# Presentation: 320526

## Relation of Speed and Speed Limits to Crashes

**National Forum on Speeding**
**Washington, D.C., June 15, 2005**
**Susan Ferguson, Ph.D.**

## Overview

- Relation of speeds to crashes
- Relation of speed to crash severity
- Relation of speed limit changes to speed and crashes
- Characteristics of excessive speeders

## Relation of speed to crashes Solomon 1964, Cirillo 1968

- Solomon studied speeds of crash-involved vehicles and overall traffic speeds on rural highways
- Cirillo conducted a similar analysis for daytime crashes occurring on interstates; crash types limited to rear-end, angle, and same-direction side-swipes

**Notes:**
It is nearly impossible to obtain a reliable measure of pre-crash speeds( today EDR’s might help)
Speeds of crashed vehicles were obtained from police reports, driver's reports, or third party estimates - sources that are subject to error and unknown reliability
Speed data collection locations and study sections were different - problems associated with overstating risk of vehicles traveling at slower speeds

Solomon’s study 
was restricted to US rural highways and did not cover freeways
Used the term accident involvement rate instead of vehicle or driver involvement rate
Covered all types of accidents 
Turning vehicles were estimated but not turning-related crashes.
Self-reported speeds

Cirillo’s study 
Covers only freeways
states data only refers to rear-end, angle and same-direction side-swipe accidents, as head-on, single-vehicle and pedestrian accidents were ignored

Speeds of crash-involved drivers in both studies were self reported or police reported, thus subject to error, bias, and unknown reliability

## Findings

- Both studies found a U-shaped relationship between vehicle speed and crash incidence
- Crash rates were lowest for drivers traveling near the mean speed, and increased with deviations above and below the mean
- Low-speed drivers were more likely to be involved in crashes than relatively high speed drivers

## U-shaped curve- Solomon/Cirillo

- Cirillo

- Solomon

## Explaining Solomon’s and Cirillo’s findings

- Solomon:
- many crashes occurred at intersections and involved stopped or slowing vehicles
- data were collected on 1950s era roads, which lacked turn lanes and passing lanes
- Cirillo:
- many crashes, especially rear-end collisions, may have been related to traffic congestion, which by definition involves stopped or slowing vehicles

**Notes:**
So question is whether slow moving vehicles are dangerous or situations requiring slowing are dangerous.

## Explaining the U-Shape

- High Crash Rates at Low Speeds(<40mph)

- Crash Rates constant (40-68mph)

- High Crash Rates (>70mph)

**Combining all we have this U shaped curve**

**Explaining the U-Shape**

**Injuries increase with speed**

**Risk of Injury Crashes and Speed, Cowley 1987**

**Notes:**
Cowley reanalyzed Solomon’s data by type of crash

From the head-on collision graph – it can be inferred that the risk of crashes increase at night-time particularly at high speeds

## West and Dunn, 1971

- Data collected on a state highway in Indiana with speed limits of 40-50 mph
- Found a U-shaped relationship like some of the earlier studies
- Crash risk was greatest for vehicles traveling more than two standard deviations above the mean speed
- Does not explain significant percent of speeding-related single vehicle crashes

**Notes:**
The Research Triangle Institute study (1970)aimed to quantify the relationship between speed and the occurrence of a crash 
It was undertaken a decade after Solomon's study. The idea was similar to Solomon’s study but some aspects of the methods were different. 
The study covered all state highways and county roads with a speed limit or a mean speed of at least 40 mph in Monroe County, Indiana, in all about 70 miles of road. 
A total of 294 crashes were included in the study.
Efforts were made to obtain pre-crash speeds that were more reliable than those abstracted from accident reports, including the use of accident investigation and of a computer-sensor system. 
For the first eight months of the study an accident investigation team determined the pre-crash speeds on the basis of physical evidence at the crash site and driver and witness reports. In the meantime, a computer-sensor system (basically a series of magnetic loop pairs connected to an on-line computer enabling collection of speeds and traffic volumes) was developed. 
The sensors were embedded at 16 points along the main highway, Indiana Highway 37. Using this system it was possible to identify accident-involved vehicles or the platoon in which they had been traveling and thereby obtain pre-crash speeds, so accident investigation was replaced by the computer-sensor system for the last few months of the study.

## West and Dunn speed curve

- Likelihood of being involved in crash flat
- within 15 mph of mean speed

**Replicated U shape like previous studies**

**Deviation from mean speed, mph**

**Notes:**
West and Dunn reported the results of RTI study.  
Information on the operation and output of the computer-sensor system was provided by West and Dunn (1971). 
In order to test the reliability of the system, measures of pre-crash speed for a group of 36 crashes were obtained using both available methods. 
It was found that in a quarter of the cases the speed of the accident-involved vehicle or the platoon in which it had been traveling could be identified confidently from the computer output (a result which seemed to be regarded as an achievement rather than as a cause for misgivings about the quality of the data). Some information was retrievable for the remaining crashes, but it was not made clear how these less certain estimates were gained or treated.
The findings of the Research Triangle Institute for state highways were only presented in terms of accident involvement rates for categories of deviation from the mean speed, calculated in a similar manner to those of Solomon. However, in recognition of the distorting influence of vehicles executing turning manoeuvres, crashes in which such a manoeuvre occurred (44% of the total cases) were excluded from the analysis. Based on data for 154 vehicles, the pattern of involvement rates was a U-shaped curve, but the elevated rates at low speeds were not nearly as pronounced as those of Solomon.
For a subset of the Research Triangle Institute data, West and Dunn elaborated on the exclusion of crashes which involved a turning vehicle: the involvement rate for vehicles with speeds of more than 15.5 mph below the mean speed was reduced by a factor of seven when such crashes were excluded, while the other rates changed only a little. This result provides an explanation for the high crash rates at low speeds in Solomon's study.

## Kloeden et al., 1997

- Risk of being involved in an injury crash was lowest for vehicles traveling near or below the median speed and increased exponentially at higher speeds

**Risk increases exponentially**

**Notes:**
Kloeden et al. (1997) compared the estimated speeds of over 150 cars involved in non-alcohol related injury crashes in 60 km/h speed zones in Australia 
The free speed of cars was measured at the same location at the same time of day and day of week. 
The pre-crash traveling speeds were based on detailed investigations of each crash scene and computer-aided crash reconstruction. 
The average and median speed of traffic was about 60 km/h ( 37 mi/h). 
Vehicles exceeding the 90th percentile speed or traveling more than 7 km/h faster (4 mi/h) than the speed limit and median speed had above average injury crash involvement rates. 
Nearly 25 percent of the cars involved in injury crashes were traveling faster than 72 km/h (45 mi/h) compared to only 2 percent of free flow traffic. 

## Shinar, TRB Special Report 254 (1998)On the relation of speeds to crashes

- There is sufficient evidence to indicate a driver’s speed is a correlate of crash involvement
- Speed deviation of crash-involved vehicles from the average appears to be positively related to crash probability, especially for arterial highways and interstates
- The principal factor...is the requirement to slow down to make turns and to enter and exit high-speed roads

**Notes:**
Also some evidence that even when turning vehicles removed from the data some effects of speed deviation, especially at the extreme ends remains.
This is not the same as evidence that if the same drivers were to increase their speed their crash probability would be reduced. That is because these are correlational not causal studies.  

## Relation of speed to crash severity

- The relationship between vehicle speed and crash severity is unequivocal and based on the laws of physics
- Velocity change in a crash (**Δ**V) is a critical measure of crash severity

**Notes:**
From an analysis of 10,000 crashes, Solomon concluded that crash severity increased rapidly at speeds in excess of 60 mi/h (96 km/h), and the probability of fatal injuries increased sharply above 70 mi/h (112 km/h). 

## Speed and crash severity (continued)

- The likelihood of being injured, and injury severity in a crash, depend on the change in speed (**Δ**V) at impact                        (Bowie and Waltz,1994)

**As ****Δ****V increases injury risk also increases**

## Speed and crash severity (continued)

- O'Day and Flora (1982) and Joksch (1993) found that the risk of a car driver being killed in a crash increased with increases in speed

**Speed**** ****Increase**

**Risk Increase**
**Exponential**

**Notes:**
Power relationship between impact speed and the energy released in a crash—the energy release is proportional to the square of the impact speed—is responsible for the sharp rise in injury probability for the vehicle

**Relation of Speed Limit Changes **
**to Speeds and Crashes**

## 1974 National Maximum Speed Limit               (NMSL) legislation

- In 1974, Congress established the NMSL of 55 mph
- mainly implemented to conserve fuel
- traffic fatalities declined 16 percent,                         from 54,052 in 1973 to 45,196 in 1974

## US DOT study

- Joint NHTSA-FHWA task force studied safety effects of the NMSL (1980)
- while the precise number of lives saved is unknown, an estimated *20,000 to 30,000 lives were saved by the NMSL from 1974-1978*
*	*

## TRB Special Report 2041984

- Concluded:
- 	The lower limit [of the NMSL] reduced both travel            speeds and fatalities, although driver speed compliance gradually eroded

## Partial repeal of the NMSL in 1987

- States granted authority to raise speed limits to 65 mph on rural interstates

## Effects of the 1987 speed limit changes Speed

- McKnight et al.,1989: 65 mph states, 48% increase in drivers exceeding 65 mph; 55 mph states, 18 % increase
- NHTSA, 1992:
- 3 mph increase in average speeds
- 4 mph increase in 85th percentile speeds
- 0.7 mph increase in standard deviation

**Notes:**
Many others Freedman & Esterlitz 1990,
	Freedman & Williams 1992
p. 245 Arguments against role of speed dispersion- that is single vehicle crashes

## Effects of the 1987 speed limit changes Crashes

- Garber and Graham,1989:
- 15% increase in fatalities on rural interstates
-   5% increase in fatalities on rural non-interstates
- McKnight et al., 1989:
- 22% increase in fatal crashes on rural interstates
-   1% increase in fatal crashes on 55mph roads

**Notes:**
Garber and Graham-1989
Analysis Method: Pooled time series regression

McKnight et al.,1989 
Analysis Method: Quasi-experimental ARIMA models, Before/After Analysis (1982-1988)- controlled for seasonal factors, economic factors, weekend travel & seat belt laws & time trends

Baum et al.-1991
Analysis Method: Before/After odds ratio (1982-1986 vs. 1989) 

## Effects of the 1987 speed limit changesCrashes (continued)

- Baum et al.,1991:
- 19% increase in fatalities on rural interstates relative to other rural roads
- NHTSA 1992:
- No increase in fatality rates on rural interstates
- 12% decrease in fatality rates on rural interstates in states that retained 55 mph

**Notes:**
Baum 1991- Adjusted for VMT & vehicle occupancy.

NHTSA 1992
Analysis Method: Before/After Analysis (1986-1990), Regression Analysis with comparison (1975-1990)

## Effects of the 1987 speed limit changes:      Crashes (continued)

- Lave and Elias,1994:
- reported a 3 to 5 percent decrease in statewide fatality rates in states that raised speed limits to 65 mph

**Notes:**
The aggregation of data across highway type can lose all power to detect the effect of the higher speed limits on a subset of highway- rural interstate fatalities are a small percentage ( approximately 6%)

 Also note that can get an increase on other rural roads attributable to the 65 mph limit

VMT used are unreliable- cannot translate to monthly rates

## Full repeal of the NMSL in 1995

- States granted full authority to establish speed limits                   on all roads

## Effects of full repeal of the NMSL Interstate and freeway speeds

- Retting and Green, 1997:
- mean speeds and 85th percentile speeds increased by 2 to 5 mph
- proportion of cars exceeding 70 mph increased by 15 to 50 percent
- speed variation, as measured by the standard deviation, increased by 5 to 15%

**Notes:**
Retting and Green -1997
Analysis Method: Before/After

## Effects of full repeal of the NMSL: Crashes

- Farmer et al., 1999:
- 15% increase in fatalities and 17% increase in fatality rates on interstates with increased speed limits
- no significant change on non-interstate roads
- Frith et al., 2002:
- 35% increase in fatalities in 70 mph states
- 38% increase in fatalities in 75 mph states

## Effects of full repeal of the NMSL Crashes (continued)

- NHTSA,1998:
- 6% increase in fatalities and 15% increase in injuries on interstates
- 2% decrease in fatalities and 3% increase in injuries on non-interstate roads

## Effects of full repeal of the NMSL Crashes (continued)

- McCarthy (TRB Report 254),1998:
- positive relationship between crash severity and speed dispersion for rural interstates
- speed dispersion is very important for fast drivers
- Cato Institute, 1999:
- raising speed limits reduced injuries significantly, increased fatalities insignificantly and had a considerable net cost benefit

**Notes:**
Calo- said deaths have gone down nationally while speed limits have gone up.
But reductions in fatalities have been happening over decades- he did not control for extraneous factors that have brought deaths down.
Consistent with looking at the effects of drought in several states by studying crop production nationwide.

## Conclusions

- Overall crash involvement as a function of travel speed generally follows a U-shaped curve
- Lower than average traffic speeds are unavoidable due to traffic conditions and vehicle maneuvers that require low speeds
- Higher than average speeds generally violate traffic laws and can be addressed through consistent law enforcement

## Conclusions (continued)

- Higher and lower than average speeds don’t have equal consequences; as crash speeds increase, so does crash severity
- The overwhelming majority of evidence suggests that reductions in speed limits reduce vehicle speeds and crashes; increases in speed limits increase speed, as well as crashes
- Excessive speeders are more likely to be male, younger, and to have poor driving records