Safety Report

Evaluation of the Rollover Propensity

of 15-passenger Vans

National 

Transportation 

Safety Board

Washington, D.C.

A

A

N TION

L TRA SPOR

ATIO

N

B OARD

SAF E T Y

N

T

E PLURIBUS UNUM 

NTSB/SR-02/03

PB2002-917005

Safety Report

Evaluation of the Rollover Propensity

of 15-passenger Vans

NTSB/SR-02/03

PB2002-917005 National Transportation Safety Board

Notation 7498 490 LíEnfant Plaza, S.W.

Adopted October 15, 2002 Washington, D.C. 20594
National Transportation Safety Board. 2002. Evaluation of the Rollover Propensity of 15-passenger

Vans. Safety Report NTSB/SR-02/03. Safety Study NTSB/SR-02/03. Washington, DC.

Abstract: Fifteen-passenger vans, which make up about 0.25 percent of the passenger vehicle fleet in the

United States, are frequently used to transport school sports teams, van pools, church groups, and other

groups. Although they are involved in a proportionate number of fatal accidents compared to their

percentage in the fleet, they are involved in a higher number of single-vehicle accidents involving rollovers

than are other passenger vehicles. Various factors have been associated with 15-passenger van rollover,

particularly occupancy level and vehicle speed. Fully loading or nearly loading a 15-passenger van causes

the center of gravity to move rearward and upward, which increases its rollover propensity and could

increase the potential for driver loss of control in emergency maneuvers. The National Highway Traffic

Safety Administration has been evaluating vehicle rollover for several years. The agency has initiated

rulemaking activities concerning vehicle rollovers, established a rollover resistance rating system, and is

currently examining dynamic testing procedures; however, these programs have not been extended to 15-

passenger vans. As a result of this safety report, the National Transportation Safety Board issued safety

recommendations to the National Highway Traffic Safety Administration and to the manufacturers of 15-

passenger vans.

The National Transportation Safety Board is an independent Federal agency dedicated to promoting aviation, railroad, highway, marine,

pipeline, and hazardous materials safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board

Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study

transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The Safety Board

makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and

statistical reviews.

Recent publications are available in their entirety on the Web at <http://www.ntsb.gov>. Other information about available publications also

may be obtained from the Web site or by contacting:

National Transportation Safety Board

Public Inquiries Section, RE-51

490 LíEnfant Plaza, S.W.

Washington, D.C. 20594

(800) 877-6799 or (202) 314-6551

Safety Board publications may be purchased, by individual copy or by subscription, from the National Technical Information Service. To

purchase this publication, order report number PB2002-917005 from:

National Technical Information Service

5285 Port Royal Road

Springfield, Virginia 22161

(800) 553-6847 or (703) 605-6000

The Independent Safety Board Act, as codified at 49 U.S.C. Section 1154(b), precludes the admission into evidence or use of Board reports

related to an incident or accident in a civil action for damages resulting from a matter mentioned in the report. 
iii Safety Report

Contents

Acronyms and Abbreviations .......................................................................................................... iv

Executive Summary .............................................................................................................................. v

Background .............................................................................................................................................. 1

The Fleet and Accidents ................................................................................................................................ 2

Rollover Propensity ....................................................................................................................................... 4

Activities Pertaining to Vehicle Rollover ..................................................................................... 8

Technology ..................................................................................................................................................... 8

Rulemaking Activities ................................................................................................................................... 8

NHTSA Rulemaking Activities ............................................................................................................ 8

Congressional Activity ........................................................................................................................ 10

Consumer Information From NHTSA ......................................................................................................... 10

Rollover Risk Rating System .............................................................................................................. 10

Consumer Advisories .......................................................................................................................... 13

Summary ................................................................................................................................................. 14

Findings ................................................................................................................................................... 16

Recommendations ............................................................................................................................... 17

Appendixes

A: Previously Issued Safety Recommendations Pertaining to Large Vans ........................................... 19

B: Vehicle Specifications, Model Year 2002 ........................................................................................ 26

C: Predicting 15-passenger Van Rollover Accidents ............................................................................ 27

D: Vehicle Dynamics of 15-passenger Vans ......................................................................................... 40

Conversion Factors for the 

International System of Units (SI) 

To convert from into multiply by

feet (ft) meters (m) 0.3048 

inches (in) centimeters (cm) 2.54 

miles (U.S. statute) kilometers (km) 1.609344 

pounds (lb) kilograms (kg) 0.4535924 
iv Safety Report

Acronyms and Abbreviations

ANPRM advance notice of proposed rulemaking

CFR Code of Federal Regulations

CG center of gravity

ESC electronic stability control

FARS Fatal Analysis Reporting System

FMVSS Federal Motor Vehicle Safety Standards

FR Federal Register

GVWR gross vehicle weight rating

NAS National Academy of Sciences

NCAP New Car Assessment Program

NHTSA National Highway Traffic Safety Administration

NTSB National Transportation Safety Board

SSF static stability factor

TREAD Transportation, Recall Enhancement, Accountability, and 

Documentation Act of 2000

VIN vehicle identification number
v Safety Report

Executive Summary

Fifteen-passenger vans, which make up about 0.25 percent of the passenger

vehicle fleet in the United States, are frequently used to transport school sports teams, van

pools, church groups, and other groups. Although they are involved in a proportionate

number of fatal accidents compared to their percentage in the fleet, they are involved in a

higher number of single-vehicle accidents involving rollovers than are other passenger

vehicles. Various factors have been associated with 15-passenger van rollover, particularly

occupancy level and vehicle speed. Because these vans are designed to carry 15

passengers, the Safety Board is particularly concerned about the relationship between

occupancy level and vehicle rollover. Fully loading or nearly loading a 15-passenger van

causes the center of gravity to move rearward and upward, which increases its rollover

propensity and could increase the potential for driver loss of control in emergency

maneuvers.

The National Highway Traffic Safety Administration (NHTSA) has been

evaluating vehicle rollover for several years. NHTSA has initiated rulemaking activities

concerning vehicle rollovers, established a rollover resistance rating system, and is

currently examining dynamic testing procedures; however, these programs have not been

extended to 15-passenger vans. The Safety Board is concerned that NHTSA has not

included 15-passenger vans in the dynamic testing or proposed rollover resistance ratings

for this class of vehicle, given their high rate of rollover involvement in single-vehicle

accidents, particularly under fully or nearly loaded conditions.

As a result of this safety report, the National Transportation Safety Board issued

new safety recommendations to the National Highway Traffic Safety Administration and

the manufacturers of 15-passenger vans.

1 Safety Report

Background

About 2:30 p.m. on February 10, 2000, a Ford 1999 E-350 XLT 15-passenger van

carrying a track coach, an athletic trainer, and eight student athletes, departed Prairie View

A&M University near Hempstead, Texas. A 21-year-old student athlete was driving, and

the van was traveling northbound on a two-lane highway near Karnack, Texas. About 6:50

p.m., the van was traveling about 82 mph as it approached a northbound Jeep Cherokee

that was signaling to turn left near the first entrance to a convenience store parking lot. A

van occupant stated that the Jeep slowed while signaling to turn left, did not complete its

left turn into the first entrance, and continued traveling forward. When the van driver

attempted to pass the Jeep on the left, the Jeep began its left turn into the second entrance

of the convenience store. The van driver tried to reverse the passing action by swerving

the van sharply to the right, and the van went out of control. No contact between the van

and the Jeep occurred. The van yawed right, then left, dropped off the pavement edge, and

began overturning. The van traveled a total of about 490 feet from where it began its yaw

to where it came to its final resting position; the distance traveled from where it hit the

pavement edge to where it came to rest was about 265 feet. Evidence on the roadway

indicated that the van overturned three full turns before coming to rest inverted. The

accident resulted in fatal injuries to the van driver and three of five ejected occupants. The

remaining six passengers, including two other ejected occupants, sustained serious

injuries.1

In recent years, the Safety Board has investigated other highly visible accidents

involving 15-passenger vans, such as the one near Karnack, Texas.2

 Although there are

many factors involved in such accidents, the Safety Board is currently concerned about the

propensity of 15-passenger vans to roll over.3

Although the Safety Board has previously addressed the safety of large vans,4 it

has not addressed the rollover propensity of the vehicles. Throughout the 1970s and

1 National Transportation Safety Board, Single-Vehicle Rollover, Texas State Highway 43, Near

Karnack, Texas, February 10, 2000, Highway Accident Brief NTSB/HAB-02/03 (Washington, DC: NTSB,

2002).

2 For example, Henrietta, Texas, May 8, 2001 (NTSB Accident No. HWY-01-FH-022); and

Randleman, North Carolina, July 1, 2001 (NTSB Accident No. HWY-01-FH-027).

3 In this report, the Safety Board limits discussion to the rollover propensity of 15-passenger vans.

Other safety factors such as driver qualifications, occupant protection, and tire blowout will be addressed in

its accident investigations.

4 For example, (a) Pattison Head Start Center School Van Run Off Bridge and Fire Near Hermanville,

Mississippi, December 17, 1981, Highway Accident Report NTSB/HAR-82/05 (Washington, DC: NTSB,

1982). (b) Crashworthiness of Large Poststandard School Buses, Safety Study NTSB/SS-87/01

(Washington, DC: NTSB, 1987). (c) Performance of Lap/Shoulder Belts in 167 Motor Vehicle Crashes,

Safety Study NTSB/SS-88/02 and NTSB/SS-88/03 (Washington, DC: NTSB, 1988), 2 Vols. (d)

Crashworthiness of Small Poststandard School Buses, Safety Study NTSB/SS-89/02 (Washington, DC:

NTSB, 1989). (e) Pupil Transportation in Vehicles Not Meeting Federal School Bus Standards, Special

Investigation Report NTSB/SIR-99/02 (Washington, DC: NTSB, 1999).
Background 2 Safety Report

1980s, the Safety Board issued recommendations to the National Highway Traffic Safety

Administration (NHTSA) and other organizations on occupant protection, emergency

egress, and crashworthiness of vans. More recently, the Safety Board issued

recommendations directed at the use of large vans for school transportation and

recommended that children should be transported in vehicles that meet Federal school bus

standards or the equivalent. (Appendix A lists previously issued Safety Board

recommendations and their current status.)

The Fleet and Accidents

Fifteen-passenger vans represent about 0.25 percent of the passenger fleet in the

United States.5 Historically, Dodge and Ford have manufactured the majority of 15-

passenger vans, with Chevrolet and GMC making up the remainder of the fleet. The

following model year 2002 makes and models are available with seating for 15

passengers: Chevrolet Express G3500 Extended; Ford Econoline Wagon E-350 XL

Extended; Ford Econoline Wagon E-350 XLT Extended; Dodge Ram Wagon 3500 Maxi;

and GMC Savana G3500 Extended (appendix B contains more detailed vehicle

information). Dodge, however, ceased production of 15-passenger vans in June 2002.

Analysis of Fatal Analysis Reporting System (FARS) data for 1991ñ2000 shows

that 15-passenger vans are involved in about 0.22 percent of the fatal accidents (1,204

vans of 557,786 vehicles) and 0.24 percent of the single-vehicle fatal accidents (499 vans

of 209,447 vehicles).6

 This is proportionate to the percent of 15-passenger vans in the

passenger vehicle fleet (0.25 percent); however, 15-passenger van accidents are involved

in a higher percentage of rollover accidents than are passenger cars and smaller vans.

About 52 percent of the 15-passenger vans involved in single-vehicle, fatal accidents

experience a rollover (as a primary or subsequent event) compared to 33 percent of the

passenger automobiles involved in such accidents (figure 1). Additionally, 81 percent of

the 15-passenger van occupant fatalities occur in single-vehicle rollover accidents (figure 2).

5 U.S. Department of Transportation, National Highway Traffic Safety Administration, ì15-Passenger

Vans Involved in Fatal Crashes: Statistical Fact Sheet,î Item in press release packet (Washington, DC:

NHTSA, April 15, 2002). In contrast, the Bureau of Transportation Statistics reports that in 1998 there were

215,496,003 registered vehicles: 131,838,538 registered passenger vehicles; 3,879,450 motorcycles;

71,330,205 other 2-axle 4-tire vehicles; 5,734,925 single-unit trucks; 1,997,345 combination trucks, and

715,540 buses (U.S. Department of Transportation, Bureau of Transportation Statistics, National

Transportation Statistics 2000, BTS01-01 (Washington, DC: BTS, April 2001)).

6 FARS is maintained by the National Highway Traffic Safety Administration (NHTSA) of the U.S.

Department of Transportation. The database is a census of all fatal crashes involving a motor vehicle

traveling on a trafficway customarily open to the public and results in the death of a person (occupant of a

vehicle or a nonmotorist) within 30 days of the crash.
Background 3 Safety Report

 Figure 1. Percent of single-vehicle fatal accidents by vehicle type and rollover involvement, 

1991ñ2000. (Data source: Fatality Analysis Reporting System of the National Highway Traffic Safety 

Administration.)

 Figure 2. Percent of occupant fatalities by vehicle type and rollover involvement, 1991ñ2000. 

(Data source: Fatality Analysis Reporting System of the National Highway Traffic Safety 

Administration.)

0

10

20

30

40

50

60

70

80

90

100

15-passenger vans

Passenger automobiles

Sport utility vehicles

Van, other

Lt. trucks

Lg. Trucks

Buses

Other

Vehicle type

Percent of single-vehicle fatal accidents

Rollover

No rollover

0

10

20

30

40

50

60

70

80

90

100

15-passenger vans

Passenger automobiles

Sport utility vehicles

Van, other

Lt. trucks

Lg. trucks

Buses

Other

Vehicle type

Percent of occupant fatalities

Rollover

No rollover
Background 4 Safety Report

Rollover Propensity

Research has shown that among other factors, accidents in rural areas, vehicles

with higher occupancy levels, vehicle speed, driver alcohol/drug involvement, and

younger driver age are associated with rollover propensity.7

 However, much of the

previous work was done on passenger vehicles and excluded 15-passenger vans.8 The

Safety Board thus conducted analyses on the FARS data for single-vehicle, fatal 15-

passenger van accidents that occurred from 1991 through 2000 and found similar results,

suggesting that occupancy level and vehicle speed are consistently associated with van

rollover (figures 3 and 4).9

 The analyses are described and discussed in appendix C.

7 (a) W. Riley Garrott, Barbara Rhea, Rajesh Subramanian, and Gary J. Heydinger, The Rollover

Propensity of Fifteen-Passenger Vans, Research Note (Washington, DC: NHTSA, April 2001). (b) T.M.

Klein, A Statistical Analysis of Vehicle Rollover Propensity and Vehicle Stability, SAE Tech. Pap. 920584

(Warrendale, PA: Society of Automotive Engineers, 1992) 135-150. (c) ìConsumer Information

Regulations; Federal Motor Vehicle Safety Standards; Rollover Resistance; Final Rule [49 CFR Part 575],î

Federal Register Vol. 66, No. 9, dated January 12, 2001: 3388-3437.

8 NHTSA informed Safety Board staff on June 4, 2002, that the agency is currently preparing a

technical report that examines single-vehicle, fatal 15-passenger van rollover accidents. According to

NHTSA, the FARS data analysis has been extensive and evaluates the effect of several factors such as speed,

number of vehicle occupants, vehicle maneuvers, age of the driver, and alcohol involvement on vehicle

rollover. NHTSA expects to publish this report in 2002.

 Figure 3. Number of single-vehicle fatal accidents by number of occupants and rollover 

involvement, 1991ñ2000. (Data source: Fatality Analysis Reporting System of the National 

Highway Traffic Safety Administration.)

9 The Safety Board notes that the validity of data for vehicle speed is often questioned.

0

10

20

30

40

50

60

70

80

90

100

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Number of occupants

Number of single-vehicle fatal accidents

Rollover

No Rollover
Background 5 Safety Report

Because these vans are designed to carry 15 passengers, the results relating

occupancy level to rollover are particularly disturbing. NHTSA research reported in 2001

that 15-passenger vans with 10 or more occupants had three times the rollover ratio than

did those with fewer than 10 occupants.10 The same analyses conducted by the Safety

Board on the FARS data yielded higher rollover ratios for all levels of occupancy levels

but similar magnitudes of increase in the rollover ratio when comparing lightly loaded to

fully loaded vans (table 1).11 Fifteen-passenger vans with 10ñ15 occupants had a rollover

ratio of 85.0 percent compared with a ratio of 28.3 percent for vans with fewer than 5

occupants. 

 Figure 4. Number of single-vehicle fatal accidents by vehicle speed and rollover involvement, 

1991ñ2000. (Data source: Fatality Analysis Reporting System of the National Highway Traffic 

Safety Administration.)

10 (a) Rollover ratio is the number of all single-vehicle rollover accidents divided by the number of all

single-vehicle accidents. (b) Garrott and others (2001) conducted their analyses using a subset of NHTSAís

State Data System. The State Data System is a census of crashes (fatal, injury, and property-damage) that

occur in 17 States (California, Florida, Georgia, Illinois, Indiana, Kansas, Maryland, Michigan, Missouri,

North Carolina, New Mexico, Ohio, Pennsylvania, Texas, Utah, Virginia, and Washington).

11 FARS data for 1991ñ2000.

0

5

10

15

20

25

30

3 8 13 18 23 28 33 38 43 48 53 58 63 68 73 78 83 88 93

Vehicle speed

Number of single-vehicle fatal accidents

Rollover

No rollover
Background 6 Safety Report

A simulation study conducted by the NHTSA illustrates some of the adverse

effects that loading can have on the handling of 15-passenger vans.12 The simulations

showed that the fully loaded 15-passenger van made a transition from understeer to

oversteer as the lateral acceleration increased.13 Further, under one simulated driving

maneuver, the fully loaded van rolled over whereas the lightly loaded (driver only) van did

not. NHTSA concluded that the computer simulations illustrated the adverse effects that a

fully loaded passenger van can have on its handling properties (sudden transition from

understeer to oversteer) and rollover propensity. Adding passengers and cargo to a 15-

12 Garrott and others, 2001.

13 A simple test illustrates the concepts of understeer and oversteer. A vehicle is driven around a circle

at a constant speed, then the speed is slowly increased. If the vehicle tends to go off the outside of the circle

so that the driver must increase steering to maintain the circle, then the vehicle is considered to be an

understeer vehicle. If the vehicle tends to go off the inside of the circle so that the driver must reduce

steering to maintain the circle, then the vehicle is considered to be an oversteer vehicle. Understeer and

oversteer can affect the stability of a vehicle; however, just because a vehicle is an oversteer vehicle does not

mean that it is uncontrollable. A more detailed discussion of understeer and oversteer and their impact on

stability and control is contained in (a) William F. Milliken and Douglas L. Milliken, ìSimplified Steady

State Stability and Control,î Chapter 5, and ìSimplified Transient Stability and Control,î Chapter 6 in Race

Car Vehicle Dynamics (Warrendale, PA: Society of Automotive Engineers, 1995) 123-229 and 231-277; and

(b) Thomas D. Gillespie, ìRollover,î Chapter 9 in Fundamentals of Vehicle Dynamics (Warrendale, PA:

Society of Automotive Engineers, 1992) 309-333. The Society of Automotive Engineersí definitions of

understeer and oversteer are given in Milliken and Milliken (1995), p. 164.

Table 1. Number of accidents, rollovers, and rollover ratio by occupancy level of 

15-passenger vans in single-vehicle accidents. 

Occupancy level 

All 

single-vehicle 

accidents 

All 

rollovers 

Rollover ratioa

(percent) 

Combined rollover ratios for 

1 to 9 occupants and for 

10 or more occupants 

(percent) 

Results from the 2001 NHTSA analysisb

(Data source: Subset of NHTSAís State Data System, 1994ñ1997): 

Fewer than 5 1,815 224 12.3 

5ñ9 77 16 20.8 

12.7 

10ñ15 55 16 29.1 

More than 15 10 7 70.0 

35.4 

Results from the 2002 NTSB analysis 

(Data source: FARS data, 1991ñ2000): 

Fewer than 5 244 69 28.3 

5ñ9 114 72 63.2 

39.4 

10ñ15 113 96 85.0 

More than 15 21 19 90.0 

85.8 

FARS = Fatality Analysis Reporting System (FARS); NHTSA = National Highway Traffic Safety Administration; 

NTSB = National Transportation Safety Board. 

a The number of all single-vehicle rollover accidents divided by the number of all single-vehicle accidents. 

b W. Riley Garrott, Barbara Rhea, Rajesh Subramanian, and Gary J. Heydinger, The Rollover Propensity of FifteenPassenger Vans, Research Note (Washington, DC: NHTSA, April 2001). 
Background 7 Safety Report

passenger van causes the center of gravity to move rearward and upward, which increases

the vehicleís rollover propensity and could increase the potential for driver loss of control

in emergency maneuvers.14 Details of the NHTSA simulations, and information about

vehicle dynamics and the effects of moving the center of gravity rearward and upward, are

provided in appendix D.

14 (a) NHTSA simulations, reported in Garrott and others, 2001. (b) NHTSA press release packet,

April 15, 2002. (c) Gillespie (1992) discusses general vehicle dynamics of how increasing the center of

gravity height affects rollover.
8 Safety Report

Activities Pertaining 

to Vehicle Rollover

Technology

Technology has been developed to assist drivers in maintaining control of the

vehicle; for example, antilock brakes, traction control, lane departure systems, and

electronic stability control (ESC) systems. Antilock brakes use speed sensors, valves,

pumps, and controllers to stop the vehicle in a safe manner. Traction control systems sense

when a tire is slipping or losing traction and automatically activate the brakes or slow

down engine speed. Lane departure systems typically alert the driver when the vehicle has

departed from the driving lane. ESC systems are computer-controlled systems that attempt

to stabilize the vehicle by monitoring a vehicleís movement and the direction the driver is

steering. If the driver inputs and the vehicle response do not correspond, computer

controls intervene to enhance the driverís ability to maintain control of the vehicle by

selectively braking individual wheel(s), or changing power applied to the wheels. Future

ESC systems will likely include inputs to steering and differential power control to the

wheels.

Some of these technologies are currently available on certain motor vehicles,

including some sport utility vehicles and minivans. Antilock brakes are currently available

on 15-passenger vans, but traction control systems, lane departure systems, and ESC

systems are not. Given the rollover propensity of 15-passenger vans, such technological

systems may have potential to assist drivers in maintaining control of these vehicles.

Rulemaking Activities

NHTSA Rulemaking Activities

NHTSA originally proposed rulemaking concerning vehicle rollovers in 1973

when it issued an advance notice of proposed rulemaking on minimum performance

rollover resistance and periodically has taken rulemaking action since then (table 2);

however, there are no rollover standards at present for any highway vehicle. In 2001,

NHTSA issued a final rule pertaining to the development of a consumer information

rollover resistance rating system based on a static measure of stability (discussed further

in the ìConsumer Informationî section of this report). NHTSA proposed additional

rulemaking related to vehicle rollover on October 7, 2002 (Federal Register (FR), Vol. 67,

No. 194). The notice is a follow-on to NHTSAís request for comments published on

July 3, 2001, which announced the agencyís plans to evaluate driving maneuver tests for

rollover resistance and to develop a dynamic test for the consumer information program.

The proposed rule contains information on a dynamic testing protocol, but it does not
Activities Pertaining to Vehicle Rollover 9 Safety Report

extend to 15-passenger vans because the Federal Motor Vehicle Safety Standards

(FMVSS) define a motor vehicle designed to carry more than 10 persons as a bus (Title 49

Code of Federal Regulations (49 CFR) Part 571.3).

Table 2. Rulemaking activities of the National Highway Traffic Safety Administration 

concerning vehicle rollovers. 

Year Activity Outcome 

April 1973 NHTSA published an advance notice of proposed 

rulemaking (ANPRM) on resistance to rollover. 

Terminated. 

September 

1986 

Congressman Wirth petitioned NHTSA to establish a 

safety standard for rollover resistance by setting a 

minimum allowable static stability factor (SSF). 

Denied. 

June 1988 Consumers Union petitioned NHTSA to establish a 

safety standard to protect occupants against 

ìunreasonable risk of rollover.î 

NHTSA embarked on research 

program but did not proceed 

with rulemaking. 

January 1992 NHTSA issued an ANPRM on various regulatory actions 

to reduce the frequency of vehicle rollovers and/or the 

number and severity of injuries resulting from vehicle 

rollovers. 

NHTSA terminated rulemaking 

in June 1994 to establish a 

standard and proposed to 

require manufacturers to label 

vehicles with information on 

their rollover stability using 

either tilt table angle or critical 

sliding velocity. 

September 

1994 

Congress prohibited NHTSA from requiring vehicle 

labeling until completion of National Academy of 

Sciences (NAS) study. 

NAS study recommended that 

NHTSA expand scope of 

consumer information it 

provides to the public. 

June 1996 NHTSA reopened comment period on proposed labeling 

rule. 

August 1996 NHTSA was petitioned by the Consumers Union to 

develop a test of vehicle emergency handling capability 

and to provide test results on new vehicles to the public 

as consumer information. 

Petition granted. NHTSA 

began new research program. 

June 2000 NHTSA published request for comments on consumer 

information program that will use the SSF to indicate 

overall rollover risk in single-vehicle crash. 

November 1, 

2000 

Congress enacted the Transportation Recall 

Enhancement, Accountability, and Documentation 

(TREAD) Act. 

NHTSA expands dynamic 

testing program. 

January 12, 

2001 

Final rule published on Consumer Information 

Regulations adding rollover resistance. 

Rollover resistance ratings 

available beginning with model 

year 2001 vehicles. 

July 3, 2001 NHTSA published request for comments on developing 

dynamic test to add to consumer information program. 

October 7, 

2002 

NHTSA published an NPRM on its dynamic testing 

protocol and proposed alternatives for using the dynamic 

test results in consumer information on the rollover 

resistance of new vehicles. 
Activities Pertaining to Vehicle Rollover 10 Safety Report

Congressional Activity

The Transportation Recall Enhancement, Accountability, and Documentation

(TREAD) Act was enacted on November 1, 2000. Among other requirements, it mandated

NHTSA to develop dynamic tests of vehicle rollover by November 1, 2002, and to

conduct rulemaking to determine how best to disseminate test results to the public.15 This

requirement applies to motor vehicles, including passenger cars, multipurpose passenger

vehicles, and trucks with a gross vehicle weight rating of 10,000 pounds or less; the

requirement does not apply to 15-passenger vans because the FMVSS define such

vehicles as buses.

Consumer Information From NHTSA

Rollover Risk Rating System

In 2001, NHTSAís New Car Assessment Program (NCAP) was expanded to

include consumer information on the rollover risk of passenger cars, and of light,

multipurpose passenger vehicles and trucks.16 The expansion does not extend to vehicles

that carry more than 10 passengers. The rollover resistance rating system, available

beginning with model year 2001 vehicles, is shown in table 3. The ratings estimate the risk

of rolling over in a single-vehicle crash; the system does not predict the likelihood of such

a crash.

15 The NPRM published by NHTSA on October 7, 2002 (Federal Register, Vol. 67, No. 194) was in

response to the TREAD Act.

16 The NCAP program was established in 1978 with the purpose of providing consumers with a

measure of the relative safety potential of vehicles in frontal crashes. NCAP information includes results

from frontal and side crash tests as well as rollover resistance ratings. The ultimate goal of the program is to

improve occupant safety by providing market incentives for vehicle manufacturers to voluntarily design

their vehicles to better protect occupants in a crash rather than by regulatory devices.

Table 3. Rollover resistance ratings for vehicles in a single-vehicle 

crash, available beginning with model year 2001 vehicles. 

Vehicle rating Crash rollover risk 

Five stars Less than 10 percent 

Four stars 10ñ20 percent 

Three stars 20ñ30 percent 

Two stars 30ñ40 percent 

One star Greater than 40 percent 

Source: National Highway Traffic Safety Administration, New Car Assessment Program. 
Activities Pertaining to Vehicle Rollover 11 Safety Report

The rollover resistance rating is based on the static stability factor (SSF). The SSF

is generated by dividing a vehicleís track width (distance between the wheels from side to

side, denoted by t) by twice the vehicleís center of gravity height (SSF=t/2h).17 The SSF

used for the NCAP rollover resistance rating is based on measurements for a driver-only

load condition.

Static measures of stability (that is, SSF, tilt table ratio, and critical sliding

velocity18) have been shown to be important factors in understanding vehicle rollover.19

NHTSA, in its 2001 research, compared static stability factors of two 7-passenger vans

and a 15-passenger van under lightly loaded and fully loaded conditions. Although the

SSF decreased for all three vans from the lightly loaded condition to the fully loaded

condition, the change was the greatest for the 15-passenger van: the SSF decreased

3 percent for one 7-passenger van, 5 percent for the other 7-passenger van, and 11 percent

for the 15-passenger van.

In response to a 2001 congressional mandate,20 the National Research Council of

the National Academy of Sciences (NAS) completed a review of NHTSAís rollover resistance rating system.21 It concluded that the SSF captures important vehicle characteristics

related to rollover propensity and is strongly correlated with the outcome of actual crashes

(rollover or no rollover). However, it also concluded that the NCAP rollover resistance

rating system, which uses numbers of stars to indicate rollover risk, is likely to be of limited use in presenting practical information to the public because (1) there were shortcomings in the statistical methodology used to derive the average rollover curve; (2) the

approximation of the rollover curve by five discrete categories is coarse and does not adequately convey the degree of resolution among vehicles provided by available crash data;

and (3) limited procedures used by NHTSA to evaluate the rating system raise questions

about the systemís effectiveness. The NAS report recommended that

1. NHTSA should vigorously pursue its ongoing research on driving maneuver

tests for rollover resistance, mandated under the TREAD Act, with the objective

of developing one or more dynamic tests that can be used to assess transient

vehicle behavior leading to rollover.

2. In the longer term, NHTSA should develop revised consumer information on

rollover that incorporates the results of one or more dynamic tests on transient

vehicle behavior to complement the information from static measures, such as

SSF.

17 Typical SSF values for passenger cars range from 1.25 to 1.45 or more. Typical SSF values for SUVs

range from 1.04 to 1.24.

18 The tilt table angle is the angle at which a vehicle will begin to tip off a gradually tilted platform. The

critical sliding velocity is the minimum velocity needed to trip a vehicle that is sliding forward.

19 (a) Garrott and others, 2001. (b) Klein, 1992. (c) NHTSAís final rule on rollover resistance (Federal

Register Vol. 66, No. 9, dated January 12, 2001: 3388-3437).

20 Department of Transportation and Related Agencies Appropriations Act of 2001 (Public Law 106ñ

346).

21 Transportation Research Board, National Research Council, An Assessment of the National Highway

Traffic Safety Administrationís Rating System for Rollover Resistance. (Prepublication copy).
Activities Pertaining to Vehicle Rollover 12 Safety Report

3. NHTSA should investigate alternative options for communicating information

to the public on SSF and its relationship to rollover. In developing revised

consumer information, NHTSA should:

ï Use a logit model as a starting point for analysis of the relationship between rollover

risk and SSF.22

ï Consider a higher-resolution representation of the relationship between rollover risk

and SSF than is provided by the current five-star rating system.

ï Continue to investigate presentation metrics other than stars.

ï Provide consumers with more information, placing rollover risk in a broader context

of motor vehicle safety.

NHTSA launched a vehicle dynamic rollover propensity research program in the

late 1990s. Phases I, II, and III evaluated a broad range of dynamic testing maneuvers that

might induce on-road, untripped rollovers. The program tested 12 vehicles (3 passenger

cars, 3 light trucks, 3 sport utility vehicles, and 3 vans23 but no 15-passenger vans) using

candidate test maneuvers designed to determine fundamental vehicle handling properties

and untripped rollover propensity measured by two-wheel lift. The program also studied

pulse brake automation in rollover maneuvers.24

As a result of the TREAD Act of 2000, NHTSA is conducting phases IV, V, and VI

of its dynamic rollover propensity program. These phases of research will continue to look

at additional testing maneuvers and examine various influences on rollover testing (for

example, the effect of outriggers, cold and hot weather testing, and surface effects). The

October 2002 NPRM describes NHTSAís proposed test maneuvers and two load

conditions. NHTSA has informed the Safety Board that 15-passenger vans will not be

included in this testing because the FMVSS define these vehicles as buses.

22 A logit model is a binary response model where the probability of an event occurring (that is,

rollover) is estimated.

23 The vans were an 8-passenger 1998 Ford E-150 Club Wagon, an 8-passenger 1998 Chevrolet Astro,

and a 7-passenger 1998 Dodge Caravan. Each vehicle was tested with a driver, instrumentation, and

outriggers.

24 (a) W. Riley Garrott, J. Gavin Howe, and Garrick Forkenbrock, An Experimental Examination of

Selected Maneuvers That May Induce On-road Untripped, Light Vehicle Rollover: Phase II of NHTSAís

1997ñ1998 Vehicle Rollover Research Program, Report No. TBD (Washington, DC: NHTSA, July 1999).

(b) W. Riley Garrott, ìNHTSA Research on Dynamic Rollover Tests,î presented at the session ìCrash

Avoidance II: Rollover, Tires, Handling, and Stability,î SAE/Government/Industry Meeting, May 13ñ15,

2002; Washington, DC.
Activities Pertaining to Vehicle Rollover 13 Safety Report

Consumer Advisories

Following several high publicity 15-passenger van accidents, NHTSA published a

consumer advisory in April 2001. The advisory contained a cautionary warning to users of

15-passenger vans because of an increased rollover risk under certain conditions. NHTSA

issued a second consumer advisory in April 2002, making the following safety tips:

ï Protect passengers with a seat belt policy;

ï Select an experienced driver;

ï Make sure the driver is not fatigued or driving too fast;

ï Properly maintain your tires; and

ï Avoid placing any load on the roofóthat increases the chance of rollover.
14 Safety Report

Summary

Fifteen-passenger vans, which make up about 0.25 percent of the passenger

vehicle fleet, are frequently used to transport school sports teams, vanpools, church

groups, and other groups. Although they are involved in a proportionate number of fatal

accidents compared to their percentage in the fleet, they are involved in a higher rate of

single-vehicle accidents involving rollovers than are other passenger vehicles.

Various factors have been associated with vehicle rollover, particularly occupancy

level and vehicle speed. Both the FARS data and a subset of State census data show that

the rollover rate for fully loaded or nearly loaded 15-passenger vans is about three times

the rollover ratio of vans with fewer than 5 passengers. Further, statistical analyses have

shown that increased occupancy level and vehicle speed (measured by either travel speed

or posted speed limit) consistently predict the increased likelihood of 15-passenger van

rollover. Other accident characteristics have also been shown to be related to vehicle

rollover but with less reliability.

Because these vans are designed to carry 15 passengers and frequently are used by

various organizations to transport many passengers to activities, the Safety Board is

particularly concerned about the relationship between occupancy level and vehicle

rollover. Fully loading or nearly loading a 15-passenger van causes the center of gravity to

move rearward and upward, which increases the vehicleís rollover propensity and could

increase the potential for driver loss of control in emergency maneuvers. Simulations

conducted by NHTSA illustrate how fully loading a 15-passenger van could adversely

affect the vehicleís handling properties in extreme maneuvers.

NHTSA has been evaluating vehicle rollover for several years. At the direction of

the TREAD Act of 2000, NHTSA expanded its dynamic testing on several vehicles, but it

does not include 15-passenger vans. Further, although NHTSA has initiated rulemaking

activities concerning vehicle rollovers, established a vehicle rollover resistance rating

system, and is currently examining dynamic testing procedures, these programs do not

extend to 15-passenger vans. Given their high rate of rollover involvement in singlevehicle accidents, particularly under fully loaded conditions for which they are designed

and are being used, the Board believes that 15-passenger vans should be included in

dynamic testing and proposed rollover resistance ratings for this class of vehicle.

Information from the dynamic testing also has the potential to develop a dynamic testing

protocol that could supplement the NCAP rollover resistance rating system. Therefore, the

Safety Board recommends that the National Highway Traffic Safety Administration

include 15-passenger vans in its dynamic testing program. The dynamic testing should test

the performance of 15-passenger vans under various load conditions.

The Safety Board recognizes that NHTSA has issued two consumer advisories

regarding the propensity of 15-passenger vans to roll over. The NCAP program also serves

as an available source of consumer information about the safety potential of vehicles in
Summary 15 Safety Report

crashes; however, the NCAP rollover resistance rating system does not currently include

15-passenger vans. The Safety Board believes that, at a minimum, the rollover resistance

rating system should be extended to include 15-passenger vans. Therefore, the Safety

Board recommends that NHTSA extend the NCAP rollover resistance program to 15-

passenger vans, especially for various load conditions. The inclusion of 15-passenger vans

in NHTSAís dynamic testing program, as described and recommended earlier in this

report, would provide valuable information by which to supplement the rollover resistance

rating system. Thus, the Board also recommends that in extending the rollover resistance

program to 15-passenger vans, NHTSA also use the dynamic testing results of 15-

passenger vans to supplement the static measures of stability in the NCAP rollover

resistance program.

Various technological systems have been developed to assist drivers in

maintaining control of the vehicle. Although some of these systems are currently available

on some vehicle types, most of them are not currently available on 15-passenger vans.

Given the rollover propensity of these vehicles, technological systems such as traction

control, lane departure systems, and particularly electronic stability control systems may

have potential to assist drivers in maintaining control of 15-passenger vans. The Safety

Board therefore recommends that NHTSA, in conjunction with the manufacturers of 15-

passenger vans, evaluate, and test as appropriate, the potential of technological systems,

particularly electronic stability control systems, to assist drivers in maintaining control of

15-passenger vans.
16 Safety Report

Findings

1. Although 15-passenger vans are involved in a proportionate number of accidents

compared to their percentage in the fleet, they are involved in a higher rate of singlevehicle accidents involving a rollover than are other passenger vehicles.

2. Statistical analyses have shown that increased occupancy level and vehicle speed

(measured by either travel speed or posted speed limit) consistently predict the

increased likelihood of 15-passenger van rollover.

3. Although the National Highway Traffic Safety Administration has initiated

rulemaking activities concerning vehicle rollovers, established a vehicle rollover

resistance rating system, and is currently examining dynamic testing procedures,

these programs do not extend to 15-passenger vans.

4. Given the rollover propensity of 15-passenger vans, technological systems such as

traction control, lane departure systems, and particularly electronic stability control

systems may have potential to assist drivers in maintaining control of these vehicles.
17 Safety Report

Recommendations

As a result of this safety report, the National Transportation Safety Board made the

following safety recommendations:

To the National Highway Traffic Safety Administration

Include 15-passenger vans in the National Highway Traffic Safety

Administration dynamic testing program. The dynamic testing should test

the performance of 15-passenger vans under various load conditions. (H02-26)

Extend the National Car Assessment Program (NCAP) rollover resistance

program to 15-passenger vans, especially for various load conditions, and

use the dynamic testing results of 15-passenger vans, as described in Safety

Recommendation H-02-26, to supplement the static measures of stability in

the NCAP rollover resistance program. (H-02-27)

Evaluate, in conjunction with the manufacturers of 15-passenger vans, and

test as appropriate, the potential of technological systems, particularly

electronic stability control systems, to assist drivers in maintaining control

of 15-passenger vans. (H-02-28)

To the manufacturers of 15-passenger vans

Evaluate, in conjunction with the National Highway Traffic Safety

Administration, and test as appropriate, the potential of technological

systems, particularly electronic stability control systems, to assist drivers in

maintaining control of 15-passenger vans. (H-02-29)

By the National Transportation Safety Board

Carol J. Carmody John A. Hammerschmidt

Acting Chairman Member

John Goglia

Member

George W. Black, Jr.

Member

Adopted October 15, 2002

19 Safety Report

Appendix A 

Previously Issued Safety Recommendations 

Pertaining to Large Vans 

Safety Recommendation No.: H-79-14 

Date Issued: April 4, 1979 

Recommendation:

In its on-going and planned investigation of van accidents: (1) study the failures of

custom highback bucket seats and anchorage systems to determine if they pose a

significant injury of safety problem; (2) study the failures of custom steering wheels

which do not meet FMVSS 203 to determine whether they pose a significant injury or

safety problem; (3) study the extent to which doors jam in collisions to determine if

corrective action is needed to prevent ejection and to enhance escape; (4) determine if

FMVSS 203 and 204 (steering wheel and steering column) should be extended to all

classes of vans or if new requirements are needed for vans; (5) determine the

feasibility of extending FMVSS 212 to all classes of vans. 

Recipient(s): Status:

National Highway Traffic 

Safety Administration ClosedóAcceptable Action 

Safety Recommendation No.: H-79-15 

Date Issued: April 4, 1979 

Recommendation:

Intensify its study to explore the feasibility of extending the passive restraint

requirements of FMVSS-208 to all classes of vans. 

Recipient(s): Status:

National Highway Traffic 

Safety Administration ClosedóAcceptable Action 

Safety Recommendation No.: H-79-17 

Date Issued: April 4, 1979 

Recommendation:

Include in its exploratory rulemaking and research activity control of crash 

aggressiveness of vans in relation to other vehicles. 
20 Safety Report

Recipient(s): Status:

National Highway Traffic 

Safety Administration ClosedóAcceptable Action 

Safety Recommendation No.: H-82-38 

Date Issued: October 6, 1982 

Recommendation:

Examine the crash performance of vans in rollovers and all accident types, through its 

crash testing and accident investigation programs, to determine if there is any

tendency for doors and other escape areas to unnecessarily jam or be blocked in lowspeed crashes. If necessary, establish additional crash performance standards for van

escape areas, especially those used for public transportation. 

Recipient(s): Status:

National Highway Traffic 

Safety Adminstration ClosedóAcceptable Action 

Safety Recommendation No.: H-89-3 

Date Issued: May 25, 1989 

Recommendation:

Purchase only school bus type vehicles which meet the Federal Motor Vehicle Safety

Standards set for school buses in April 1977. 

Recipient(s): Status:

Church Associations and other 

special activity groups ClosedóNo Longer Applicable 

Safety Recommendation No.: H-83-39 

Date Issued: September, 28, 1983 

Recommendation:

Review State laws and regulations, and take any necessary legislative action, to

ensure that passengers in small (more than 10 passengers and less than 10,000

GVWR) school buses and school vans are required to use available restraint systems 

whenever the vehicle is in motion; ensure that all users of such vehicles are aware of

and comply with these provisions. 

Recipient(s): Status:

States and the 

District of Columbia ClosedóAcceptable Action 

Appendix A
21 Safety Report

Safety Recommendation No.: H-89-40 

Date Issued: September 28, 1983 

Recommendation:

Review State laws and regulations, and take any necessary legislative action, to

ensure that vehicles designed to carry more than 10 passengers and weighing less

than 10,000 pounds GVWR, used to transport children to and from school, schoolrelated events, camp, daycare center, or similar purposes meet all Federal Motor

Vehicle Safety Standards applicable to small school buses. 

Recipient(s): Status:

States and the 

District of Columbia Various 

Safety Recommendation No.: H-89-47 

Date Issued: March 19, 1990 

Recommendation:

Conduct research, including computer simulation and sled crash tests using hybrid III

dummies if needed, to determine the relationship between restraining barrier design 

and injuries to unrestrained and lapbelted passengers of different sizes on small

school buses (gross vehicle weight rating of 10,000 pounds or less). Research should

focus on the height, width, padding, location, and anchorage strength of the barrier, 

and the spacing between the barrier and front seats. Amend Federal Motor Vehicle

Safety Standard 222 ìSchool Bus Passenger Seating and Crash Protectionî as needed.

Recipient(s): Status:

National Highway Traffic 

Safety Adminstration ClosedóAcceptable Alternate Action 

Safety Recommendation No.: H-89-49 

Date Issued: March 19, 1990 

Recommendation:

Collect and evaluate accident data on the crash performance of the roof and

emergency exits on small school buses (gross vehicle weight rating of 10,000 pounds 

or less) in rollovers. Data should not be limited to van based buses. Based on analysis,

ascertain whether it is appropriate to amend Federal Motor Vehicle Safety Standard

220, ìSchool Bus Rollover Protection,î to make roof performance tests for small 

school buses (gross vehicle weight of 10,000 pounds or less) to be identical in all

aspects to those now required of large school buses (gross vehicle weight rating of

more than 10,000 pounds). If such tests are not appropriate, modify the test for small 

school buses to stress the roof more than the present force application plate test does. 

Appendix A
22 Safety Report

Recipient(s): Status:

National Highway Traffic 

Safety Administration ClosedóAcceptable Action 

Safety Recommendation No.: H-89-50 

Date Issued: March 19, 1990 

Recommendation:

Collect and evaluate accident data involving small school buses to ascertain whether

school buses with a gross vehicle weight rating of 10,000 pounds or less should be

required to meet joint strength requirements Federal Motor Vehicle Safety Standard

221, ìSchool Bus Body Joint Strength.î 

Recipient(s): Status:

National Highway Traffic 

Safety Administration ClosedóAcceptable Action 

Safety Recommendation No.: H-89-51 

Date Issued: March 19, 1990 

Recommendation:

Specify in new rulemaking or in an amendment to Federal Motor Vehicle Safety

Standard 206, ìDoor Locks and Door Retention Components,î a requirement for a

positive latch locking mechanism on the passenger loading doors of small school

buses (gross vehicle weight rating of 10,000 pounds or less) to eliminate the

possibility of inadvertent door opening during a frontal crash or roll over. Work with

school bus and school van manufacturers to develop the performance standards. 

Recipient(s): Status:

National Highway Traffic 

Safety Administratiin ClosedóAcceptable Alternate Action 

Safety Recommendation No.: H-89-53 

Date Issued: March 19, 1990 

Recommendation:

Work with the National Highway Traffic Safety Administration to develop

performance standards for a locking mechanism for the boarding doors of school

buses with a gross vehicle weight rating of 10,000 pounds or less to eliminate the

possibility of inadvertent door opening during frontal or rollover crash. 

Appendix A
23 Safety Report

Recipient(s): Status:

Members of the School Bus 

Manufacturers Institutute and 

Manufacturers of Van Conversion 

School Buses Various 

Safety Recommendation No.: H-89-54 

Date Issued: March 19, 1990 

Recommendation:

Provide retrofit kits for small school buses (gross vehicle weight rating of 10,000 

pounds or less) currently without positive latch door control locking mechanisms. 

Recipient(s): Status:

Members of the School Bus 

Manufacturers Institute and 

Manufacturers of Van Conversion 

School Buses ClosedóAcceptable Action 

Safety Recommendation No.: H-91-13 

Date Issued: April 10, 1991 

Recommendation:

Enact legislation that requires occupants of all passenger automobiles, vans, and light

trucks to use lap/ shoulder belt systems at seating positions equipped with such belt 

systems. 

Recipient(s): Status:

The 12 States Without 

Mandatory Seatbelt Use Laws ClosedóAcceptable Action 

Safety Recommendation No.: H-99-20 

Date Issued: July 6, 1999 

Recommendation:

Require that head start children be transported in vehicles built to Federal school bus 

structural standards or the equivalent. 

Recipient(s): Status:

U.S. Department of Health 

and Human Services ClosedóAcceptable Action 

Appendix A
24 Safety Report

Safety Recommendation No.: H-99-21 

Date Issued: July 6, 1999 

Recommendation:

Incorporate and mandate the use of the guidelines from the National Highway Traffic

Safety Administrationís guideline for the safe transportation of pre-school age 

children in school buses into the rules for the transportation of Head Start children. 

Recipient(s): Status:

U.S. Department of Health 

and Human Services ClosedóAcceptable Action 

Safety Recommendation No.: H-99-22 

Date Issued: July 6, 1999 

Recommendation:

Require that all vehicles carrying more than 10 passengers (buses) and transporting

children to and from school and school-related activities, including, but not limited to, 

Head Start programs and day care centers, meet the school bus structural standards or

the equivalent as set forth in 49 Code of Federal Regulations Part 571. Enact 

regulatory measures to enforce compliance with the revised statutes. 

Recipient(s): Status:

States, U.S. territories, and 

the District of Columbia Various 

Safety Recommendation No.: H-99-23 

Date Issued: July 6, 1999 

Recommendation:

Review your State and local laws and, if applicable, revise to eliminate any

exclusions or exemptions pertaining to the use of age-appropriate restraints in all seat 

belt equipped vehicles carrying more than 10 passengers (buses) and transporting 

school children. 

Recipient(s): Status:

States, U.S. territories, and 

the District of Columbia Various 

Appendix A
25 Safety Report

Safety Recommendation No.: H-99-24 

Date Issued: July 6, 1999 

Recommendation:

Adopt the National Highway Traffic Safety Administrationís guideline for the safe

transportation of pre-school age children in school buses, distribute the guideline to

all school bus operators transporting preschool-age children to and from school or 

school-related activities, and encourage those operators to implement the guideline. 

Recipient(s): Status:

States, U.S. territories, and 

the District of Columbia Various 

Safety Recommendation No.: H-99-25 

Date Issued: July 6, 1999 

Recommendation:

Inform their members about the circumstances of the accidents discussed in this

special investigation report and urge that they use school buses or buses having

equivalent occupant protection to school buses to transport children. 

Recipient(s): Status:

Church Associations Various 

Safety Recommendation No.: H-99-26 

Date Issued: July 6, 1999 

Recommendation:

Inform your members of the circumstances of the East Dublin, Georgia, accident and

of the added safety benefits of transporting children by school bus, and encourage

them to use buses built to Federal school bus structural standards or equivalent to

transport children. 

Recipient(s): Status:

The Community Transportation 

Association of America OpenóAcceptable Response 

Appendix A
26 Safety Report

Appendix B

Vehicle Specifications,

Model Year 2002

Table Bñ1. Specifications of 15-passenger vans, model year 2002. 

Vehicle specifications 

Ford 

Econoline 

Wagon E-350 

XL Extended

Ford 

Econoline 

Wagon E-350 

XLT Extended

Dodge Ram 

Wagon 3500 

Maxi 

Chevrolet 

Express 

G3500 

Extended 

GMC Savana

G3500 

Extended 

Exterior dimensions: 

Curb weight, 

 automatic (lb.) 

6,425 6,500 5,570 5,957 5,962 

Wheelbase (in.) 138 138 127.6 155 155 

Length (in.) 231.9 231.9 231.2 238.8 238.8 

Width (in.) 79.3 79.3 79.8 79.4 79.4 

Height (in.) 84.1 84.1 79.9 82.8 82.8 

Ground clearance (in.) 7 7 8.4 8.5 8.5 

Interior dimensions: 

Standard seating 15 15 15 12 12 

Optional seating 12 12 12 15 15 

Payload and towing: 

Maximum towing (lb.) 10,000 10,000 7,750 10,000 10,000 

Maximum payload (lb.) 3,194 3,151 3,560 3,543 3,538 

Maximum GVWR 9,400 9,400 9,200 9,500 9,500 

Steering and suspension: 

Turning diameter (left) 48 48 52.4 53.5 53.5 

Turning diameter (right) 48 48 52.4 53.5 53.5 

Safety features: 

Antilock brakes Std Std Std Std Std 

Four-wheel antilock 

 brakes 

Std Std Opt Std Std 

Depowered air bag Std Std Std Std Std 

Passenger air bag Std Std Std Std Std 

Head air bag NA NA NA NA NA 

Side air bag NA NA NA NA NA 

Child safety seats NA NA NA NA NA 

Traction control NA NA NA NA NA 

Stability control NA NA NA NA NA 

Rollover test results Not tested Not tested Not tested Not tested Not tested 

GVWR = gross vehicle weight rating; NA = not available; Opt = optional equipment; Std = standard equipment. 

Source: <http://carpoint.msn.com> and <http://www.edmunds.com> accessed April 29,2002. 
27 Safety Report

Appendix C

Predicting 15-passenger Van Rollover Accidents

Method

Data for years 1991ñ2000 from the Fatality Analysis Reporting System (FARS)

were used for the Safety Boardís analyses to predict 15-passenger van rollover accidents.

The FARS, maintained by the National Highway Traffic Safety Administration (NHTSA),

is a census of all fatal crashes involving a motor vehicle traveling on a trafficway

customarily open to the public and results in the death of a person (occupant of a vehicle

or a nonmotorist) within 30 days of the crash.

Accidents selected for analysis involved single-vehicle, fatal 15-passenger van

accidents (n=499). Single-vehicle accidents were examined to eliminate accidents

involving rollovers caused by collision of vehicles. Fifteen-passenger van body types were

identified through the vehicle identification number (VIN).1

The dependent variable, rollover involvement, is defined in FARS as a three-level

variable: no rollover (n=240), rollover as a first event (n=168), and rollover as a

subsequent event (n=91). The dependent variable was recoded into a dichotomous

variable indicating that the vehicle was not involved in a rollover (code=0) or was

involved in a rollover (code=1) as a primary or subsequent event. Independent variables in

the analysis included occupancy level, travel speed, driver age, driver drinking, roadway

profile, pavement type, roadway surface condition, roadway function, driver sex, driving

maneuver, and roadway alignment.2

 Vehicle information (wheelbase and vehicle length,

width, and height) was also used in the analysis.3

For each variable, missing data values were recoded to system missing values.4

Although the number of occupants ranged from 1 to 26, values greater than 15 were

considered missing data (n=21). The Safety Board considered these vans to be loaded with

occupants beyond the number for which the vehicles were intended. Because it is

inappropriate to model nominal scaled variables as interval scaled variables, the multilevel

categorical variables were recoded into dichotomous variables.5

 Vehicle length, width, and

height were treated as continuous variables, although it is noted that the variability was

1 The VIN codes for 15-passenger vans were provided by NHTSA. The VINs do not identify whether

a van is used as a cargo van or a passenger van.

2 These variables were used directly from FARS.

3 Information on wheelbase and vehicle length, width, and height, was obtained from the World Wide

Web sites <http://carpoint.msn.com> and <http://www.edmunds.com>.

4 FARS assigns a value to missing data (that is, ë9í or ë99í), which skews the continuous data and

creates a new category with categorical data.
28 Safety Report

limited. Wheelbase was recoded into a dichotomous variable (wheelbase = 127 inches;

wheelbase > 127 inches) to assess whether a shorter wheelbase was indicative of rollover.

Logistic regression was used to predict the probability of vehicle rollover. Logistic

regression allows for the examination of the effect of several independent variables

simultaneously on the dichotomous variable of rollover involvement. Initially, stepwise

logistic procedures were used to identify significant predictors of 15-passenger van

rollover.6

 Based on the stepwise regression results, hierarchical logistic regressions were

performed. 

Four basic models, described below, were developed and tested using both

stepwise and hierarchical logistic regression.

Model 1. The predictor variables were travel speed, occupancy level, wheelbase,

vehicle length, vehicle width, vehicle height, driver drinking, driver sex, driver age,

roadway function, driving maneuver, relation to the roadway, roadway profile, roadway

surface condition, and roadway alignment.7

Model 2. The predictor variables were the same as in model 1 except posted travel

speed was substituted for travel speed. The substitution was made because the validity of

travel speed is often questioned. Use of posted speed limit, however, also raises questions,

including whether or not the vehicle was traveling at, above, or below the speed limit.

Models 3 and 4. The vehicle variables (wheelbase and vehicle length, weight, and

height) were excluded because these variables (a) were obtained from sources other than

FARS, (b) were not shown to be significantly related to rollover based on the bivariate

correlations, (c) had limited variance, and (d) limited the sample size. Model 3 used the

variable travel speed; model 4 used the variable posted speed limit.

Results

Tables Cñ1 and Cñ2 provide distribution and descriptive information. The

correlation matrix (table Cñ3) shows several significant relationships with rollover.

Specifically, rollovers are associated with accidents on graded or hillcrest roadways, rural

roadways, female drivers, straight driving maneuvers, curved roadways, increased

occupancy level, increased travel speed, and off-roadway occurrences. Additionally,

several of the variables are correlated with one another. 

5 An alternative procedure would be to create design variables (dummy variables); however, because

some values of the multilevel variables contained few or no cases, it was decided to dichotomize the

variables.

6 The stepwise procedure either includes or excludes variables in the model based on a fixed statistical

algorithm.

7 Pavement type was excluded because of small cell size.

Appendix C
29 Safety Report

Table Cñ1. Distribution of the categorical independent variables, by rollover 

involvement, in the National Transportation Safety Board analysis of FARS 

data on single-vehicle, fatal 15-passenger van rollover accidents for years 

1991ñ2000 (n=499). 

Variable 

Number of nonrollover 

accidents (code=0) 

Number of rollover 

accidents (code=1) 

Roadway profile (n=494): 

Level (0) 203 175 

Grade, hillcrest (1) 35 81 

Pavement type (n=486): 

Concrete, blacktop (0) 227 250 

Slag, gravel (1) 2 7 

Roadway surface condition (n=498): 

Wet, snow, ice (0) 49 70 

Dry (1) 190 189 

Roadway function (n=498): 

Rural (0) 83 207 

Urban (1) 157 51 

Driver sex (n=497): 

Female (0) 38 66 

Male (1) 202 191 

Driving maneuver (n=417): 

Curve (0) 181 180 

Straight (1) 15 41 

Roadway alignment (n=499): 

Curve (0) 26 68 

Straight (1) 214 191 

Driver drinking (n=499): 

No (0) 224 236 

Yes (1) 16 23 

Wheelbase 127" (n=499): 

No (0) 126 152 

Yes (1) 114 107 

Relation to roadway (n=493): 

Off roadway (0) 71 193 

On roadway (1) 167 62 

FARS = Fatality Analysis Reporting System, maintained by the National Highway Traffic Safety 

Administration. 

Appendix C
30 Safety Report

The initial stepwise logistic model based on 15 predictor variables, including

travel speed, was based on 107 cases.8

 The stepwise regression concluded in three steps:

travel speed was entered in step one, occupancy level was included in step two, and

roadway function was included in the third step. Thus, the final stepwise regression model

included three significant predictor variables: travel speed, occupancy level, and roadway

function, as shown in table Cñ4.9

 The probability of rollover involvement is defined as

1/1-e-z, where z = -4.643-1.410 (roadway function) + 0.081(travel speed) + 0.184

(occupancy level).

8 Cases with missing data are deleted on a listwise basis.

9 (a) The Wald statistic was used to determine significance of the coefficients. The Wald statistic is

based on a chi-square distribution; it is the square of the ratio of the coefficient to its standard error. (b) The

odds ratio indicates the change in odds for a case when the value of that variable increases by 1; thus, when

occupancy increases by one, the odds of a rollover are increased by a factor of 1.202.

Table Cñ2. Descriptive statistics of the continuous independent variables, by 

rollover involvement, in the National Transportation Safety Board analysis of 

FARS data on single-vehicle, fatal 15-passenger van rollover accidents for 

years 1991ñ2000 (n=499). 

Variable n Minimum Maximum Mean 

Standard 

deviation 

Occupancy level: 471 1 15 5.61 4.46 

Nonrollover accidents 234 1 15 3.35 3.07 

Rollover accidents 237 1 15 7.85 4.50 

 

Travel speed: 242 3 95 54.08 18.79 

Nonrollover accidents 94 20 95 40.36 19.36 

Rollover accidents 148 3 90 62.79 12.03 

Vehicle length: 278 222.9 238.8 229.24 3.99 

Nonrollover accidents 120 222.9 238.8 229.71 3.92 

Rollover accidents 158 222.9 238.8 229.51 3.95 

Vehicle width: 291 78.8 79.9 79.52 0.30 

Nonrollover accidents 125 78.8 79.9 79.55 0.30 

Rollover accidents 166 78.8 79.9 79.50 0.30 

Vehicle height: 264 79.9 84.1 81.71 1.79 

Nonrollover accidents 114 79.9 84.1 81.54 1.70 

Rollover accidents 150 79.9 84.1 81.83 1.86 

Driver age: 498 15 86 38.96 14.13 

Nonrollover accidents 240 17 77 40.06 14.23 

Rollover accidents 258 15 86 37.93 13.99 

FARS = Fatality Analysis Reporting System, maintained by the National Highway Traffic Safety 

Administration. 

Appendix C
	
	
	

     	          	  	








 

!

 

!"

 

!

"

"

 

!"



"



! 









""

!

"

! 

"



""



 

#$

"









!



!







!



!



!



 



!"

" 



"

!"

"



""





%&''
		

 













!"







!"







 





""

!

"



"

!

"

!



&
'	



   







!



 



!"







! 



! 

"

!

"

 

"

!

" 





(	$%


     

 

!







!"



!



!

"



"

! 

"



" 

 



(	$	
)
&$


      !

 





!





 



"



" 

! 

" 



"" 





	)




       !

 





!



!

""

!

"



"

!

"





(	$	
*	
)


        

 

!





""

!

"



"

!

"

!



+,-%".


        







!

""

!

"

"

"

!

"





/''&
'$


           

 "" 



"

!

"



"

!



0$%


           

""



 

!

 



"

!

"

1,	'
),
"

              !"

" "



"

!

"

1,	'	,
 

               !

" "

!

"

1,	',	),


                !

" " 

(	$)


              

 



	



              

$


              
32 Safety Report

Overall, 83 percent of the 15-passenger van accidents were correctly classified. As

shown in the classification table (table Cñ5), 64 van accidents were correctly classified as

involving a rollover, and 25 vans were correctly classified as not being involved in a

rollover accident. A total of 18 accidents were incorrectly classified: 12 accidents that

were not involved in a rollover but were predicted to involve a rollover; and 6 accidents

that were involved in rollovers but rollover was not predicted.

The Nagelkerke R2

 for the last step of model 1 is 0.58.10 The model -2LL is 57.907,

p < 0.05, indicating that the logistic regression with the three coefficients (travel speed,

occupancy level, and roadway function) is significantly different from the model

containing only the constant (X2

 = 137.988).

10 The -2 times the log of the likelihood (-2LL) values and the Nagelkerke R2

 are other measures of fit.

Similar to the R2 in linear regression, the Nagelkerke R2 attempts to quantify the proportion of explained variation in the

logistic regression model.

Table Cñ4. Logistic regression model 1 (n=107). 

Variables in 

the equation 

Estimated 

coefficient 

Standard 

error 

Wald 

statistic Significance 

Odds 

ratio 

Step 1: 

Travel speed 0.098 0.020 24.373 0.000 1.103 

Constant -4.780 1.124 18.070 0.000 0.008 

Step 2: 

Travel speed 0.082 0.020 16.956 0.000 1.085 

Occupancy level 0.211 0.072 8.652 0.003 1.234 

Constant -5.140 1.143 20.240 0.000 0.006 

Step 3: 

Travel speed 0.081 0.021 14.986 0.000 1.085 

Occupancy level 0.184 0.072 6.554 0.010 1.202 

Roadway function -1.410 0.687 4.209 0.040 0.244 

Constant -4.643 1.186 15.321 0.000 0.010 

Table Cñ5. Classification table for model 1. 

 Event predicted 

Event 

observed No rollover Rollover 

Percent correctly 

classified 

No rollover (n=37) 25 12 67.6 

Rollover (n=70) 6 64 91.4 

 Overall percent 83.2 

Appendix C
33 Safety Report

The second logistic regression (model 2) was conducted substituting posted speed

limit for travel speed, and the n increased to 203 cases. The correlation between posted

speed limit and travel speed is 0.69, p < 0.05, and similar results were expected between

the two analyses. As such, posted speed limit and occupancy level were significantly

related to rollover. Relation to the roadway and roadway profile were also significant

predictors of vehicle rollover (table Cñ6). Overall, 85.2 percent of the cases were correctly

classified. About 60 percent of the variation in 15-passenger van rollover is explained by

the logistic regression model (Nagelkerke R2

=0.596). The -2LL is 119.531, p < 0.05,

showing that the logistic equation is significantly different from the model containing only

the constant.

Models 3 and 4 were conducted using a smaller set of predictor variables,

excluding the vehicle variables. In model 3, travel speed was used (n=188); in model 4,

posted speed limit was substituted (n=364). Four significant predictors were in model 3:

travel speed, occupancy level, roadway alignment, and relation to the roadway (table Cñ

7). About 80 percent of the cases were correctly classified. The Nagelkerke R2

 is 0.544.

Table Cñ6. Logistic regression results for model 2 (n=203). 

Variables in 

the equation 

Estimated 

coefficient 

Standard 

error 

Wald 

statistic Significance 

Odds 

ratio 

Step 1: 

Speed limit 0.099 0.015 45.394 0.000 1.104 

Constant -5.211 0.840 38.460 0.000 0.005 

Step 2: 

Speed limit 0.069 0.015 20.042 0.000 1.072 

Occupancy level 0.235 0.051 20.913 0.000 1.265 

Constant -4.870 0.852 32.641 0.000 0.008 

Step 3: 

Relation to roadway -1.711 0.412 17.244 0.000 0.181 

Speed limit 0.053 0.017 9.917 0.002 1.054 

Occupancy level 0.247 0.054 20.998 0.000 1.281 

Constant -3.329 0.951 12.265 0.000 0.036 

Step 4: 

Speed limit 0.057 0.017 10.596 0.001 1.058 

Occupancy level 0.244 0.055 19.37 0.000 1.276 

Relation to roadway -1.495 0.431 12.040 0.001 0.224 

Roadway profile 1.078 0.477 5.117 0.240 2.940 

Constant -3.902 1.028 14.406 0.000 0.020 

Appendix C
34 Safety Report

Model 4 resulted in a five-variable model (table Cñ8) with about 82 percent of the

cases correctly classified. The five significant variables are posted speed limit, relation to

the road, occupancy level, roadway alignment, and roadway profile. The Nagelkerke R2

 is

0.596.

Table Cñ7. Logistic regression results for model 3 (n=188). 

Variables in 

the equation 

Estimated

coefficient 

Standard 

error 

Wald 

statistic Significance 

Odds 

ratio 

Step 1: 

Travel speed 0.090 0.014 40.301 0.000 1.094 

Constant -4.524 0.797 32.205 0.000 0.011 

Step 2: 

Occupancy level 0.189 0.047 16.198 0.000 1.208 

Travel speed 0.077 0.014 29.382 0.000 1.080 

Constant -4.941 0.825 35.845 0.000 0.007 

Step 3: 

Occupany level 0.197 0.048 16.582 0.000 1.218 

Roadway alignment -1.313 0.510 6.634 0.010 0.269 

Travel speed 0.080 0.015 29.300 0.000 1.083 

Constant -4.134 0.889 21.614 0.000 0.016 

Step 4: 

Relation to roadway -0.861 0.424 4.123 0.042 0.423 

Occupancy level 0.200 0.049 16.481 0.000 1.222 

Roadway alignment -1.232 0.508 5.887 0.015 0.292 

Travel speed 0.072 0.015 21.474 0.000 1.074 

Constant -3.426 0.948 13.072 0.000 0.033 

Appendix C
35 Safety Report

In each of the stepwise logistic regression models, the speed-related variable

(travel speed or posted speed limit) and occupancy level were consistently identified as

predictors of 15-passenger van rollover. Depending on the model, other variables were

also shown to be significant predictors of van rollovers. Because the speed-related

variables and occupancy level were consistently predictive of rollover, a set of hierarchical

logistical regressions were conducted where the speed-related variables and occupancy

level were entered on the first block and the remaining variables were entered on the

second block.

Table Cñ9 shows the results for the hierarchical logistic regressions. Again, the

difference between the first two models is the speed-related variable. Model 1, using travel

speed and occupancy level at block 1, resulted in a Nagelkerke R2

 of 0.543, and 82.2

percent of the cases were correctly classified. Travel speed and occupancy level

significantly predicted vehicle rollover. The addition of the remaining predictor variables

in the second block did not significantly improve prediction, X2

(13, n=107) =16.483, p >

0.05. Model 2, which used speed limit as a proxy for travel speed, had an increased sample

size of 207. The Nagelkerke R2

 is 0.452, and 79.7 percent of the cases are correctly

classified. The entry of the additional variables significantly improves prediction, X2

(13,

n=207)=41.447, p < 0.05. The Nagelkerke R2

 increases to 0.613, and 85.5 percent of the

Table Cñ8. Logistic regression results for model 4 (n=364). 

Variables in 

the equation 

Estimated 

coefficient 

Standard 

error 

Wald 

statistic Significance 

Odds 

ratio 

Step 1: 

Speed limit 0.112 0.012 87.718 0.000 1.118 

Constant -5.920 0.668 78.534 0.000 0.003 

Step 2: 

Relation to roadway -1.485 0.279 28.398 0.000 0.226 

Speed limit 0.102 0.013 64.904 0.000 1.108 

Constant -4.781 0.723 43.757 0.000 0.008 

Step 3: 

Relation to roadway -1.587 0.299 28.247 0.000 0.205 

Speed limit 0.078 0.013 34.822 0.000 1.081 

Occupancy level 0.177 0.037 22.569 0.000 1.193 

Constant -4.406 0.726 36.860 0.000 0.012 

Step 4: 

Relation to roadway -1.453 0.308 22.313 0.000 0.234 

Speed limit 0.085 0.014 36.785 0.000 1.089 

Occupancy level 0.176 0.038 21.745 0.000 1.193 

Roadway alignment -1.074 0.367 8.551 0.003 0.342 

Constant -4.024 0.755 28.409 0.000 0.018 

Step 5: 

Relation to roadway -1.365 0.314 18.922 0.000 0.255 

Roadway profile 0.840 0.352 5.689 0.017 2.317 

Speed limit 0.088 0.014 37.560 0.000 1.092 

Occupancy level 0.175 0.038 21.280 0.000 1.191 

Roadway alignment -0.929 0.378 6.047 0.014 0.395 

Constant -4.538 0.807 31.958 0.000 0.011 

Appendix C
36 Safety Report

cases are correctly classified. Occupancy level remains a significant predictor of van

rollover; relation to the roadway (off roadway) is added as a significant predictor of

rollover. Posted speed limit is not a significant predictor.


2	',	')	%	')%%	
%&%





 !"


#
$

%&&
%


'



(



% '
)
&
%%

*




+,-

345)**"6 7'
'%%'''%%		6""

3"45)**"6 7'
'%%'''%%		6

	
		

	 

0$%      

/''&
'$  " "  " 

+,-% "    " "

1,	'
), ! "  "  

1,	'	, ! "   

1,	',	), "   "" "  "

(	$	
*	
) ! "  "  

&
'	
 !"   "   ""

(	$% !  "  ""

(	$	
)
&$ !     

	
 !"      

	 !""  " "  

%&''
		
      

(	$)  "  "    

	)

 !   " "


%
 !" ""    

+,-

345)**"6"7'
'%%'''%%		6

3"45)**"6 7'
'%%'''%%		6

	
		

	 

3		       

/''&
'$ "  ""   

+,-% !      

8
), !      

+	, !"    "  

2	), !" "  " " "

(	$	
*	
)     " " 

&
'	
 !   "  

(	$%      

(	$	
)
&$ "  "   "

	
 ! "   " 

	       ""

%&''
		
 "   "  

(	$)    " " 

	)

 !      


%
       9:"

Appendix C
37 Safety Report

As in the stepwise regressions, models 3 and 4 were conducted using a smaller set

of predictor variables, which excluded the vehicle-related variables, and used either travel

speed (model 3) or speed limit (model 4) to indicate speed of vehicle. The results of model

3 indicate that travel speed and occupancy level significantly predict vehicle rollover. The

addition of roadway alignment significantly improves prediction (X2

(9, n=188)=17.498, p

< 0.05). The Nagelkerke R2 increases to 0.569 (from 0.493), and the number of correctly

classified cases increases from 78.2 to 80.3. Block 1 of model 4, where posted speed limit

and occupancy level are entered, has a Nagelkerke R2

 of 0.371 and correctly classified

74.9 percent of the cases. Variables added in block 2 improve prediction (X2

(9,

n=379)=77.711, p < 0.05), the Nagelkerke R2

 increases to 0.549, and 82.1 percent of the

cases are correctly classified. Significant predictors include occupancy level, posted speed

limit, roadway function (rural accidents increase odds of being involved in a rollover),

relation to the roadway (off-road accidents increase odds of being involved in a rollover),

and roadway profile (accidents on a grade or hillcrest increase odds of being involved in a

rollover).

Table Cñ9. Hierarchical logistic regression results. (Continued)

Model 3 (n=188) 

Step 1: Nagelkerke R2 = 0.493; percent cases correctly classified = 78.2 

Step 2: Nagelkerke R2 = 0.569; percent cases correctly classified = 80.3 

Variables in the equation: 

Travel speed .080 .017 21.244 .000 1.083 

Occupancy level .184 .052 12.348 .000 1.202 

Driver drinking -.435 .695 .391 .532 .647 

Roadway function -.501 .499 1.006 .316 .606 

Driver sex -.611 .548 1.246 .264 .543 

Driving maneuver -1.415 1.062 1.777 .183 .243 

Relation to roadway -.674 .469 2.067 .151 .510 

Roadway profile .135 .457 .088 .767 1.145 

Roadway surface condition -.151 .503 .090 .764 .860 

Driver age .011 .015 .568 .451 1.011 

Roadway alignment -2.279 .964 5.588 .018 .102 

Constant -2.559 1.447 3.125 .077 .077 

Model 4 (n=379) 

Step 1: Nagelkerke R2 = 0.371; percent cases correctly classified 74.9 

Step 2: Nagelkerke R2 = 0.549; percent cases correctly classified 82.1 

Variables in the equation: 

Speed limit .028 .010 8.064 .005 1.028 

Occupancy level .205 .037 31.302 .000 1.227 

Driver drinking .294 .497 .350 .554 1.342 

Roadway function -1.135 .304 13.980 .000 .321 

Driver sex -.265 .352 .566 .452 .767 

Driving maneuver .110 .596 .034 .853 1.117 

Relation to roadway -1.402 .303 21.469 .000 .246 

Roadway profile .749 .337 4.946 .026 2.115 

Roadway surface condition -.142 .331 .183 .669 .868 

Driver age -.004 .009 .185 .667 .996 

Roadway alignment -.594 .500 1.410 .235 .552 

Constant -.915 .911 1.008 .315 .401 

Appendix C
38 Safety Report

Discussion

Four logistic regression models were developed and tested using stepwise logistic

regression and hierarchical logistic regression. The primary difference among the logistic

models was the selection of the speed-related variable, which was defined either as travel

speed (models 1 and 3) or posted speed limit (models 2 and 4), and the exclusion of the

vehicle-related variables (models 3 and 4).

The use of posted speed limit rather than travel speed does not make the results

more reliable. Although the sample size increases when using the posted speed limit, the

Nagelkerke R2

 and the number of correctly classified cases are generally lower than when

using travel speed. The validity of travel speed data is often questioned because the posted

speed limit is cited as the travel speed and self reports of travel speed are not reliable. The

posted speed limit does not indicate the actual speed of the vehicle, which may be at,

lower, or above the posted speed limit.

The vehicle variables (length, width, height, and wheelbase) were originally

included in the model because it was expected that they may have an effect (or a combined

effect) on 15-passenger van rollover. However, the vehicle variables were not significantly

predictive of rollover. It is noted that these variables had limited variance and small

sample size.

In each of the stepwise regression equations, the speed-related variables and the

occupancy level were significant predictors of 15-passenger van rollover. Additional

variables were also shown to be significant depending on the model (table Cñ10

summarizes the regression results). Because occupancy level and the speed-related

variables were shown to be related to rollover in all of the models, hierarchical logistic

regression procedures were used to determine how much additional variance the other

variables accounted for in rollover, after occupancy level and the speed-related variables

were entered into the equation. The results indicate that in models 2, 3, and 4, (a) the

addition of other variables significantly improves prediction, and (b) the percentage of

correctly classified cases increases; however, the improvements are not necessarily

substantial.

Appendix C
39 Safety Report

The Safety Board acknowledges potential confounding with the findings. The

analyses were conducted on fatal accidents, which frequently include higher speed

accidents. The Board also recognizes the limitations of the vehicle speed variables (travel

speed and posted speed limit). The VIN does not indicate whether the van is a cargo van or

a passenger van because the final configuration of the van is usually completed at the

vehicle dealer. However, in summary, the results indicate that increased vehicle speed and

occupancy level consistently predict the increased likelihood of 15-passenger van rollover

and that other factors may improve prediction of van rollover.

Table Cñ10. Summary of stepwise and hierarchical logistic results of significant 

predictors of 15-passenger van rollover. 

 Item Model 1 Model 2 Model 3 Model 4 

 

Variables 

entered into 

model 

Travel speed 

Occupancy level 

Wheelbase 

Vehicle length 

Vehicle width 

Vehicle height 

Roadway function 

Driving maneuver 

Relation to roadway 

Roadway profile 

Roadway surface 

 condition 

Roadway alignment 

Driver drinking 

Driver sex 

Driver age 

Posted speed limit 

Occupancy level 

Wheelbase 

Vehicle length 

Vehicle width 

Vehicle height 

Roadway function 

Driving maneuver 

Relation to roadway 

Roadway profile 

Roadway surface 

 condition 

Roadway alignment 

Driver drinking 

Driver sex 

Driver age 

Travel speed 

Occupancy level 

Roadway function 

Driving maneuver 

Relation to roadway 

Roadway profile 

Roadway surface 

 condition 

Roadway alignment 

Driver drinking 

Driver sex 

Driver age 

Posted speed limit 

Occupancy level 

Roadway function 

Driving maneuver 

Relation to 

 roadway 

Roadway profile 

Roadway surface 

 condition 

Roadway 

 alignment 

Driver drinking 

Driver sex 

Driver age 

 

Significant 

predictors in 

stepwise 

logistic 

regression 

Travel speed 

Occupancy level 

Roadway function 

Posted speed limit 

Occupancy level 

Relation to roadway 

Roadway profile 

Travel speed 

Occupancy level 

Relation to roadway 

Roadway alignment 

Posted speed limit 

Occupancy level 

Relation to 

 roadway 

Roadway profile 

Roadway 

 alignment 

 

Significant 

predictors in 

hierarchical 

logistic 

regression 

Travel speed 

Occupancy level 

Occupancy level 

Relation to roadway 

Travel speed 

Occupancy level 

Roadway alignment 

Posted speed limit 

Occupancy level 

Roadway function 

Relation to 

 roadway 

Roadway profile 

 

Appendix C
40 Safety Report

Appendix D

Vehicle Dynamics of 15-passenger Vans

The National Highway Traffic Safety Administration (NHTSA) conducted a

simulation study to show the effects of occupant loading on the handling of 15-passenger

vans.11 NHTSA included two steering maneuvers in its simulation study: one was

designed to measure the understeer/oversteer characteristics of the vehicle, and the other

was a steering reversal maneuver.12 To show the effects of occupant loading, simulation

runs were performed at a driver-only load condition and also when the van was fully

loaded to its gross vehicle weight (GVW).13 Tire and suspension properties for the

simulations were not measured but were based on existing parametric data to roughly

approximate those of a 15-passenger van. Measured values from a 2000 Ford E-350 were

used for center of gravity (CG) location and inertia properties. According to these

measurements for the Ford E-350, loading the van to its GVW increased the CG height of

the van about 4 inches and moved the CG rearward about 18 inches, placing 65 percent of

the weight on the rear tires of the van (table Dñ1). The increase in CG height and rearward

movement of the CG could result if any 15-passenger van were loaded to the GVW

because other 15-passenger vans have similar seating configurations to the Ford E-350.

11 W. Riley Garrott, Barbara Rhea, Rajesh Subramanian, and Gary J. Heydinger, The Rollover

Propensity of Fifteen-Passenger Vans, Research Note (Washington, DC: NHTSA, April 2001).

12 A simple test illustrates the concepts of understeer and oversteer. A vehicle is driven around a circle

at a constant speed, then the speed is slowly increased. If the vehicle tends to go off the outside of the circle

so that the driver must increase steering to maintain the circle, then the vehicle is considered to be an

understeer vehicle. If the vehicle tends to go off the inside of the circle so that the driver must reduce

steering to maintain the circle, then the vehicle is considered to be an oversteer vehicle. Understeer and

oversteer can affect the stability of a vehicle; however, just because a vehicle is an oversteer vehicle does not

mean that it is uncontrollable. A more detailed discussion of understeer and oversteer and their impact on

stability and control is contained in (a) William F. Milliken and Douglas L. Milliken, ìSimplified Steady

State Stability and Control,î Chapter 5, and ìSimplified Transient Stability and Control,î Chapter 6 in Race

Car Vehicle Dynamics (Warrendale, PA: Society of Automotive Engineers, 1995) 123-229 and 231-277; and

(b) Thomas D. Gillespie, ìRollover,î Chapter 9 in Fundamentals of Vehicle Dynamics (Warrendale, PA:

Society of Automotive Engineers, 1992) 309-333. The Society of Automotive Engineersí definitions of

understeer and oversteer are given in Milliken and Milliken (1995), p. 164.

13 The driver-only load condition was the weight of the van with a weight equivalent to a 50th

percentile male dummy in the driverís seat and no cargo. The fully loaded condition was the weight of the

van with weights equivalent to a 50th percentile male dummy in every seating position, plus ballast

(simulated luggage) in the rear cargo space.
41 Safety Report

In the first simulated maneuver, the understeer/oversteer characteristics of a lightly

loaded 15-passenger van and a fully loaded 15-passenger van were calculated using a

slowly increasing steer rate of 5o per second and a constant vehicle speed of 30 mph. The

simulation showed that the fully loaded 15-passenger van made a transition from understeer to oversteer as the lateral acceleration increased. This result is important because an

oversteer vehicle requires different driver inputs than does an understeer vehicle in order

to stabilize it in a steady-state cornering maneuver. NHTSAís report also noted the safety

concern:

The fact that a heavily laden vehicleís understeer characteristics are similar to its

lightly loaded condition at low lateral accelerations but different at higher lateral

accelerations is a topic of concern. This sort of transition is known to cause safety

problems, particularly for drivers who normally only drive smaller passenger

vehicles and who are unfamiliar with a loaded fifteen passenger vanís

responsiveness and limits.

In the second set of simulations, the vans were placed through a steering reversal

maneuver. In the maneuver, the vans were driven at a steady speed of 30 mph, and a

steering input of about 180o of right steer was followed abruptly by 180o of steer input to

the left. In the simulations, the fully loaded van rolled whereas the lightly loaded (driver

only) van did not. NHTSAís examination of the results found that rollover was preceded

by a high sideslip angle, indicating a reduction in the rear axleís cornering capability.

The NHTSA report concluded that the computer simulations illustrated the

adverse effects that a fully loaded passenger van can have on the vehicleís handling

properties (sudden transition from understeer to oversteer) and rollover propensity.

However, the simulation results should be viewed with caution because the suspension

and tire properties used by NHTSA were default values and not specific to any particular

Table Dñ1. Measured parameters of the 2000 Ford E-350 XLT 

Super Duty 15-passenger van. 

Load condition 

Center of gravity 

height (inches) 

Longitudinal distance 

from front axle 

to vehicle center 

of gravity 

Percent weight 

on rear axle 

Driver onlya 31.9 72.4 52.4 

Fully loaded to GVWb 35.9 90.3 65.3 

GVW = gross vehicle weight. 

a The weight of the van with a weight equivalent to a 50th percentile male dummy in the driverís seat and 

no cargo. 

b The weight of the van with weights equivalent to a 50th percentile male dummy in every seating position, 

plus ballast (simulated luggage) in the rear cargo space. 

Source: W. Riley Garrott, Barbara Rhea, Rajesh Subramanian, and Gary J. Heydinger, The Rollover 

Propensity of Fifteen-Passenger Vans, Research Note (Washington, DC: U.S. Department of 

Transportation, National Highway Traffic Safety Administration, April 2001). 

Appendix D
42 Safety Report

van. The default measures of the suspension and tire properties may not replicate true

measures; therefore, simulated test results may vary from actual results.

Analyses of data on occupancy level and rollover conducted by NHTSA and the

Safety Board show that 15-passenger vans with 10 or more occupants have much higher

rollover ratios than do those with fewer occupants.14 NHTSAís measurements of the Ford

E-350 indicate that adding passengers and cargo to a 15-passenger van causes the CG to

move rearward and upward. Increasing the CG height increases the rollover propensity,

measured using the static stability factor (SSF).15 In its simulation study, NHTSA

measured the inertial properties of a fully loaded 15-passenger van versus a lightly loaded

van. These measurements found that a decrease in stability under the fully loaded

condition correlated to an increase in the rollover risk of about 40 percent.16 Basic vehicle

dynamics also indicate that increasing the CG height can make the vehicle more prone to

rollover.17 Moving the CG rearward and upward and adding weight to the rear tires can

cause the vehicle to handle differently and make it more prone to sideslip.18 It could also

decrease lateral stability in extreme maneuvers, as the NHTSA steering reversal

simulations indicate. These changes in handling may make it more difficult for the driver

to control the vehicle in some emergency situations. The magnitude of the change in

handling and rollover potential depends on a number of factors including the amount of

change in the CG location, weight, suspension properties, tire properties, type of

maneuver, and driver response.

NHTSAís simulations illustrate how fully loading a 15-passenger van could

adversely affect handling in extreme maneuvers and increase the vanís rollover

propensity. Given the general vehicle dynamics, NHTSAís measurements of CG locations

and SSF of a lightly loaded and a fully loaded 15-passenger van, and NHTSAís simulation

results, the Safety Board is concerned that fully loading or nearly loading a 15-passenger

van causes the center of gravity to move rearward and upward, which increases the

vehicleís rollover propensity and could increase the potential for driver loss of control in

emergency maneuvers.

14 (a) Rollover ratio is the number of all single-vehicle rollover accidents divided by the number of all

single-vehicle accidents. (b) NHTSAís analysis, conducted on data for 1994ñ1997, is reported in Garrott and

others, 2001. The Safety Boardís analysis, conducted on FARS data for 1991ñ2000, is discussed in the

ìBackgroundî section and appendix C of this report (Evaluation of the Rollover Propensity of 15-passenger

Vans, NTSB/SR-02/03).

15 The SSF is generated by dividing a vehicleís track width (distance between the wheels from side to

side, denoted by t) by twice the vehicleís center of gravity height (SSF=t/2h).

16 This figure is based on NHTSAís Rollover Ratio versus Static Stability Factor regression trend line.

NHTSA uses this trend line to provide consumers information on the rollover risk of passenger cars, and of

light, multipurpose passenger vehicles and trucks. This trend line is based solely on SSFs measured for a

driver-only load condition because this is the most common load condition in which private consumer

vehicles are driven.

17 Gillespie, 1992.

18 Milliken and Milliken, 1995.