
effectiveness in these non-control group involvements relative to the
control-group involvements can then be calculated
\[1-{(459/183)/(510/166)} = 18 percent\]. The effect of ESC on all fatal
crash involvements for passenger cars can finally be determined by
multiplying the effectiveness in non-control group involvements by the
proportion of the total crash involvements that were relevant \[18 x
(510/676) = 14 percent\]. The effect of ESC on all crash involvements is
not statistically significant because the effect on all non-control
group crash involvements is not.

As previously discussed, *"all run-off-road" and "other single-vehicle"
involvements used in this study are what we have categorized in the 2004
study as "single-vehicle crashes" -- which did not include pedestrian,
bicycle, and animal crashes*. For comparison purpose (though the control
groups used in the 2004 study and in this study are not the same), we
also included in the analysis tables the effectiveness estimates of ESC
in reducing *single-vehicle crashes except those involving pedestrians,
bicycles, or animals*. The results from this study (shown in Table 12)
suggest that ESC reduced fatal single-vehicle crash involvements of
passenger cars by 36 percent -- if pedestrian, bicycle, and animal
crashes were excluded -- and the reduction is statistically significant.

***[Table 12]{.underline}: Effect of ESC on Fatal Crashes in Passenger
Cars***

(FARS: 1997-2004)

  ------------------------------- ------------ ------------ ------------------- ----------------
  **Type of fatal crash           **Vehicles   **Vehicles   **Effectiveness**   **\*Chi-square
  involvement**                   with no      with ESC**                       Value**
                                  ESC**                                         

                                  **Fatal      **Fatal      **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***166***    ***183***                        

  **All run-off-road**            **217**      **154**      **36**              **8.62**

  **Side impact with fixed        **41**       **23**       **49**              **5.89**
  object**                                                                      

  **Rollover**                    **36**       **12**       **70**              **12.71**

  **Other run-off-road**          **140**      **119**      **23**              **2.50**

  **Pedestrian/bicycle/animal**   **46**       **69**       **-36**             **1.99**

  **Other single vehicle**        **6**        **3**        **55**              **1.28**

  **Culpable multi-vehicle**      **176**      **157**      **19**              **1.91**

  **Other multi-vehicle\*\*\***   **65**       **76**       []{.mark}           []{.mark}

  **All non-control group         **510**      **459**      **18**              **2.64**
  involvements**                                                                

  **All crash involvements**                                **14**              

  Total                           676          642                              

                                                                                

  **Single-vehicle crashes\*\***  **223**      **157**      **36**              **9.04**
  ------------------------------- ------------ ------------ ------------------- ----------------

\* For the 2x2 table formed by this row and the "control group" row.

\*\* excludes pedestrian, bicycle, animal crashes.

\*\*\* includes involvements that are not culpable but are not part of
the control-group.

*[Fatal crashes in LTVs]{.underline}*

For LTVs (the majority of the analysis sample consists of SUVs), ESC is
highly effective in reducing fatal run-off-road crashes (70 percent
effectiveness, chi-square statistic 29.90), especially rollovers (88
percent) -- Table 13. In this analysis, ESC has little observed effect
on crashes that involved pedestrians, bicycles, or animals. Culpable
involvements in fatal multi-vehicle crashes significantly declined (34
percent effectiveness; chi-square statistic 3.64) in LTVs with ESC. The
overall effectiveness in all fatal crash involvements is a positive 28
percent -- which indicates a substantial benefit with ESC in reducing
all fatal crashes in LTVs. Similar to the result from the 2004 study,
ESC is highly effective in reducing single-vehicle crashes involving
LTVs -- as shown in Table 13 by the positive 63 percent effectiveness,
and 23.32 chi-square statistic.

***[Table 13]{.underline}: Effect of ESC on Fatal Crashes in LTVs***

(FARS: 1997-2004)

  ------------------------------- ------------ ------------ ------------------- ----------------
  **Type of fatal crash           **Vehicles   **Vehicles   **Effectiveness**   **\*Chi-square
  involvement**                   with no      with ESC**                       Value**
                                  ESC**                                         

                                  **Fatal      **Fatal      **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***153***    ***95***                         

  **All run-off-road**            **191**      **36**       **70**              **29.90**

  **Side impact with fixed        **15**       **6**        **36**              **0.78**
  object**                                                                      

  **Rollover**                    **106**      **8**        **88**              **37.56**

  **Other run-off-road**          **70**       **22**       **49**              **6.16**

  **Pedestrian/bicycle/animal**   **56**       **37**       **-6**              **0.06**

  **Other single vehicle**        **6**        **9**        **-142**            **2.78**

  **Culpable multi-vehicle**      **108**      **44**       **34**              **3.64**

  **Other multi-vehicle\*\*\***   **69**       **40**                           

  **All non-control group         **430**      **166**      **38**              **8.96**
  involvements**                                                                

  **All crash involvements**                                **28**              

  Total                           583          261                              

                                                                                

  **Single-vehicle crashes\*\***  **197**      **45**       **63**              **23.32**
  ------------------------------- ------------ ------------ ------------------- ----------------

\* For the 2x2 table formed by this row and the "control group" row.

\*\* excludes pedestrian, bicycle, animal crashes.

\*\*\* includes involvements that are not culpable but are not part of
the control-group.

*[Effectiveness confidence bounds in various fatal crash
involvements]{.underline}*

Now that we have discussed ESC effectiveness in various fatal crash
involvements in passenger cars and LTVs, let us discuss further the
confidence bounds for certain effectiveness estimates that are
statistically significant. The entries in Table 11 will be used to
produce an approximate sense of sampling errors and confidence bounds.
As previously explained, the two numbers in the table, a and b, are
counts of non-relevant crash involvements (control group) while the
other two numbers, c and d, represent counts of relevant involvements in
various crash modes -- in vehicles that are equipped with ESC versus
earlier versions of similar make-models that are not. These numbers can
be considered independent Poisson variates.[^13] As discussed, the
effectiveness statistic in various crashes

$$E = 1 - \hat{\theta} = 1 - \left\lbrack (dc) \div (ba) \right\rbrack$$

is based sample odds ratio, $\hat{\theta}$.

The sampling distribution of the odds ratio can be highly skewed even
for moderately large sample sizes. On the other hand, the log transform
of the sample odds ratio, log $\hat{\theta}$, has a less skewed sampling
distribution and thus, is more symmetric. For large sample sizes, the
distribution can be approximate to a normal distribution with a mean of
log $\theta$ and a standard deviation often referred to as asymptotic
standard error and denoted by ASE, of

$$\text{ASE}(\text{log \{}\hat{\theta}) = \sqrt{\left\lbrack \left( \frac{1}{a} \right) + \left( \frac{1}{b} \right) + \left( \frac{1}{c} \right) + \left( \frac{1}{d} \right) \right\rbrack}$$

Taking 1.645 standard deviations on either side of the log
$\hat{\theta}$ yields approximate confidence bounds (two-sided α = .10,
i.e., 90 percent bounds):

$$\text{log \{}\hat{\theta} \pm 1\text{.}\text{645}\text{ASE}(\text{log \{}\hat{\theta})$$

The confidence intervals can then be transformed back by using the
exponential function to form the confidence interval for the odds ratio
and subtracting from 1 to calculate effectiveness estimates.

Tables 14 and 15 show not only the estimates and chi-square statistics
but also the confidence bounds of the effects of ESC for the following
crash involvements: all run-off-road, rollover, culpable multi-vehicle,
single-vehicle (excluding pedestrian, bicycle, animal crashes), and all
crash involvements -- in passenger cars and LTVs, respectively.

In Table 14, the effectiveness statistic in all run-off-road crashes is
1 -- 0.6438 = 0.3562, which is based on a sample odds ratio (154/217) /
(183/166) = 0.6438. The natural log of the sample odds ratio equals log
(0.64) = -0.4404 and its asymptotic standard error equals \[1/166 +
1/183 + 1/217 + 1/154\] ^.5^ = 0.1503. A 90 percent confidence interval
for the sample log odds ratio
equals$- 0\text{.}\text{4404} \pm 1\text{.}\text{645}(0\text{.}\text{1503}),$or
(-0.6877, -0.1932). The corresponding confidence interval for the sample
odds ratio is

$$\left\lbrack \text{exp}( - 0\text{.}\text{6877})\text{,exp}( - 0\text{.}\text{1932}) \right\rbrack = (0\text{.}\text{5027},0\text{.}\text{8243})\text{.}$$

Consequently, the confidence bounds for the effectiveness estimate is

$\left\lbrack (1 - 0\text{.}\text{8243}) \ast \text{100},(1 - 0\text{.}\text{5027}) \ast \text{100} \right\rbrack =$
(18 percent, 50 percent).

Using the same approach, 69 percent effectiveness in rollovers yields
approximate confidence bounds (46 to 83 percent fatality reduction). A
non-significant 36 percent increase in crashes involving pedestrians,
bicycles, or animals suggests that the effectiveness of ESC is between
-95 and 5 percent. The reduction estimates are between -4 and 37 percent
for all culpable involvements in fatal multi-vehicle crashes. For all
fatal crash involvements in passenger cars, the confidence bounds for
the reduction (14 percent) are estimated to be 0 to 26 percent. For a 36
percent reduction in single-vehicle crashes involving passenger cars
(excluding pedestrian, bicycle, animal crashes), the confidence bounds
are between 18 and 50 percent.

Due to higher chi-square statistics for the effectiveness estimates
(smaller standard deviations) in certain fatal crash involvements in
LTVs (specifically in all run-off-roads and rollovers), the confidence
bounds are tighter in LTVs than in passenger cars. For instance, in all
fatal run-off-roads, the confidence bounds for the fatality reduction
are 56 to 80 percent in LTVs as compared to 18 to 50 percent in
passenger cars. Similarly, the effect of ESC in reducing fatal rollover
crashes is so large that the confidence bounds range from 77 to 94
percent. Since the observed effect of ESC in reducing crashes involving
pedestrians, bicycles, or animals crashes is negligible and
non-significant, the confidence interval is between -60 and 29 percent.
The reduction in culpable fatal multi-vehicle crash involvements in
LTVs, on the other hand, is not as large as the reductions in various
run-off-road involvements; thus, the confidence bounds are between 6 and
54 percent. For all types of fatal crashes in LTVs, the 28 percent
effectiveness yields approximate confidence bounds, 13 to 41 percent
fatality reduction. Last but not least, the reduction in single-vehicle
crashes (not including pedestrian, bicycle, animal crashes) for LTVs is
expected to be somewhere between 48 and 74 percent.

***[Table 14]{.underline}: Effectiveness Confidence Bounds***

in Certain Fatal Crash Involvements in Passenger Cars

  ------------------------------- ------------ ------------ ------------------- -------------- --------------
  **Type of fatal crash           **Vehicles   **Vehicles   **Effectiveness**   **Confidence   **Chi-square
  involvement**                   with no      with ESC**                       Bounds**       Value**
                                  ESC**                                                        

                                  **Fatal      **Fatal      **(%)**             **(%)**        
                                  Crashes**    Crashes**                                       

  ***Control group***             ***166***    ***183***                                       

  **All run-off-road**            **217**      **154**      **36**              **18 to 50**   **8.62**

  **Rollover**                    **36**       **12**       **70**              **46 to 83**   **12.71**

  **Pedestrian/bicycle/animal**   **46**       **69**       **-36**             **-95 to 5**   **1.99**

  **Culpable multi-vehicle**      **176**      **157**      **19**              **-4 to 37**   **1.91**

  **All crash involvements**      []{.mark}    []{.mark}    **14**              **0 to 26**    

  **Single vehicle crashes\***    **223**      **157**      **36**              **18 to 50**   **9.04**
  ------------------------------- ------------ ------------ ------------------- -------------- --------------

\* excludes pedestrian, bicycle, and animal crashes.

***[Table 15]{.underline}: Effectiveness Confidence Bounds***

***in Certain Fatal Crash Involvements in LTVs***

  ------------------------------- ------------ ------------ ------------------- -------------- --------------
  **Type of fatal crash           **Vehicles   **Vehicles   **Effectiveness**   **Confidence   **Chi-square
  involvement**                   with no      with ESC**                       Bounds**       Value**
                                  ESC**                                                        

                                  **Fatal      **Fatal      **(%)**             **(%)**        
                                  Crashes**    Crashes**                                       

  ***Control group***             ***153***    ***95***                                        

  **All run-off-road**            **191**      **36**       **70**              **56 to 79**   **29.90**

  **Rollover**                    **106**      **8**        **88**              **77 to 94**   **37.56**

  **Pedestrian/bicycle/animal**   **56**       **37**       **-6**              **-60 to 29**  **0**

  **Culpable multi-vehicle**      **108**      **44**       **34**              **6 to 54**    **3.64**

  **All crash involvements**      []{.mark}    []{.mark}    **28**              **13 to 41**   

  **Single vehicle crashes\***    **197**      **45**       **63**              **48 to 74**   **23.32**
  ------------------------------- ------------ ------------ ------------------- -------------- --------------

\* excludes pedestrian, bicycle, and animal crashes.

Thus far, we have seen that ESC is highly effective in reducing fatal
crashes -- not only in passenger cars but also in LTVs. Let us now look
at the results from the State analyses to see if these effectiveness
rates still hold for the State data analyzed. The State data samples
include fatal and non-fatal crash involvements although the vast
majority of the involvements are non-fatal (i.e., property damage,
possible injury, non-incapacitating and incapacitating injury). With the
State data, separate analyses were performed in each of the selected
States. Let us start with the analysis results in California.

*[Crashes in California]{.underline}*

*(Passenger cars)*

As previously explained, the California data sample only includes
Mercedes-Benz vehicles, because we could not identify other vehicles due
to lack of VIN information. As shown in Table 16, run-off-road crashes
in California were reduced by 66 percent in passenger cars equipped with
ESC, and the reduction (relative to the control group) is statistically
significant with a chi-square value of 125.22. Rollover crashes alone
were significantly decreased (76 percent effectiveness; chi-square
statistic 15.54). Side impacts with fixed objects were included in
"other run-off-road" involvements because we could not identify those
crashes based on the available information coded in the data files.
Crashes that involved pedestrians, bicycles, or animals in California
were decreased by 73 percent, and the decrease is statistically
significant. The reduction (22 percent) of culpable involvements in
multi-vehicle crashes is also statistically significant with chi-square
statistic 19.32. For all non-control group involvements, the decrease is
32 percent, and it is statistically significant (a chi-square value of
61.99). When considering all crash involvements in passenger cars in
California, the result showed that the benefit is quite strong for ESC
-- 15 percent.

***[Table 16]{.underline}: Effect of ESC on Crashes in Mercedes-Benz
Passenger Cars***

(California: 2001-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***1688***   ***2510***                       

  **All run-off-road**            **331**      **167**      **66**              **125.22**

  **Rollover**                    **25**       **9**        **76**              **15.54**

  **Other run-off-road**          **306**      **158**      **65**              **113.08**

  **Pedestrian/bicycle/animal**   **10**       **4**        **73**              **5.65**

  **Other single vehicle**        **48**       **60**       **16**              **0.78**

  **Culpable multi-vehicle**      **874**      **1017**     **22**              **19.32**

  **Other multi-vehicle\***       **151**      **183**      []{.mark}           []{.mark}

  **All non-control group         **1414**     **1431**     **32**              **61.99**
  involvements**                                                                

  **All crash involvements**                                **15**              

  Total                           3102         3941                             
  ------------------------------- ------------ ------------ ------------------- --------------

\*includes involvements that are not culpable but are not part of the
control-group.

*(LTVs -- mostly SUVs)*

As shown in Table 17, ESC is even more effective (81 percent) in
reducing run-off-road crashes in LTVs than in passenger cars, and the
reduction is statistically significant. Particularly, LTVs equipped with
ESC decreased the risk of rolling over by 78 percent. Likewise, those
vehicles also reduced the risk of hitting a pedestrian, bicycle, or an
animal by 26 percent although the effect is not statistically
significant. Culpable involvements of LTVs in multi-vehicle crashes in
California were reduced by nearly one-fourth (24 percent). The reduction
was even more significant in all culpable involvements as evidenced by
the 40 percent effectiveness and 9.66 chi-square value. In general, ESC
is equally as effective in reducing all crash involvements (16 percent)
in LTVs as in passenger cars.

***[Table 17]{.underline}: Effect of ESC on Crashes in Mercedes-Benz
LTVs***

(California: 2001-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***106***    ***575***                        

  **All run-off-road**            **20**       **21**       **81**              **29.62**

  **Rollover**                    **5**        **6**        **78**              **7.18**

  **Other run-off-road**          **15**       **15**       **82**              **24.13**

  **Pedestrian/bicycle/animal**   **1**        **4**        **26**              **0.07**

  **Other single vehicle**        **6**        **15**       **54**              **2.57**

  **Culpable multi-vehicle**      **57**       **235**      **24**              **2.29**

  **Other multi-vehicle\***       **8**        **48**                           

  **All non-control group         **92**       **323**      **35**              **7.60**
  involvements**                                                                

  **All crash involvements**                                **16**              

  Total                           198          898                              
  ------------------------------- ------------ ------------ ------------------- --------------

\*includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Florida]{.underline}*

*(Passenger cars)*

All run-off-road crashes in Florida were decreased by 28 percent for
passenger cars -- Table 18. The risk of a rollover or a side impact with
fixed object was also lowered in vehicles with a factory installed ESC.
In fact, the relative decreases were 14 percent and 43 percent for side
impacts with fixed objects and rollovers, respectively. Furthermore, the
effects are statistically significant for all run-off-roads and
rollovers. Collisions with pedestrians, bicycles, or animals and
culpable involvements in multi-vehicle crashes also benefited from ESC
as indicated in Table 18 with statistically significant 49 percent and
13 percent effectiveness, respectively. All culpable crashes decreased
by 13 percent, and the decrease is statistically significant. On the
whole, ESC is effective in reducing all crash involvements by 5 percent.

*(LTVs -- mostly SUVs)*

Table 19 shows that ESC reduced all run-off-road crashes in LTVs by 66
percent with chi-square statistic 50.08. The reduction is equally
substantial in rollovers (78 percent effectiveness and a chi-square
value of 16.83). Crashes that involved a pedestrian, bicycle, or an
animal were also decreased (20 percent effectiveness), but the relative
effect is not statistically significant. On the other hand, the effect
of ESC in reducing all culpable involvements in multi-vehicle crashes is
statistically significant as evidenced by the 10 percent effectiveness
and a chi-square value of 3.44. Similarly, ESC is quite effective in
reducing all non-control group crashes (a 13 percent reduction and
chi-square statistic of 8.74). The overall effect of ESC in reducing all
crash involvements in Florida is slightly higher for LTVs -- 6 percent
reduction.

***[Table 18]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Florida: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***5104***   ***5441***                       

  **All run-off-road**            **422**      **324**      **28**              **18.59**

  **Side impact with fixed        **63**       **58**       **14**              **0.64**
  object**                                                                      

  **Rollover**                    **33**       **20**       **43**              **4.06**

  **Other run-off-road**          **326**      **246**      **29**              **16.03**

  **Pedestrian/bicycle/animal**   **82**       **45**       **49**              **13.13**

  **Other single vehicle**        **401**      **471**      **-10**             **1.88**

  **Culpable multi-vehicle**      **2259**     **2105**     **13**              **13.96**

  **Other multi-vehicle\***       **906**      **856**                          

  **All non-control group         **4070**     **3801**     **12**              **19.71**
  involvements**                                                                

  **All crash involvements**                                **5**               

  Total                           9174         9242                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

***[Table 19]{.underline}: Effect of ESC on Crashes in LTVs***

(Florida: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***2993***   ***1655***                       

  **All run-off-road**            **257**      **48**       **66**              **50.08**

  **Side impact with fixed        **36**       **13**       **35**              **1.74**
  object**                                                                      

  **Rollover**                    **57**       **7**        **78**              **16.83**

  **Other run-off-road**          **164**      **28**       **69**              **35.93**

  **Pedestrian/bicycle/animal**   **34**       **15**       **20**              **0.53**

  **Other single vehicle**        **298**      **135**      **18**              **3.41**

  **Culpable multi-vehicle**      **1227**     **609**      **10**              **3.44**

  **Other multi-vehicle\***       **517**      **314**      []{.mark}           []{.mark}

  **All non-control group         **2333**     **1121**     **13**              **8.74**
  involvements**                                                                

  **All crash involvements**                                **6**               

  Total                           5326         2776                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Illinois]{.underline}*

*(Passenger cars)*

Statistically significant reductions in all run-off-road crashes (53
percent), especially in rollovers (80 percent) were also found in
Illinois for passenger cars equipped with ESC -- Table 20. The results
also showed significant declines not only in pedestrian, bicycle, and
animal crashes (26 percent) but also in crash involvements where the
vehicle was considered the culpable party in a multi-vehicle crash (16
percent). All non-control group involvements were reduced (13 percent)
as well. When we looked at all crash involvements in Illinois for
passenger cars, we found that ESC, by and large, is still fairly
effective -- 7 percent reduction.

***[Table 20]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Illinois: 1997-2002)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***3548***   ***2636***                       

  **All run-off-road**            **420**      **146**      **53**              **60.63**

  **Rollover**                    **27**       **4**        **80**              **11.15**

  **Other run-off-road**          **393**      **142**      **51**              **25.53**

  **Pedestrian/bicycle/animal**   **247**      **136**      **26**              **7.49**

  **Other single vehicle**        **662**      **528**      **-7**              **1.24**

  **Culpable multi-vehicle**      **2002**     **1253**     **16**              **17.57**

  **Other multi-vehicle\***       **1244**     **899**                          

  **All non-control group         **4575**     **2962**     **13**              **15.56**
  involvements**                                                                

  **All crash involvements**                                **7**               

  Total                           8123         5598                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*(LTVs -- mostly SUVs)*

Although the reduction in rollovers in Illinois -- which is credited to
ESC -- was only a few percentage points higher (87 percent versus 80
percent) in LTVs than in passenger cars, the decrease in all
run-off-road crashes was substantially higher in LTVs than in passenger
cars (80 percent versus 53 percent, respectively) -- Tables 20-21. The
analysis results also revealed potential increase (-20 percent) in
pedestrian, bicycle, and animal crashes, but such increase (if any) is
not statistically significant since the chi-square statistic (0.88) does
not exceed 3.84. Culpable involvements in multi-vehicle crashes were
also improved (due to ESC) by 28 percentage points, and the improvement
is statistically significant. All non-control group crash involvements
were reduced by 22 percent as well, and the reduction is also
statistically significant -- Table 21. Once again, ESC is quite
successful in reducing all crash involvements (13 percent) for LTVs.

***[Table 21]{.underline}: Effect of ESC on Crashes in LTVs***

(Illinois: 1997-2002)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***1252***   ***959***                        

  **All run-off-road**            **147**      **22**       **80**              **59.71**

  **Rollover**                    **41**       **4**        **87**              **21.44**

  **Other run-off-road**          **106**      **18**       **78**              **40.18**

  **Pedestrian/bicycle/animal**   **60**       **55**       **-20**             **0.88**

  **Other single vehicle**        **217**      **165**      **1**               **0.00**

  **Culpable multi-vehicle**      **756**      **416**      **28**              **19.71**

  **Other multi-vehicle\***       **484**      **332**                          

  **All non-control group         **1664**     **990**      **22**              **18.52**
  involvements**                                                                

  **All crash involvements**                                **13**              

  Total                           2916         1949                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Kentucky]{.underline}*

*(Passenger Cars)*

As for Kentucky, Table 22 shows a significant 47 percent reduction in
all run-off-road crash involvements of passenger cars equipped with ESC.
Of that, the decrease in rollovers is 73 percent, although the
effectiveness is statistically not significant (1.72 chi-square
statistic) due to limited number of involvements in our data sample
(i.e., 7 crashes total; 6 pre-ESC crashes and 1 post-ESC crash). The
effectiveness in passenger car involvements with pedestrians, bicycles,
or animals in Kentucky is negative (-4 percent) and non-significant
(chi-square statistic of 0.02). As for culpable involvements of
passenger cars in multi-vehicle crashes in Kentucky, the effect is
positive but non-significant as well. Likewise, the effect of ESC on
non-control group involvements is positive (10 percent). As a whole, ESC
still has a positive influence on all crash involvements in Kentucky for
passenger cars.

***[Table 22]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Kentucky: 1997-2002)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***771***    ***482***                        

  **All run-off-road**            **117**      **39**       **47**              **10.80**

  **Rollover**                    **6**        **1**        **73**              **1.72**

  **Other run-off-road**          **111**      **38**       **45**              **9.59**

  **Pedestrian/bicycle/animal**   **37**       **24**       **-4**              **0.02**

  **Other single vehicle**        **30**       **19**       **-1**              **0.00**

  **Culpable multi-vehicle**      **409**      **231**      **10**              **1.02**

  **Other multi-vehicle\***       **165**      **112**                          

  **All non-control group         **758**      **425**      **10**              **1.68**
  involvements**                                                                

  **All crash involvements**                                **5**               

  Total                           1529         907                              
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*(LTVs -- mostly SUVs)*

As for LTVs, ESC is highly effective in reducing all-run-off road
crashes, especially rollovers -- Table 23. The reductions are
statistically significant with the effectiveness equals to 77 percent
for all run-off-roads and 92 percent for rollovers. On the other hand,
crashes that involved pedestrians, bicycles, or animals, were increased
by 21 percent, but the increase is once again statistically not
significant. Although the effectiveness in culpable involvements in
multi-vehicle crashes is also not significant, it is nevertheless a
strong positive effect (17 percent). The effect of ESC on all
non-control group crashes is also positive (17 percent). All in all, ESC
is very effective in reducing all LTV crash involvements in Kentucky as
shown by the 9 percent reduction.

***[Table 23]{.underline}: Effect of ESC on Crashes in LTVs***

(Kentucky: 1997-2002)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  crashes**    crashes**                        

  ***Control group***             ***463***    ***208***                        

  **All run-off-road**            **95**       **10**       **77**              **20.73**

  **Rollover**                    **28**       **1**        **92**              **10.08**

  **Other run-off-road**          **67**       **9**        **70**              **12.15**

  **Pedestrian/bicycle/animal**   **22**       **12**       **-21**             **0.28**

  **Other single vehicle**        **16**       **6**        **17**              **0.14**

  **Culpable multi-vehicle**      **257**      **96**       **17**              **1.60**

  **Other multi-vehicle\***       **106**      **62**                           

  **All non-control group         **496**      **186**      **17**              **2.27**
  involvements**                                                                

  **All crash involvements**                                **9**               

  Total                           959          394                              
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Missouri]{.underline}*

*(Passenger Cars)*

All run-off-road crash involvements in Missouri were also decreased (44
percent) for passenger cars equipped with ESC -- Table 24. Similarly,
crash involvements with pedestrians, bicycles, or animals as well as
culpable involvements in multi-vehicle crashes and all non-control group
crashes were reduced by 48 percent, 8 percent, and 17 percent,
respectively. Except for the reduction in culpable involvements in
multi-vehicle crashes, all other reductions are statistically
significant. We found that ESC was 10 percent effective in reducing all
passenger car crash involvements.

*(LTVs -- mostly in SUVs)*

A continuing trend in crash reductions -- specifically in all
run-off-road crashes -- is observed for LTVs in Missouri -- Table 25.
The reduction is significant with 80 percent effectiveness and
chi-square statistic of 39.51. ESC also reduced crashes that involved
pedestrians, bicycles, or animals, but the reduction is statistically
non-significant because the chi-square statistic is close to 0. The
results also suggested that ESC provides little benefits (if any) to
LTVs -- particularly, in culpable involvements in multi-vehicle crashes,
but for all non-control group crash involvement, the benefits are
significant (a 16 percent reduction). Overall, all crash involvements
decreased by 9 percent.

***[Table 24]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Missouri: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***1141***   ***915***                        

  **All run-off-road**            **232**      **105**      **44**              **21.09**

  **Side impact with fixed        **21**       **20**       **-19**             **0.30**
  object**                                                                      

  **Rollover**                    **14**       **2**        **82**              **6.59**

  **Other run-off-road**          **197**      **83**       **47**              **22.24**

  **Pedestrian/bicycle/animal**   **58**       **24**       **48**              **7.43**

  **Other single vehicle**        **161**      **98**       **24**              **4.15**

  **Culpable multi-vehicle**      **757**      **560**      **8**               **1.28**

  **Other multi-vehicle\***       **313**      **225**                          

  **All non-control group         **1521**     **1012**     **17**              **9.65**
  involvements**                                                                

  **All crash involvements**                                **10**              

  Total                           2662         1927                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

***[Table 25]{.underline}: Effect of ESC on Crashes in LTVs***

(Missouri: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***562***    ***283***                        

  **All run-off-road**            **150**      **15**       **80**              **39.51**

  **Side impact with fixed        **13**       **0**        **100**             **6.50**
  object**                                                                      

  **Rollover**                    **31**       **0**        **100**             **13.34**

  **Other run-off-road**          **106**      **15**       **72**              **22.08**

  **Pedestrian/bicycle/animal**   **27**       **12**       **12**              **0.12**

  **Other single vehicle**        **71**       **36**       **-1**              **0.00**

  **Culpable multi-vehicle**      **401**      **204**      **-1**              **0.01**

  **Other multi-vehicle\***       **158**      **76**                           

  **All non-control group         **807**      **343**      **16**              **3.04**
  involvements**                                                                

  **All crash involvements**                                **9**               

  Total                           1369         626                              
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Pennsylvania]{.underline}*

*(Passenger cars)*

For passenger cars in Pennsylvania, ESC has quite an impact on all
run-off-road involvements (40 percent effectiveness, 21.69 chi-square
statistic), especially in rollovers (66 percent effectiveness, 5.23) --
as shown in Table 26. Although it is not statistically significant,
observed ESC effectiveness in pedestrian, bicycle, and animal crashes is
negative -- as illustrated in Table 26 by the negative 23 percent
effectiveness. Pennsylvania is the first State whose analysis shows a
negative ESC effectiveness in culpable involvements in multi-vehicle
crashes, but the effect is statistically non-significant. Furthermore,
ESC has little effect in reducing all crash involvements (1 percent) in
Pennsylvania for passenger cars.

***[Table 26]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Pennsylvania: 1997-2001, 2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***983***    ***553***                        

  **All run-off-road**            **431**      **146**      **40**              **21.69**

  **Side impact with fixed        **53**       **18**       **40**              **3.36**
  object**                                                                      

  **Rollover**                    **26**       **5**        **66**              **5.23**

  **Other run-off-road**          **352**      **123**      **38**              **16.61**

  **Pedestrian/bicycle/animal**   **62**       **43**       **-23**             **1.04**

  **Other single vehicle**        **31**       **24**       **-38**             **1.34**

  **Culpable multi-vehicle**      **731**      **454**      **-10**             **1.53**

  **Other multi-vehicle\***       **282**      **183**                          

  **All non-control group         **1537**     **850**      **2**               **0.06**
  involvements**                                                                

  **All crash involvements**                                **1**               

  Total                           2520         1403                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*(LTVs -- mostly SUVs)*

Table 27 shows that ESC significantly reduced all run-off-roads in
Pennsylvania by 63 percent for LTVs and was even more effective in
reducing rollovers -- as evidenced by the 86 percent effectiveness and
18.26 chi-square statistic. We see possible harm with ESC for crash
involvements with pedestrians, bicycles, or animals; however, the effect
is not statistically significant. This analysis showed that ESC has
negligible effect on culpable involvements of LTVs in multi-vehicle
crashes in Pennsylvania, but it has quite an effect on all non-control
group involvements (22 percent) as well as on crash involvements (14
percent). These effects are statistically significant.

***[Table 27]{.underline}: Effect of ESC on Crashes in LTVs***

(Pennsylvania: 1997-2001, 2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***355***    ***210***                        

  **All run-off-road**            **193**      **42**       **63**              **28.64**

  **Side impact with fixed        **19**       **8**        **29**              **0.63**
  object**                                                                      

  **Rollover**                    **48**       **4**        **86**              **18.26**

  **Other run-off-road**          **126**      **30**       **60**              **17.71**

  **Pedestrian/bicycle/animal**   **19**       **19**       **-69**             **2.49**

  **Other single vehicle**        **13**       **3**        **61**              **2.27**

  **Culpable multi-vehicle**      **226**      **132**      **1**               **0.01**

  **Other multi-vehicle\***       **129**      **70**                           

  **All non-control group         **580**      **266**      **22**              **4.97**
  involvements**                                                                

  **All crash involvements**                                **14**              

  Total                           935          476                              
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in Wisconsin]{.underline}*

*(Passenger cars)*

Similar to all run-off-road involvements of passenger cars in other
States, the involvements in Wisconsin were significantly decreased -- 34
percent effectiveness and 9.58 chi-square statistic -- Table 28. In
addition to involvements with pedestrians, bicycles, or animals,
culpable involvements in multi-vehicle crashes as well as in all

non-control group crashes decreased -- for passenger cars equipped with
ESC versus those not equipped with ESC -- as shown by the 23 percent, 16
percent, and 17 percent, respectively. The decreases are significant in
the culpable involvements in multi-vehicle crashes and in all
non-control group crashes. ESC is effective overall in Wisconsin for
passenger cars -- as demonstrated in Table 28 with an 11 percent
reduction of all crash involvements.

*(LTVs -- mostly SUVs)*

Likewise, LTVs with ESC are much less at risk of being involved in
run-off-road crashes when compared to vehicles with no ESC -- as shown
in Table 29 by the 68 percent effectiveness and 32.45 chi-square value.
In addition to these single-vehicle crash involvements, other culpable
involvements in multi-vehicle crashes were also significantly decreased
by 27 percent -- though the decrease is not as substantial as in the
single-vehicle crashes. ESC had negligible effect on crashes that
involved pedestrians, bicycles, or animals as evidenced by the negative
4 percent effectiveness and 0.02 chi-square statistic. All in all, ESC
is very effective in reducing all crash involvements in Wisconsin -- 23
percent for LTVs.

***[Table 28]{.underline}: Effect of ESC on Crashes in Passenger Cars***

(Wisconsin: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***705***    ***516***                        

  **All run-off-road**            **214**      **104**      **34**              **9.58**

  **Rollover**                    **13**       **4**        **58**              **2.42**

  **Other run-off-road**          **201**      **100**      **32**              **8.19**

  **Pedestrian/bicycle/animal**   **120**      **68**       **23**              **2.49**

  **Other single vehicle**        **57**       **25**       **40**              **4.39**

  **Culpable multi-vehicle**      **645**      **395**      **16**              **4.28**

  **Other multi-vehicle\***       **178**      **147**                          

  **All non-control group         **1214**     **739**      **17**              **6.14**
  involvements**                                                                

  **All crash involvements**                                **11**              

  Total                           1919         1255                             
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

***[Table 29]{.underline}: Effect of ESC on Crashes in LTVs***

(Wisconsin: 1997-2003)

  ------------------------------- ------------ ------------ ------------------- --------------
  **Type of crash involvement**   **Vehicles   **Vehicles   **Effectiveness**   **Chi-square
                                  with no      with ESC**                       Value**
                                  ESC**                                         

                                  **All        **All        **(%)**             
                                  Crashes**    Crashes**                        

  ***Control group***             ***359***    ***231***                        

  **All run-off-road**            **162**      **33**       **68**              **32.45**

  **Rollover**                    **50**       **4**        **88**              **21.51**

  **Other run-off-road**          **112**      **29**       **60**              **17.15**

  **Pedestrian/bicycle/animal**   **42**       **28**       **-4**              **0.02**

  **Other single vehicle**        **26**       **10**       **40**              **1.85**

  **Culpable multi-vehicle**      **310**      **146**      **27**              **5.68**

  **Other multi-vehicle\***       **128**      **63**                           

  **All non-control group         **668**      **280**      **35**              **15.16**
  involvements**                                                                

  **All crash involvements**                                **23**              

  Total                           1027         511                              
  ------------------------------- ------------ ------------ ------------------- --------------

\* includes involvements that are not culpable but are not part of the
control-group.

*[Crashes in all selected States]{.underline}*

At this point, we have comprehensively discussed separate analysis
results of the effect of ESC in reducing relevant crash involvements of
passenger cars and LTVs -- relative to the control-group involvements --
in each selected State. Let us now try to assess the overall effect of
ESC across these seven States. The easiest way to obtain a single
estimate would be to pool the cases from the States and perform a single
analysis. We are reluctant to analyze the data that way because:

Different States have different crash-reporting thresholds. States with
low thresholds have more reported crash cases per capita, and would
account for an unjustifiably large share of the pooled data.

The make-model mix varies from State to State.

The distribution of crash types varies from State to State, as do the
definitions of the crash types.

Instead, we will use the *weighted* mean of the sample log odds ratios
from the seven States as the best indicator of the central tendency of
the data, and the *weighted* standard error of the log odds ratios as a
basis for judging statistical significance and estimating confidence
bounds, which will be discussed in the next section. In this section, we
will concentrate only on the overall effects of the systems.[^14]

The reason for using the mean of the log odds ratios is due to the facts
that it is (1) less affected by extreme values, (2) useful as a measure
of central tendency for certain positively skewed distributions, and (3)
an appropriate measure to use for averaging rates (e.g., crash
reduction/increase). The weighted mean is used to take into account the
difference in the sampling distributions among the seven States. The
weighted value for each State is simply the reciprocal of the sum of the
reciprocals of the four frequencies from the 2x2 contingency table. The
weighted mean of the log odds ratios is determined using the PROC MEANS
procedure in SAS from the seven log odds ratios and the weighted value
for each of the seven States.

*(Passenger cars)*

We will first discuss the effects on passenger cars and then on LTVs.
Table 30 shows not only the individual percentage reductions in the
ratios of relevant involvements to control-group involvements -- in
passenger cars with ESC versus cars of similar

make-models without ESC -- in each State, but also the overall
reductions -- which are determined by computing the weighted mean of the
sample log odds ratios. For example, for all run-off-road crashes in
California involving passenger cars (Table 16), the odds ratio equals
\[(167/331)$$ (2510/1688)\] = 0.3393. The log odds ratio is log (0.3393)
=

-1.0809. The weighted value equals

$$1 \div \left\{ \left( 1\text{1688} \right) + \left( 1\text{2510} \right) + \left( 1\text{331} \right) + \left( 1\text{167} \right) \right\} = \text{100}\text{.}\text{00}$$

Using the same approach, the log odds ratios for Florida, Illinois,
Kentucky, Missouri, Pennsylvania, and Wisconsin, are -0.3282, -0.7595,
-0.6289, -0.5720, -0.5073, and

-0.4095, respectively. The corresponding weighted values are 171.36,
101.10, 26.62, 63.28, 83.37, and 56.67. The computed weighted mean of
the log odds ratios from SAS program is -0.5969. The weighted mean of
the overall effectiveness is estimated simply by first taking the
antilogarithm of -0.5969 and then subtracting \[exp (-0.5969) = 0.5505\]
from 1, which yields 45 percent effectiveness.

As illustrated in Table 30 in the gray-highlighted column, ESC is most
effective in reducing all run-off-road crashes by 45 percent,
particularly in rollovers by 64 percent. Other single-vehicle crash
involvements -- where ESC also had an impact -- are those that involved
pedestrians, bicycles, or animals, even though the results are
inconsistent across States. Nevertheless, the impact is still positive:
26 percent effectiveness. The table shows negative overall effectiveness
(-2 percent) of ESC in reducing "other single-vehicle" crashes (such as
collisions with parked cars, impacts with thrown or falling objects,
undercarriage scrapes, or first-event fires), and the results are also
inconsistent among the States. Culpable involvements in multi-vehicle
crashes received some benefit from ESC as evidenced by the 13 percent
effectiveness. When all crashes (including culpable, non-culpable, and
the control-group involvements) were taken into account, ESC is still
quite effective -- as shown by the 8 percent effectiveness. In the State
analyses, we also analyzed the effectiveness of ESC on single-vehicle
crashes that did not involve pedestrians, bicycles, or animals. As shown
in Table 30, the reductions of single-vehicle crashes were observed in
each of the States, and the overall reduction (which is the weighted
mean of the reductions in seven States) is 26 percent.

***[Table 30]{.underline}***: The Effects and Mean of the Effects of ESC
on Various Crash Involvements of Passenger Cars - By State

+------------+------+------+------+------+------+------+------+------+
| **Type of  | **   | **   | **   | **   | **   | **   | **   | **\* |
| crash      | CA** | FL** | IL** | KY** | MO** | PA** | WI** | Mean |
| inv        |      |      |      |      |      |      |      | of   |
| olvement** |      |      |      |      |      |      |      | Stat |
|            |      |      |      |      |      |      |      | es** |
+------------+------+------+------+------+------+------+------+------+
|            | *    | *    | *    | *    | *    | *    | *    | *    |
|            | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff |
|            | ecti | ecti | ecti | ecti | ecti | ecti | ecti | ecti |
|            | vene | vene | vene | vene | vene | vene | vene | vene |
|            | ss** | ss** | ss** | ss** | ss** | ss** | ss** | ss** |
|            |      |      |      |      |      |      |      |      |
|            | **(  | **(  | **(  | **(  | **(  | **(  | **(  | **(  |
|            | %)** | %)** | %)** | %)** | %)** | %)** | %)** | %)** |
+------------+------+------+------+------+------+------+------+------+
| ***Control |      |      |      |      |      |      |      |      |
| group***   |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **All      | 66   | 28   | 53   | 47   | 44   | 40   | 34   | 45   |
| run-       |      |      |      |      |      |      |      |      |
| off-road** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **         | 76   | 43   | 80   | 73   | 82   | 66   | 58   | 64   |
| Rollover** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Other    | 65   | 29   | 51   | 45   | 47   | 38   | 32   | 45   |
| run-       |      |      |      |      |      |      |      |      |
| off-road** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Pedestr  | 73   | 49   | 26   | -4   | 48   | -23  | 23   | 26   |
| ian/bicycl |      |      |      |      |      |      |      |      |
| e/animal** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Other    | 16   | -10  | -7   | -1   | 24   | -38  | 40   | -2   |
| single     |      |      |      |      |      |      |      |      |
| veh        |      |      |      |      |      |      |      |      |
| icle\*\*** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Culpable | 22   | 13   | 16   | 10   | 8    | -10  | 16   | 13   |
| multi      |      |      |      |      |      |      |      |      |
| -vehicle** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **All      | 15   | 5    | 7    | 5    | 10   | 1    | 11   | 8    |
| crash      |      |      |      |      |      |      |      |      |
| invo       |      |      |      |      |      |      |      |      |
| lvements** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
|            |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Single   | 60   | 9    | 16   | 37   | 36   | 35   | 35   | 26   |
| vehicle    |      |      |      |      |      |      |      |      |
| crashes**  |      |      |      |      |      |      |      |      |
|            |      |      |      |      |      |      |      |      |
| **         |      |      |      |      |      |      |      |      |
| (excluding |      |      |      |      |      |      |      |      |
| p          |      |      |      |      |      |      |      |      |
| edestrian, |      |      |      |      |      |      |      |      |
| bike,      |      |      |      |      |      |      |      |      |
| animal     |      |      |      |      |      |      |      |      |
| crashes)** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+

*\* indicates the weighted mean of the effects in all 7 States.*

*\*\* includes collisions with parked vehicles.*

*(Light-Trucks -- mostly SUVs)*

ESC was even more effective in reducing run-off-road crashes (relative
to the control-group involvements) for LTVs, especially in rollover
involvements -- as shown in Table 31 by the 72 percent and 85 percent
effectiveness, respectively. In calculating the weighted mean effect for
the rollover reductions in seven States, the crash involvements in
Illinois and neighboring Missouri were pooled because a 100 percent
reduction in Missouri yields an undefined log odds ratio. ESC had a
negative effect on pedestrian, bicycle, or animal crashes. In LTVs,
culpable involvements in other single- and multi-vehicle crashes were
also decreased by 12 percent and 16 percent, respectively. Overall, ESC
was also effective in reducing all crash involvements in LTVs. The
reduction in single-vehicle crashes (excluding those involving
pedestrians, bicycles, or animals) involving LTVs is not as high as the
reduction in all run-off-road crashes (48 percent versus 72 percent.

***[Table 31]{.underline}***: The Effects and Mean of the Effects of ESC
on Various Crash Involvements in LTVs - By State

+------------+------+------+------+------+------+------+------+------+
| **Type of  | **   | **   | **   | **   | **   | **   | **   | **\* |
| crash      | CA** | FL** | IL** | KY** | MO** | PA** | WI** | Mean |
| inv        |      |      |      |      |      |      |      | of   |
| olvement** |      |      |      |      |      |      |      | Stat |
|            |      |      |      |      |      |      |      | es** |
+------------+------+------+------+------+------+------+------+------+
|            | *    | *    | *    | *    | *    | *    | *    | *    |
|            | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff | *Eff |
|            | ecti | ecti | ecti | ecti | ecti | ecti | ecti | ecti |
|            | vene | vene | vene | vene | vene | vene | vene | vene |
|            | ss** | ss** | ss** | ss** | ss** | ss** | ss** | ss** |
|            |      |      |      |      |      |      |      |      |
|            | **(  | **(  | **(  | **(  | **(  | **(  | **(  | **(  |
|            | %)** | %)** | %)** | %)** | %)** | %)** | %)** | %)** |
+------------+------+------+------+------+------+------+------+------+
| ***Control |      |      |      |      |      |      |      |      |
| group***   |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **All      | 81   | 66   | 80   | 77   | 80   | 63   | 68   | 72   |
| run-       |      |      |      |      |      |      |      |      |
| off-road** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Roll     | 78   | 78   | 87   | 92   | 100  | 86   | 88   | 85   |
| over\*\*** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Other    | 82   | 69   | 78   | 70   | 72   | 60   | 60   | 69   |
| run-       |      |      |      |      |      |      |      |      |
| off-road** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Pedestr  | 26   | 20   | -20  | -21  | 12   | -69  | -4   | -11  |
| ian/bicycl |      |      |      |      |      |      |      |      |
| e/animal** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Other    | 54   | 18   | 1    | 17   | -1   | 61   | 40   | 12   |
| single     |      |      |      |      |      |      |      |      |
| vehicle**  |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Culpable | 24   | 10   | 28   | 17   | -1   | 1    | 27   | 16   |
| multi      |      |      |      |      |      |      |      |      |
| -vehicle** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **All      | 16   | 6    | 13   | 9    | 9    | 14   | 23   | 10   |
| crash      |      |      |      |      |      |      |      |      |
| invo       |      |      |      |      |      |      |      |      |
| lvements** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
|            |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+
| **Single   | 74   | 40   | 33   | 68   | 54   | 63   | 64   | 48   |
| vehicle    |      |      |      |      |      |      |      |      |
| crashes    |      |      |      |      |      |      |      |      |
| (excluding |      |      |      |      |      |      |      |      |
| p          |      |      |      |      |      |      |      |      |
| edestrian, |      |      |      |      |      |      |      |      |
| bike,      |      |      |      |      |      |      |      |      |
| animal     |      |      |      |      |      |      |      |      |
| crashes)** |      |      |      |      |      |      |      |      |
+------------+------+------+------+------+------+------+------+------+

*\* indicates the weighted mean of the effects in all 7 States.*

*\*\* Data in IL and MO were pooled to allow computation of a log-odds
ratio.*

*[Effectiveness confidence bounds and statistical significance in
various crash involvements]{.underline}*

Now, let us discuss confidence intervals for certain effectiveness
estimates that are considered significant crash reductions in passenger
cars and LTVs -- Tables 32 and 33. In the State analysis results, we
will calculate confidence bounds using weighted mean of the log odds
ratios, weighted standard errors based on the variation of the seven
States, and take 1.943 standard errors for two-sided
$\alpha = 0\text{.}\text{10}$, i.e., 90 percent bounds, because 1.943 is
the 95^th^ percentile of the *t*-distribution with (7-1)=6 degrees of
freedom. A general notation for calculating the confidence interval for
the overall effect is illustrated below by the formula[^15]:

$$Confidence interval = (overall
effect)${}_{\text{weightedmeanof}\text{log}\text{odds}}$ $\pm$
*t*${}_{\text{.}\text{05},N - 1}$ (SE)

where

1.  (overall effect)${}_{\text{weightedmeanof}\text{log}\text{odds}}$ is
    determined using the approach discussed in the previous section.

2.  *t*${}_{\text{.}\text{05},N - 1}$$$is the *t* value for a two-sided
    test (90 percent bounds) and *N*-1 degrees of freedom.

3.  *N* is the number of individual effects.

4.  SE is the weighted standard error of the log odds ratios obtained
    from the PROC MEANS procedure in SAS (i.e., the weighted standard
    deviation of these ratios divided by $\sqrt{7}$, the square root of
    the number of States).

Let us take one example (all run-off-road crash involvements of
passenger cars) and discuss the statistics in detail. As discussed in
the previous section, the weighted mean of the log odds ratios is
-0.5969. The weighted standard error of the log odds ratios (computed
from SAS) is 0.1068. Applying these statistics to the above
*t*-confidence interval equation produces

90% confidence interval = -0.5969 $\pm$`<!-- -->`{=html}1.943 (0.1068) =
-08044 to -0.3894

which translates to (0.4474, 0.6775) in odds ratio by taking the
antilogarithm of the confidence interval estimates (-0.8044, -0.3894).
The confidence interval for the overall effect of ESC on all
run-off-road crashes involving passenger cars is (32%, 55%). Using the
same approach, we found the following confidence intervals for other
critical reductions in passenger cars: 50 to 75 percent in rollover
crashes, 8 to 41 percent in crashes that involved a pedestrian, bicycle,
or an animal, 7 to 18 percent in culpable involvements in multi-vehicle
crashes, and 5 to 10 percent in all crashes. For single-vehicle crash
involvements (not including collisions with a pedestrian, bicycle, or an
animal), the expected effectiveness is between 10 and 40 percent. The
overall effectiveness estimates listed in Table 32 are statistically
significant because the confidence bounds are greater than 0.

For LTVs, the confidence bounds for the reductions (listed in Table 33)
were also calculated using the above method. Table 33 shows that the
weighted mean of the effectiveness of ESC in reducing all LTV
run-off-crashes is 72 percent, and the 90 percent confidence interval is
65 to 77 percent. As previously discussed, because the crash
involvements in Illinois and neighboring Missouri were pooled for the
analysis of rollovers (Table 31), the number of individual effects *(N)*
is reduced to 6, and the critical *t*-value for 6-1 = 5 degrees of
freedom is 2.015[^16]. Hence, the confidence interval for the rollover
reduction is (79%, 90%). A negative 11 percent effectiveness in crashes
involving pedestrians, bicycles, or animals implies that the expected
effectiveness estimate is between -31 and 6 percent. The confidence
bounds for the decrease in culpable involvements in multi-vehicle
crashes are 7-23 percent. The confidence bounds for the reduction in all
crashes in LTVs are between 6 and 14 percent. When we look at all
single-vehicle crashes except the pedestrian, bicycle, and animal
crashes, the confidence interval for the reduction is (35%, 58%). In
general, the spread of the sample distribution determines the range of
the confidence interval. In other words, if the effectiveness estimates
are consistent across all studied States, then the spread of the sample
distribution is expected to be small, and hence, the range of the
confidence interval is expected to be small as well -- as evidenced by
the estimates and the confidence bounds listed in Tables 32-33. Similar
to the effects on passenger cars, the weighted mean effects on LTVs for
run-off-road, rollover, culpable multi-vehicle, and all crash
involvements are statistically significant because the confidence bounds
are all positive. The overall reduction of single-vehicle crashes
excluding pedestrian, bicycle, and animal crashes is also statistically
significant. Only the overall effect on crashes that involved a
pedestrian, bicycle, or an animal is not statistically significant
because the estimates from the individual States are inconsistent.

***[Table 32]{.underline}***: The Effects and Mean of the Effects of ESC
of Various Crash Involvements of Passenger Cars - By State

<table style="width:100%;">
<colgroup>
<col style="width: 10%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Type of crash involvement</strong></td>
<td><strong>CA</strong></td>
<td><strong>FL</strong></td>
<td><strong>IL</strong></td>
<td><strong>KY</strong></td>
<td><strong>MO</strong></td>
<td><strong>PA</strong></td>
<td><strong>WI</strong></td>
<td><strong>*Mean of States</strong></td>
<td><strong>Confidence bounds</strong></td>
</tr>
<tr class="even">
<td></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
</tr>
<tr class="odd">
<td><strong>All run-off-road</strong></td>
<td>66</td>
<td>28</td>
<td>53</td>
<td>47</td>
<td>44</td>
<td>40</td>
<td>34</td>
<td><strong>45</strong></td>
<td>32 to 55</td>
</tr>
<tr class="even">
<td><strong>Rollover</strong></td>
<td>76</td>
<td>43</td>
<td>80</td>
<td>73</td>
<td>82</td>
<td>66</td>
<td>58</td>
<td><strong>64</strong></td>
<td>50 to 75</td>
</tr>
<tr class="odd">
<td><p><strong>Pedestrian/</strong></p>
<p><strong>bike/animal</strong></p></td>
<td>73</td>
<td>49</td>
<td>26</td>
<td>-4</td>
<td>48</td>
<td>-23</td>
<td>23</td>
<td><strong>26</strong></td>
<td>8 to 41</td>
</tr>
<tr class="even">
<td><strong>Culpable multi-vehicle</strong></td>
<td>22</td>
<td>13</td>
<td>16</td>
<td>10</td>
<td>8</td>
<td>-10</td>
<td>16</td>
<td><strong>13</strong></td>
<td>7 to 18</td>
</tr>
<tr class="odd">
<td><strong>All crash involvements</strong></td>
<td>15</td>
<td>5</td>
<td>7</td>
<td>5</td>
<td>10</td>
<td>1</td>
<td>11</td>
<td><strong>8</strong></td>
<td>5 to 10</td>
</tr>
<tr class="even">
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Single-vehicle crashes**</strong></td>
<td>60</td>
<td>9</td>
<td>16</td>
<td>37</td>
<td>36</td>
<td>35</td>
<td>35</td>
<td><strong>26</strong></td>
<td>10 to 40</td>
</tr>
</tbody>
</table>

*\* indicates the weighted mean of the effectiveness estimates of 7
States.*

*\*\* excludes pedestrian, bicycle, animal crashes.*

*[Notes:]{.underline}*

*1) Bold -- weighted mean effect is statistically significant.*

*2) Plain type -- weighted mean effect is not statistically
significant.*

***[Table 33]{.underline}***: The Effects and Mean of the Effects of ESC
on Various Crash Involvements in LTVs -- By State

<table style="width:100%;">
<colgroup>
<col style="width: 10%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 9%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Type of crash involvement</strong></td>
<td><strong>CA</strong></td>
<td><strong>FL</strong></td>
<td><strong>IL</strong></td>
<td><strong>KY</strong></td>
<td><strong>MO</strong></td>
<td><strong>PA</strong></td>
<td><strong>WI</strong></td>
<td><strong>*Mean of States</strong></td>
<td><strong>Confidence bounds</strong></td>
</tr>
<tr class="even">
<td></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
<td><p><strong>Effectiveness</strong></p>
<p><strong>(%)</strong></p></td>
</tr>
<tr class="odd">
<td><strong>All run-off-road</strong></td>
<td>81</td>
<td>66</td>
<td>80</td>
<td>77</td>
<td>80</td>
<td>63</td>
<td>68</td>
<td><strong>72</strong></td>
<td>65 to 77</td>
</tr>
<tr class="even">
<td><strong>Rollover**</strong></td>
<td>78</td>
<td>78</td>
<td>87</td>
<td>92</td>
<td>100</td>
<td>86</td>
<td>88</td>
<td><strong>85</strong></td>
<td>79 to 90</td>
</tr>
<tr class="odd">
<td><p><strong>Pedestrian/</strong></p>
<p><strong>bike/animal</strong></p></td>
<td>26</td>
<td>20</td>
<td>-20</td>
<td>-21</td>
<td>12</td>
<td>-69</td>
<td>-4</td>
<td>-11</td>
<td>-31 to 6</td>
</tr>
<tr class="even">
<td><strong>Culpable multi-vehicle</strong></td>
<td>24</td>
<td>10</td>
<td>28</td>
<td>17</td>
<td>-1</td>
<td>1</td>
<td>27</td>
<td><strong>16</strong></td>
<td>7 to 23</td>
</tr>
<tr class="odd">
<td><strong>All crash involvements</strong></td>
<td>16</td>
<td>6</td>
<td>13</td>
<td>9</td>
<td>9</td>
<td>14</td>
<td>23</td>
<td><strong>10</strong></td>
<td>6 to 14</td>
</tr>
<tr class="even">
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Single-vehicle crashes***</strong></td>
<td>74</td>
<td>40</td>
<td>33</td>
<td>68</td>
<td>54</td>
<td>63</td>
<td>64</td>
<td><strong>48</strong></td>
<td>35 to 58</td>
</tr>
</tbody>
</table>

\* indicates the weighted mean of the effectiveness estimates of 7
States.

\*\* A combined effectiveness calculated from crash involvements in IL
and MO as 1 State (Tables 21 and 25). E = {1-\[((4+0) /
(959+283))$$((41+31) / (1252+562))}=0.92.

Only 6 States (*N=6)* are included for this calculation because the data
in IL and MO data were pooled.

\*\*\* excludes pedestrian, bicycle, animal crashes.

*[Notes:]{.underline}*

*1) Bold -- weighted mean effect is statistically significant.*

*2) Plain type -- weighted mean effect is not statistically
significant.*

**LOGISTIC REGRESSION ANALYSIS**

Logistic regression permits a refinement and check of the basic
analyses. Logistic regression by the GENMOD procedure[^17] was used to
estimate the effect of passenger car ESC on the probability that a crash
involvement was relevant (specifically the involvement of a vehicle
running off the road) as opposed to being non-relevant (i.e., the
control-group involvement), while controlling for other factors.
Estimating the impact of ESC in reducing single-vehicle run-off-road
crashes could be confounded by factors related to the driver, vehicle,
roadway, or other circumstances or by a proportionately different
make-model mix in the vehicles without ESC and the vehicles with ESC. To
make certain that the effect of ESC is estimated accurately, variables
were included in the logistic regression to control for those external
factors, other than ESC, that could possibly influence the ratio of
run-off-road to control-group crash involvements.

Here is one example of how demographic factors (i.e., driver
characteristics) could confound the effect of ESC. Since ESC is more
commonly installed on higher-priced vehicles, ESC-equipped vehicles are
more likely to be driven by a certain group of drivers -- perhaps more
affluent and older -- than by other segments of the driving population.
Although it is unlikely that the majority of the passenger cars used in
our analysis sample were driven by 16-24 year-olds, we included this
group of drivers in our model because they could potentially confound
the effect of ESC in reducing run-off-road crashes. This driver-age
category generally has a higher percentage of drivers considered more
aggressive in their driving than others, and thus, their vehicles are at
a higher risk of being involved in run-off-road crashes. Furthermore,
based on historical trends of drivers involved in crashes by sex and
crash severity,[^18] it is a common stereotype and many researchers have
attempted to test the hypothesis that -- male drivers are more likely to
be involved in crashes because they are generally more aggressive in
their driving than female drivers. Thus, driver gender was included in
the regression model as well.

Other external factors include whether or not the crash occurred: (1) at
night, (2) on a rural roadway, or (3) on a freeway. These factors could
influence the proportion of run-off-road crash involvements because a
driver, in those environment or situations, is more likely to lose
control of a vehicle, and hence, ESC is more likely to be activated. The
vehicle make-model group was also included in the regression model. The
age of the vehicle was initially considered, but we did not include this
factor in the final regression analysis, because its effect was not
statistically significant when (1) all independent variables mentioned
above were included in the regression model and even when (2) only
variables for ESC and vehicle age were included.[^19]

The data points in the logistic regressions for passenger cars and LTVs
are the FARS [driver]{.underline} fatality cases, each given a weight
factor of 1. Cases with unreported age, gender, or roadway function
class (rural or urban) are excluded; so are the drivers younger than 14
years or older than 97 years. The dependent variable ROR (for
run-off-road) -- a dichotomous variable with a binomial distribution --
equals 1 if the crash involvement was a single-vehicle run-off-road and
2 if it was non-relevant (control group). The key independent variable,
ESCS indicates whether or not the crash-involved vehicle had ESC as
standard equipment: 1 if ESC was installed and 0 if it was not. The
other independent variables are:

Gender, expressed as DRVMALE (= 1 if the driver is male and 0 if the
driver is female).

The driver's age, expressed as D16_24 (= 1 if the driver is 16-24 years
old and 0 otherwise).

Time of day, expressed as NITE (= 1 if 7:00 p.m. -- 5:59 a.m., 0 if 6:00
a.m. -- 6:59 p.m.)

Roadway function class (RURAL, FREEWAY). RURAL has the value 1 if the
crash occurred on a rural roadway, 0 if the crash occurred on an urban
roadway. FREEWAY has the value 1 if the crash occurred on an interstate,
freeway, or expressway and 0 otherwise.

The vehicle make-model group, expressed as a categorical variable
(CAR_MFG).

The regression for the analysis of passenger cars is based on 708 FARS
fatality cases. The likelihood-ratio (LR) statistics for TYPE 3
analysis[^20] are shown in Table 34. TYPE 3 analysis was used because of
the inclusion of the CLASS variable CAR_MFG in the model. After we
controlled for the important quantifiable human parameters -- driver's
age, gender -- plus other variables that are strongly associated with
single-vehicle run-off-road crash involvements, the effect of ESC on
fatal run-off-road involvements (relative to the control-group
involvements) in passenger cars is statistically significant with the LR
chi-square statistic being 18.39.[^21] In fact, the model predicted that
ESC reduced fatal single-vehicle run-off-road crashes by
1 ‑ exp(-0.7838) = 54 percent because the regression coefficient
estimate for ESCS is ‑0.7838. The results from the 2x2 contingency table
analysis (Table 12) showed a 36 percent reduction in all fatal
run-off-road crash involvements relative to the same control-group
involvements used in the regression analysis. Thus, if we compare those
two results, we see that the 2x2 contingency table approach may be more
conservative than the regression analysis method. Hence, the regression
analysis gives us increased confidence that the effectiveness estimates
obtained from our 2x2 contingency table analyses are not biased in favor
of ESC.

The regression results from the passenger car analysis supported our
assumptions that: (1) male drivers are at a higher risk of being
involved in fatal run-off-road crashes than females, and (2) the 16-24
year-old group of drivers is also at a higher risk of being involved in
fatal run-off-road crashes when compared to other groups of drivers.
These factors are statistically significant -- as indicated by the LR
statistics shown in Table 34 (chi-square values 35.45 for DRVMALE and
19.65 for D16_24). Other independent variables (specifically those
related to the environment) that also had statistically significant
effects on fatal run-off-road crashes are those that indicate whether or
not the crash occurred at night (NITE) or on a rural roadway (RURAL) --
as evidenced by the LR chi-square statistics 46.13 and 19.23,
respectively. The effect of another factor FREEWAY -- which indicates
whether or not the crash occurred on a freeway, interstate, or
expressway -- is borderline-significant (chi-square = 6.46; Pr\>chisq =
0.0111). The vehicle make-model group (CAR_MFG) is, however,
statistically significant -- as shown by the 23.14 LR chi-square
statistic and 0.0008 Pr\>chisq.

***[Table 34]{.underline}***: Likelihood-ratio Statistics for TYPE 3
Analysis of the Independent Variables (FARS: 1997-2004) - Passenger Cars

  ----------------- ----------------- ----------------- -----------------
  **Source**        **Degrees of      **Chi-Square**    **Pr \> ChiSq**
                    Freedom**                           

  ESCS              1                 18.39             \<.0001

  DRVMALE           1                 35.45             \<.0001

  D16_24            1                 19.65             \<.0001

  NITE              1                 46.13             \<.0001

  RURAL             1                 19.23             \<.0001

  FREEWAY           1                 6.46              0.0111

  CAR_MFG           6                 23.14             0.0008
  ----------------- ----------------- ----------------- -----------------

For the analysis of LTVs, the regression is based on 471 FARS fatality
cases. In this analysis, the model predicted that ESC reduced fatal
single-vehicle run-off-road crashes involving LTVs by 68 percent because
the regression coefficient estimate for ESCS is

-1.1320. Similar to the effects of ESC and other independent variables
in the passenger car regression analysis, the effects of those variables
in the LTV analysis (shown in Table 35) are also statistically
significant except for the effect of the vehicle make-model group.
Again, the effectiveness estimate obtained from the regression analysis
is similar to the observed estimate from the 2x2 contingency table
analysis -- 68 percent versus 70 percent, respectively (Table 13).

***[Table 35]{.underline}***: Likelihood-ratio Statistics for TYPE 3
Analysis of the Independent Variables (FARS: 1997-2004) - LTVs

  ----------------- ----------------- ----------------- -----------------
  **Source**        **Degrees of      **Chi-Square**    **Pr \> ChiSq**
                    Freedom**                           

  ESCS              1                 21.31             \<.0001

  DRVMALE           1                 11.89             0.0006

  D16_24            1                 10.75             0.0010

  NITE              1                 7.51              0.0061

  RURAL             1                 18.37             \<.0001

  FREEWAY           1                 21.83             \<.0001

  CAR_MFG           2                 1.19              0.5511
  ----------------- ----------------- ----------------- -----------------

**COMPARISON OF 2-CHANNEL AND 4-CHANNEL ESC SYSTEMS**

We also compared crash reduction with 2-channel versus 4-channel ESC
systems for reasons already explained in the "background" section. The
4-channel ESC systems have a control algorithm and means to apply all
four brakes [individually]{.underline} whereas the 2-channel systems do
not. In other words, the 2-channel systems are capable of applying brake
torque only to the two front wheels. Hence, the latter systems only have
the capability to correct oversteer whereas the former systems are
capable of correcting not only oversteer but also understeer.

Table 35 shows passenger car fatalities (single vehicle run-off-road
versus control-group involvements) in vehicles with and without ESC --
by system type (2-channel versus 4-channel systems). We subdivided our
FARS data into two groups of make-models: those equipped with 2-channel
ESC systems (all GM models selected for the study except the Corvette)
and those equipped with 4-channel ESC systems (all non-GM models
selected for the study plus the Corvette). The make-models used in this
analysis (2- versus 4-channel ESC) were identical to those listed in
Tables 2 through 7. There were no make-models that had received both
types. The crash reduction with 2-channel ESC is compared to cars of the
same make-models, but in the model years before they received ESC -- and
likewise to the make-models that received 4-channel ESC.

In the 2-channel systems (Category 1), there were 56 fatalities in
single-vehicle run-off-road crashes and 56 fatalities in non-relevant
crashes (control group) -- in vehicles without ESC, a risk ratio of
1.000. In vehicles with ESC, there were 16 run-off-road and 41
non-relevant fatalities, a risk ratio of 0.390. That is a significant 61
percent reduction in single-vehicle run-off-road fatalities in 2-channel
systems relative to the control-group. In the 4-channel systems
(Category 2), single-vehicle run-off-road fatalities also decreased, but
the decrease is 39 percent and statistically significant. Additional
analyses were conducted to refine and properly characterized these
results, as discussed below.

We can test if the fatality reduction is significantly greater in
category 1 (2-channel) than in category 2 (4-channel) by performing a
three-dimensional contingency table analysis. The difference in
effectiveness is not statistically significant, as evidenced by the
chi-square of the three-way interaction term when the CATMOD procedure
of SAS is applied to the three-way table.[^22] In fact, the chi-square
statistic for the difference in the reduction in fatal single-vehicle
run-off-road crashes between the two ESC systems is 1.35, which suggests
that the larger observed fatality reduction with 2-channel systems
(shown in Table 35) is not significantly different from the observed
reduction with the 4-channel system. The reductions in fatal
single-vehicle run-off-road fatalities should be interpreted with
caution because of the small number of fatalities in both samples
(2-channel and 4-channel systems) -- Table 35. As we have found from the
FARS and State data analyses (discussed in detail in the previous
sections), the single-vehicle run-off-road fatality reduction (due to
ESC) for passenger cars is 36 percent (Table 12), and the run-off-road
crash reduction (mostly non-fatal crashes) is 45 percent (Table 30).
Based on these results, both fatal and non-fatal single-vehicle
run-off-road reductions are expected to be somewhere in this range --
whether the reduction is associated with 2-channel or 4-channel systems.
Hence, a 61 percent reduction in fatal single-vehicle run-off-road
involvements in vehicles with 2-channel systems is probably an
unrealistic estimate and most certainly influenced by the small sample.
Consequently, the difference in the reduction in fatal single-vehicle
run-off-road crashes between the two ESC systems is found to be
statistically non-significant (chi-square value of 1.35) -- due to the
small samples.

Given that the N of FARS cases is insufficient for statistically
meaningful results, we strongly believe, in this instance, that findings
based on State data are more relevant and should be given greater
weight.

***[Table 35]{.underline}***: Effectiveness of ESC as the Percentage
Reduction in Fatal Single Vehicle Run-Off-Road Crash Involvements
Relative to the Control-Group Involvements - by ESC System Type - for
Passenger Cars

  ------------------------ ---------------- ----------------- --------- -------------
                           **All            **Control-group   **Risk    **ESC
                           run-off-road     involvements**    Ratio**   reduction**
                           involvements**                               

  **[Category 1: 2-channel                                              
  ESC                                                                   
  systems]{.underline}**                                                

  **Vehicles not equipped  56               56                1.000     61%
  with 2-channel**                                                      

  **Vehicles equipped with 16               41                0.390     
  2-channel**                                                           

  **[Category 2: 4-channel                                              
  ESC                                                                   
  systems]{.underline}**                                                

  **Vehicles not equipped  165              105               1.571     39%
  with 4-channel**                                                      

  **Vehicles equipped with 139              144               0.965     
  4-channel**                                                           
  ------------------------ ---------------- ----------------- --------- -------------

Tables 36 shows not only the reduction in all single-vehicle
run-off-road crashes -- which consist of mostly non-fatal involvements
-- in each selected State, but also the weighted mean of the reductions
in all States (similar to the calculation method used in Tables 30 and
31) -- in passenger cars for 2-channel and 4-channel ESC systems. To
test for the significance of the difference in effectiveness in six
States, a four-dimensional CATMOD analysis was used. In theory, a
four-dimensional CATMOD analysis involves aggregating data from all the
States, using all possible combinations of the four variables in the
model. However, this approach could create biased results due to: (1)
different crash-reporting thresholds among the States, (2) variation of
make-model mix from State to State, as well as (3) variation in the
distribution of crash types from State to State. Thus, to adjust for
those biases, a CATMOD analysis was performed on 6 x 4 x 2 table of
State by ESC system by crash involvement. The dichotomous dependent
variable is ROR with values (= 1 for single vehicle run-off-road
involvements; = 2 for the control-group involvements). The independent
variables are STATE (a categorical variable with six categories: 1, 2,
3, 4, 5, and 6 for six States -- Florida, Illinois, Kentucky, Missouri,
Pennsylvania, and Wisconsin, respectively), FOUR_CH (a dichotomous
variable with values = 1 for 4-channel systems; = 2 for 2-channel
systems), and ESC (another dichotomous variable with values = 1 for
vehicles with ESC: 2-channel or 4-channel; = 2 for vehicles without
ESC). The model included the following terms: 1) ROR\*FOUR_CH, 2)
ROR\*ESC, 3) ROR\*STATE, 4) ROR\*FOUR_CH\*STATE, 5) ROR\*ESC\*STATE, and
6) ROR\*FOUR_CH\*ESC.[^23] The chi-square statistic for the
ROR\*FOUR_CH\*ESC term was 4.69, which is statistically significant at
the 0.05 level but not significant at the 0.025 level. In other words,
the difference in effectiveness between the 2-channel and 4-channel
systems -- specifically the larger observed reductions in single-vehicle
run-off-road involvements with 4-channel systems versus the smaller
observed reductions with 2-channel systems -- are statistically
significant in the State data.

***[Table 36]{.underline}***: Effectiveness of 2-Channel versus
4-Channel ESC Systems as the Percentage Reduction in All Single-Vehicle
Run-Off-Road Crashes in Passenger Cars -- By State Run-Off-Road Crash
Reduction (%) in

  ----------- ------------- ---------- -------------- -------------- ------------------ --------------- ------------
              **Florida**   Illinois   **Missouri**   **Kentucky**   **Pennsylvania**   **Wisconsin**   **\*Mean of
                                                                                                        the
                                                                                                        reductions
                                                                                                        in 6
                                                                                                        States**

  **With      0             49         31             37             50                 11              30
  2-channel                                                                                             
  ESC                                                                                                   
  systems**                                                                                             

  **With      31            57         48             50             41                 47              43
  4-channel                                                                                             
  ESC                                                                                                   
  systems**                                                                                             
  ----------- ------------- ---------- -------------- -------------- ------------------ --------------- ------------

\* indicates the weighted mean of the reductions in 6 States.

Another approach that we used to compare 2-channel and 4-channel systems
is to *pool* crash involvements (specifically single-vehicle
run-off-roads and non-relevant involvements) from the States and
estimate the significance of the difference in the overall effectiveness
estimate of the two systems. Table 37 shows the sum of crash
involvements from six States and the effectiveness estimates for the two
systems. Again, the 4-channel systems are more effective in reducing
single-vehicle run-off-road crashes than the 2-channel systems as shown
by the 48 percent and 33 percent effectiveness, respectively. When the
CATMOD procedure is applied to the three-way table, the chi-square of
the three-way interaction term is 5.41, which is statistically
significant not only at the 0.05 level but also at the 0.025 level.
Regardless of how we assess the statistical significance of the
difference in the effects of the two ESC systems in the State data
analysis, the results from two different methods of analysis
(non-parametric and parametric approach) consistently showed that the
4-channel systems reduced more single-vehicle crashes than the 2-channel
systems.

***[Table 37]{.underline}***: Effectiveness of ESC as the Percentage
Reduction in Single-Vehicle Run-Off-Road Crash Involvements Relative to
the Control-Group Involvements from Six States - by ESC System Type -
for Passenger Cars

  ------------------------ ---------------- ----------------- --------- -------------
                           **All            **Control-group   **Risk    **ESC
                           run-off-road     involvements**    Ratio**   reduction**
                           involvements**                               

  **[Category 1: 2-channel                                              
  ESC                                                                   
  systems]{.underline}**                                                

  **Vehicles not equipped  437              3304              0.132     33%
  with 2-channel**                                                      

  **Vehicles equipped with 145              1638              0.089     
  2-channel**                                                           

  **[Category 2: 4-channel                                              
  ESC                                                                   
  systems]{.underline}**                                                

  **Vehicles not equipped  1399             8948              0.156     48%
  with 4-channel**                                                      

  **Vehicles equipped with 719              8905              0.081     
  4-channel**                                                           
  ------------------------ ---------------- ----------------- --------- -------------

**CONCLUSIONS**

Based on the analysis results from the 2x2 contingency tables, ESC
appears to be extremely successful in reducing not only fatal crashes
but also other crash involvements. Tables 38 and 39 summarize the
effectiveness of ESC -- as the percentage reduction in the ratios of
relevant crash involvements to non-relevant involvements (i.e., the
control-group) -- in passenger cars and LTVs with ESC versus those
without ESC, for certain crash involvements that are considered
relevant.

In Table 38, ESC reduced all fatal single-vehicle run-off-road crash
involvements by 36 percent in passenger cars and 70 percent in LTVs. The
table also includes effectiveness estimates of fatal single-vehicle
crashes excluding pedestrian, bicycle, animal crashes for passenger cars
and LTVs -- 36 percent and 63 percent, respectively. These results are
similar to those in the 2004 study -- which showed that ESC was
effective in reducing fatal single-vehicle crashes by 30 percent in
passenger cars and 63 percent in SUVs. The control group used in the
2004 study included all multi-vehicle crash involvements, whereas this
study used non-relevant involvements -- where the crash-involved vehicle
was stopped, parked, traveled at a speed of less than 10 mph, or
non-culpable party in a multi-vehicle crash on a dry road -- as the
control group. As mentioned in the 2004 study, using multi-vehicle
crashes as the control group, when it is possible that multi-vehicle
crashes are being reduced by ESC, actually means that the true
effectiveness of ESC could be higher than what we had estimated in the
2004 study for fatal single-vehicle crashes. Based on the results from
both studies, we may conclude with confidence that the effectiveness
still hold for all fatal single-vehicle crashes especially for all
run-off-road involvements in passenger cars and LTVs. Furthermore, the
logistic regression analysis results showed that ESC reduced fatal
run-off-road involvements by 54 percent for passenger cars and 68
percent for LTVs, controlling for certain external factors that could
confound the effects of ESC. Initially, the vehicle "age" effect was
included in the regression analysis, but the effect was not
statistically significant even when only independent variables for ESC
and vehicle age were considered and other factors were not. Thus, this
factor was not included in the final regression model. While the
regression results showed that the age of vehicles had no significant
effect on crash involvements (specifically fatal run-off-road
involvements), it is important that this factor be evaluated in future
analyses as ESC equipped vehicles get older and its influence on crash
involvements might become more significant.

Moreover, in this study, we found that rollover involvements in fatal
crashes decreased by 70 percent in passenger cars and 88 percent in
LTVs. Hence, ESC is very effective in reducing fatal single-vehicle
run-off-road crashes and extremely beneficial in rollover involvements.
The reductions are statistically significant. Furthermore, multi-vehicle
crash involvements were also reduced -- in passenger cars and in LTVs
with ESC -- as shown by the positive effectiveness of 19 percent
(although not statistically significant) and 34 percent, respectively --
Table 38. The observed effects of ESC on pedestrian, bicycle, and animal
crashes are not statistically significant, and they are inconsistent --
as evidenced by the negative 36 percent and negative 6 percent
effectiveness for passenger cars and LTVs, respectively. If we excluded
pedestrian, bicycles, and animal crashes, ESC is very successful in
reducing single-vehicle crashes -- 36 percent for passenger cars and 63
percent for LTVs. The results are similar to those found in the 2004
study. Even with the inconsistent results for ESC in crashes that
involved pedestrians, bicycles, or animals, ESC is still highly
effective in reducing all fatal crash involvements (including the
control-group involvements as well as other non-relevant involvements)
by 14 percent in passenger cars and 28 percent in LTVs.

***[Table 38]{.underline}***: Effectiveness of ESC as the Percentage
Reduction in Certain Fatal Relevant Crash Involvements in Passenger Cars
and LTVs

  --------------------------- ----------------------- -----------------------
  **Types of Crash            **Passenger Cars**      **LTVs**
  Involvement**                                       

  Single-vehicle crashes\*    **36%**                 **63%**

  All run-off-road            **36%**                 **70%**

  Rollover                    **70%**                 **88%**

  Pedestrian/bicycle/animal   -36%                    -6%

  Culpable multi-vehicle      19%                     **34%**

  All crashes                 14**%**                 **28%**
  --------------------------- ----------------------- -----------------------

*\* excludes pedestrian, bicycle, animal crashes.*

*[Notes:]{.underline}*

*1) Bold -- effect is statistically significant.*

*2) Plain type -- effect is not statistically significant.*

Table 39 shows the weighted mean of the effects of ESC in all selected
States (California, Florida, Illinois, Kentucky, Missouri, Pennsylvania,
and Wisconsin) on certain relevant crash involvements (which consist of
mostly non-fatal crashes) in passenger cars and LTVs. The notations
listed in the table below are similar to those in Table 38. Similar to
the results in the fatal crash analysis, ESC is highly effective in
reducing all single-vehicle run-off-road crashes, especially rollovers.
In fact, the reductions are 45 percent in passenger cars and 72 percent
in LTVs for all run-off-road crashes -- and 64 percent and 85 percent,
respectively, for rollovers. Also, the observed reductions in
single-vehicle crashes (not including pedestrian, bicycle, animal
crashes) for passenger cars and LTVs are 26 percent and 48 percent,
respectively, which are comparable (to some extent) to the results from
the 2004 study (35 percent for passenger cars and 67 percent for SUVs).
The results for ESC-equipped vehicles in crashes that involved a
pedestrian, bicycle, or animal are inconsistent -- as shown by the 26
percent and negative 11 percent effectiveness in passenger cars and
LTVs, respectively. This contrasts with the fatal crashes, where the
results were less favorable for cars. Culpable multi-vehicle crash
involvements decreased by 13 percent in passenger cars and 16 percent in
LTVs, and the decreases are statistically significant. When we looked at
all crash involvements, we found significant reductions with ESC -- 8
percent reduction in passenger cars and 10 percent reduction in LTVs.

*[Table 39]{.underline}*: Effectiveness of ESC as the Percentage
Reduction in Certain Relevant Crash Involvemetns in Passenger Cars and
LTVs

  --------------------------- ----------------------- -----------------------
  **Types of Crash            **\*Passenger Cars**    **\*LTVs**
  Involvement**                                       

  Single-vehicle crashes\*\*  **26%**                 **48%**

  All run-off-road            **45%**                 **72%**

  Rollover                    **64%**                 **85%**

  Pedestrian/bicycle/animal   **26%**                 -11%

  Culpable multi-vehicle      **13%**                 **16%**

  All crashes                 **8%**                  **10%**
  --------------------------- ----------------------- -----------------------

*\* indicates the weighted mean of the effectiveness estimates of 7
States.*

*\*\* excludes pedestrian, bicycle, animal crashes.*

*[Notes:]{.underline}*

*1) Bold -- mean effect is statistically significant.*

*2) Plain type -- mean effect is not statistically significant.*

All in all, ESC significantly reduced single-vehicle crashes especially
run-off-road and rollover involvements. In all likelihood, ESC may also
be helpful in reducing culpable involvements in multi-vehicle crashes.
At this time we do not have enough data for statistically meaningful
results on the effect (if any) of ESC on pedestrian, bicycle, and animal
crashes. Thus, we will continue to monitor the effect on those crashes
in the future.

Although there is a fairly large variation in ESC effectiveness
(possibly because the estimates somewhat depend on the choice of the
control group) between different studies (as previously discussed in the
"Background" Section), ESC is still highly effective in all studies. ESC
is likely to have the largest effect on crashes involving severe
injuries. Minor crashes are less likely to be rollovers. Thus, including
a large number of damage only crashes or slight injury crashes in this
study would likely reduce the effectiveness of ESC. In other words, the
effectiveness of ESC may even be higher than estimated if the large
number of damage-only crashes are excluded in the analysis. Although the
sample used in this study is based on mostly luxury vehicles, we believe
that ESC would still be highly effective across the entire on-road fleet
based on the results from other studies (specifically those in Europe)
where a large population of various vehicle classes were used in
analyzing the effectiveness of ESC.

**REFERENCES**

Aga, M. and Okada, A. (2003) Analysis of Vehicle Stability Control
(VSC)'s Effectiveness from Accident Data, Paper Number 541, *Proceedings
of the 18^th^ International Technical Conference on the Enhanced Safety
of Vehicles*.

Agresti, A. (2002) *Categorical Data Analysis*, Wiley, New York,
pp.16-25.

Allison, P.D. (1999) Logistic Regression Using the SAS System: Theory
and Application, Cary, NC: SAS Institute Inc., pp.82-83.

Dang, J. (2004) *Preliminary Results Analyzing the Effectiveness of
Electronic Stability Control (ESC) Systems*, NHTSA Evaluation Note No.
DOT HS 809 790, Washington, DC.

Farmer, C. (2004) Effect of Electronic Stability Control on Automobile
Crash Risk, *Traffic Injury Prevention*, Vol 5, pp. 317-325.

Green, P. and Woodrooffe, J. (2006) *The Effectiveness of Electronic
Stability Control on Motor Vehicle Crash Prevention*, Report Number
UMTRI-2006-12, University of Michigan Transportation Research Institute,
Ann Arbor, MI.

*Handbook of Probability and Statistics with Tables*, Second Edition,
McGraw-Hill, Inc., USA, 1970, pp. 244-245.

Lie, A., Tingvall, C., Krafft, M., and Kullgren, A. (2006) The
Effectiveness of ESC (Electronic Stability Control) in Reducing Real
Life Crashes and Injuries, *Traffic Injury Prevention*, Vol 7, pp.
38-43.

Lie, A., Tingvall, C., Krafft, M., and Kullgren, A. (2004) The
Effectiveness of ESP (Electronic Stability Program) in Reducing Real
Life Accidents, *Traffic Injury Prevention,* Vol 5, pp. 37-41.

Morgan, C. (2001) *The Effectiveness of Retroreflective Tape on Heavy
Trailers*, NHTSA Technical Report No. DOT HS 809 222, Washington, DC,
pp. 29-37.

*SAS/STAT^®^ User's Guide*, Vol. 1, Version 6, 4^th^ Ed., SAS Institute,
Cary, NC, 1990.

*Traffic Safety Facts,* 2004, NHTSA Technical Report No. DOT HS 809 919,
Washington, DC, pp.19-20.

[^1]: Aga, M. and Okada, A. (2003) Analysis of Vehicle Stability Control
    (VSC)'s Effectiveness from Accident Data, Paper Number 541,
    *Proceedings of the 18^th^ International Technical Conference on the
    Enhanced Safety of Vehicles*.

[^2]: Dang, J. (2004) *Preliminary Results Analyzing the Effectiveness
    of Electronic Stability Control (ESC) Systems*, NHTSA Evaluation
    Note No. DOT HS 809 790, Washington, D.C.

[^3]: Farmer, C. (2004) Effect of Electronic Stability Control on
    Automobile Crash Risk, *Traffic Injury Prevention*, Vol 5, pp.
    317-325.

[^4]: Lie, A., Tingvall, C., Krafft, M., and Kullgren, A. (2004) The
    Effectiveness of ESP (Electronic Stability Program) in Reducing Real
    Life Accidents, *Traffic Injury Prevention,* Vol 5, pp. 37-41.

[^5]: Lie, A., Tingvall, C., Krafft, M., and Kullgren, A. (2006) The
    Effectiveness of ESC (Electronic Stability Control) in Reducing Real
    Life Crashes and Injuries, *Traffic Injury Prevention*, Vol 7, pp.
    38-43.

[^6]: Green, P. and Woodrooffe, J. (2006) *The Effectiveness of
    Electronic Stability Control on Motor Vehicle Crash Prevention*,
    Report Number UMTRI-2006-12, University of Michigan Transportation
    Research Institute, Ann Arbor, MI.

[^7]: This book is published yearly by NHTSA.

[^8]: This book is published yearly by Ward's Communication, Inc.

[^9]: We did examine those make-models that had ESC as optional
    equipment, under the assumption that all these vehicles did not have
    ESC, and these additional data did not make a big difference in the
    overall effectiveness.

[^10]: Charles Farmer, in his peer review of this report, recommended
    the exclusion of 2003-2004 Toyota 4Runners.

[^11]: This manual is published yearly by the National Insurance Crime
    Bureau (NICB).

[^12]: For the analysis of fatal crashes obtained from the FARS
    database, *[all crash involvements]{.underline}* refer (in this
    study) to all types of [fatal]{.underline} crash involvements. For
    the analysis of police-reported crashes obtained from the State data
    files, *[all crash involvements]{.underline}* also refer to all
    types of crash involvements including not only [fatal]{.underline}
    but also [non-fatal]{.underline} crash involvements (i.e., property
    damage, possible injury, non-incapacitating and incapacitating
    injury).

[^13]: Agresti, A. (2002) *Categorical Data Analysis*, Wiley, New York,
    pp.16-25.

[^14]: Charles Farmer recommended this approach in his peer review.

[^15]: *Handbook of Probability and Statistics with Tables*, Second
    Edition, McGraw-Hill, Inc., USA, 1970,

    pp. 244-245.

[^16]: *Handbook of Probability and Statistics with Tables*, Second
    Edition, McGraw-Hill, Inc., USA, 1970, p. 383.

[^17]: Allison, P.D. (1999) *Logistic Regression Using the SAS System:
    Theory and Application*, Cary, NC: SAS Institute Inc., pp.82-83.

[^18]: *Traffic Safety Facts,* 2004, NHTSA Technical Report No. DOT HS
    809 919, Washington D.C., 2004, pp.19-20.

[^19]: Charles Farmer, in his peer review, recommended analysis (2) as
    an additional way to check if vehicle age is a factor. Claes
    Tingvall also emphasized the potential importance of vehicle age in
    the current analysis, and especially in future analyses as the
    vehicles without ESC get older.

[^20]: Allison, P.D. (1999) *Logistic Regression Using the SAS System:
    Theory and Application*, Cary, NC: SAS Institute Inc., p. 27.

[^21]: Results are statistically significant at the 0.05 level when the
    LR chi-square statistic is greater than 3.84.

[^22]: For a CATMOD analysis of crash data with statistically
    significant three-way terms see Morgan, C., *The Effectiveness of
    Retroreflective Tape on Heavy Trailers*, NHTSA Technical Report No.
    DOT HS 809 222, Washington, 2001, pp. 29-37, summarized in Kahane
    (2004), pp. 43-44; *SAS/STAT^®^ User's Guide*, Vol. 1, Version 6,
    4^th^ Ed., SAS Institute, Cary, NC, 1990.

[^23]: The four-way interaction term is not included because we do not
    anticipate that the effectiveness gap between 4-channel and
    2-channel systems would vary from State to State.