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NHTSA -An Analysis of Speeding-Related Crashes

February 11, 2009

PDF 26 pages:

http://www.nhtsa.dot.gov/portal/nhtsa_static_file_downloader.jsp?file=/staticfiles/DOT/NHTSA/NCSA/Content/Reports/2009/811090.pdf

An Analysis of Speeding-Related Crashes: Definitions and the Effects of Road Environments

DOT HS 811 090

February 2009

16. Abstract

Speeding is reported in the Fatality Analysis Reporting System (FARS) as a driver-level attribute that combines “driving too fast for conditions” or “in excess of posted speed limit.” There is a growing need to parse out these two factors, especially for those designing countermeasures. The report, using data from the State Data System quantifies the extent of these two aspects related to speeding using data from six States whose police accident reports actually parse these out.

The result of this analysis shows that this really depends on the severity of the crash. In fatal crashes, about 55 percent of all speeding-related crashes were due to “exceeding posted speed limits” as compared to the 45 percent that were due to “driving too fast for conditions.” The comparable percentages for speeding-related injury crashes were 26 percent versus 74 percent and those for PDO (property-damage-only) crashes were 18 percent versus 82 percent.

The second aspect examined in this study is how these crashes, which related to the factors “driving too fast for conditions” or “exceeding posted speed limit,” were affected by roadway environments. It shows that the speeding-related crashes that were due to “driving too fast for conditions” were more likely to have occurred on roads with higher speed limits (50+ mph) as compared to other crashes. Roadway environments analyzed also include: roadway surface conditions, roadway alignment, and intersection/intersection-related roadway segment.

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1. Executive Summary

Speeding is one of the most common contributing factors of traffic crashes. Data extracted from the Fatality Analysis Reporting System (FARS) show that the driver-level attribute “driving too fast for conditions or in excess of posted speed limit” is the critical contributing factor in more than 99 percent of all speeding-related fatal crashes, as de-fined by the National Highway Traffic Safety Administration. A marginal number of drivers were determined to be speeding through citations of speeding violations reported to FARS.

In this study, two aspects related to speeding-related crashes are examined. One is how each of the two individual factors, “driving too fast for conditions” (DTFFC) and “exceeding posted speed limit” (EPSL), contributed to speeding-related crashes. The other aspect is how these speeding-related crashes, which related to the factor DTFFC or EPSL were affected by road environments. Road environments examined include: posted speed limit, road surface conditions, road alignment, and road intersection/intersection-related. Data from NHTSA’s State Data System (SDS) was used in the analyses.

Some highlights of the findings of this analysis are presented below:

In fatal crashes, about 55 percent of all speeding-related crashes were due to EPSL as compared to the 45 percent that were due to DTFFC.

A marginal number (about 0.4 percent) of all fatal crashes were determined to be speeding-related through citations of speeding violations issued to the driver.

In speeding-related crashes that resulted in one or more injuries, about 26 percent of the crashes were due to EPSL as compared to the 74 percent that were due to DTFFC.

In speeding-related property-damage-only crashes, about 18 percent of the crashes were due to EPSL as compared to the 82 percent that were due to DTFFC.

The percentage of all crashes that were speeding-related (DTFFC or EPSL) varied considerably among the States, from about 6 percent to about 20 percent of all crashes. The variance of the population density, road speed limit, weather conditions, economic status, education level, etc. among the States might have played a role in this difference.

Speeding-related crashes that were due to DTFFC were more likely to have occurred on roads with higher speed limits (50+ mph) as compared to other crashes. Speeding-related crashes that were due to EPSL occur on either lower speed limit (less than 50 mph) roads or higher speed limit (50+ mph) roads as compared to other crashes.

In speeding-related crashes that were due to DTFFC, the relative proportions of crashes that occurred under adverse road surface conditions

(“Snowy/Slushy/Icy/Slippery” and “Wet”) were much higher during cooler months

(December to March), as compared to other crashes. This seasonality was relatively weak as a contributing factor in speeding-related crashes that were due to the factor EPSL.

The relative proportion of crashes that occurred on the curved sections of the road was much higher in speeding-related (DTFFC or EPSL) crashes. There was no important variation of this relative proportion across the month of the year.

Speeding-related (DTFFC or EPSL) crashes were more likely to have occurred on non-intersection/non-intersection-related stretches of roads. There was no important variation of this relative proportion across the month of the year.

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2.1 Background

Speeding is one of the most prevalent factors contributing to traffic crashes. It reduces a driver’s ability to steer safely around curves or objects in the roadway, extends the distance necessary to stop a vehicle, and increases the distance a vehicle travels while a driver reacts to a dangerous situation. Higher crash speeds also reduce the ability of vehicle, restraint system, and roadway hardware such as guardrails, barriers, and impact attenuators to protect vehicle occupants.

In 2007, about 31 percent of all fatal crashes were speeding-related resulting in 13,040 fatalities. The economic cost to society of speeding-related crashes is estimated by NHTSA to be $40.4 billion per year.1

NHTSA defines a crash to be speeding-related if any driver involved in the crash is charged with a speeding-related offense or if a police officer indicates that racing, driving too fast for conditions, or exceeding the posted speed limit was a contributing factor in the crash.1 2

In Table 1, the speeding-related fatal crashes by NHTSA definition during 2001-2007 are displayed. Data was extracted from the Fatality Analysis Reporting System (FARS). It shows that driver-level DTFFC or EPSL are the critical contributing factors in more than 99 percent of all speeding-related fatal crashes. Also, the number of violation charges is-sued to speeding drivers is small in fatal crashes.

In FARS, there was no information to parse out the relative contributions the factors DTFFC or EPSL made to speeding-related fatal crashes. From research and engineering aspects, it is particularly relevant to examine how these two individual factors contributed to speeding-related crashes. While FARS does not parse out these two factors, some States in NHTSA’s State Data System (SDS) have information that would facilitate such an analysis.3 Six States (Kansas, Kentucky, Maryland, Montana, North Carolina, and Wisconsin) had specific information on whether the driver was traveling too fast for con-ditions or exceeding the posted speed limit. Also, information on road environments was available in the data from these six States.

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6. Conclusions

Two aspects related to speeding crashes were examined in this study. The first aspect is how these two individual factors, DTFFC and EPSL, contributed, relatively, to speeding-related crashes. The other aspect is how these crashes that related to DTFFC or EPSL were affected by road environments. Data from NHTSA’s SDS was used in the analyses. In fatal crashes, the percentage of speeding-related crashes due to ESPL are overall higher than those due to DTFFC. In speeding-related injury and property-damage-only crashes, the percentage of speeding-related crashes due to ESPL are overall lower than those due to DTFFC. There exist important differences in the percentage of all crashes that were speeding-related among these six States. Speeding-related crashes due to DTFFC are more likely to have occurred on higher speed limit roads (50+ mph) as compared to other crashes. The relative proportion of crashes that occurred under adverse road surface conditions is much higher in speeding-related crashes that were due to DTFFC during cooler months (December to March). That is, in the winter season, it could be easy for a vehicle to ex-ceed a safe travel speed under adverse road surface conditions. The safe travel speed might need to be lower than the posted speed limit. The relative proportion of crashes that occurred on curved roads is much higher in speed-ing-related (DTFFC or EPSL) crashes. There is no important variation of this relative proportion across the month. Speeding-related crashes were more likely to have occurred on non-intersection/non intersection-related roads. There is no important variation of this relative proportion across the months.

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