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Effects of Speed on Accident Frequency?

Posted: Fri Jun 19, 2009 11:05 am
by MSI
CASR Speed Study
McHenry Software participated in a study of crashes with the Road Accident Research Unit (RARU) of the University of Adelaide, Australia. They now go by the name Centre for Automotive Safety research (CASR)

The Road Accident Research Unit at Adelaide University with the support of the Australian Transport Safety Bureau has conducted a study into the effect of free travelling speed on the risk of involvement in a casualty crash in rural South Australia. A case control study method was used where the distribution of speeds of casualty crash involved passenger vehicles were compared with the distribution of speeds of non crash involved passenger vehicles. It was found that the risk of involvement in a casualty crash more than doubled when travelling 10 km/h faster.

The relationship between free travelling speed and the risk of involvement in a casualty crash in 80 km/h or greater speed limit zones in rural South Australia was quantified using a case control study design.
The crashes involving the 83 case passenger vehicles were investigated at the scene by the Road Accident Research Unit and reconstructed using the latest computer aided crash reconstruction techniques. The 830 control passenger vehicles were matched to the cases by location, direction of travel, time of day, and day of week and their speeds were measured with a laser speed gun.

It was found that the risk of involvement in a casualty crash increased more than exponentially with increasing free travelling speed above the mean traffic speed and that travelling speeds below the mean traffic speed were associated with a lower risk of being involved in a casualty crash. The effect of hypothetical speed reductions on all of the 167 crashes investigated indicated large potential safety benefits from even small reductions in rural travelling speeds.
See the Full report

Re: Effects of Speed on Accident Frequency?

Posted: Mon Aug 31, 2009 4:12 pm
by brian
The results of this study were also reanalysed using a more sophisticated method:
John Lambert has written a number of papers criticising this speed study and drawing different conclusions from the data.
Here are CASR comments on his latest paper:

Re: Effects of Speed on Accident Frequency?

Posted: Fri Jan 01, 2016 1:48 pm
by MSI
2015 FHWA Speed study:
Synthesis of Safety Research Related to Speed and Speed Management

    • This document provides a review of safety research related to speed and speed management. This review builds upon a similar synthesis prepared in 1982. This synthesis highlights the relationships among vehicle speed and safety; factors influencing speeds; and the effects on speed and crashes of speed limits, speed enforcement, traffic calming and other engineering measures intended to manage speed.
      Despite the substantial social and technological changes that have occurred since the original speed synthesis was published, vehicle speed remains an important public policy, engineering, and traffic safety issue. Speed is cited as a related factor in 30 percent of fatal crashes and 12 percent of all crashes (Bowie and Walz, 1994). Based on on–scene investigations of over 2,000 crashes in Indiana by teams of trained technicians, excessive speed for conditions was identified as the second most frequent causal factor out of approximately 50 driver, vehicle, and environmental factors (Treat et al., 1977).
      Excessive vehicle speed reduces a driver's ability to negotiate curves or maneuver around obstacles in the roadway, extends the distance necessary for a vehicle to stop, and increases the distance a vehicle travels while the driver reacts to a hazard.
      The following pages present the results of a systematic review of the literature concerning safety research related to speed and speed management. Initial listings of citations were generated using multiple keyword filters on several bibliographic databases. The most productive databases were those of the National Technical Information Service (NTIS), the Knight–Ridder Transportation Resources Index, and the Transportation Research Information Service (TRIS). The initial inventory of approximately 700 citations was supplemented by searches of the Institute of Transportation Engineers (ITE) index and more than 100 items that either predated the on–line data bases or otherwise were known to be pertinent.
    • There is evidence that crash risk is lowest near the average speed of traffic and increases for vehicles traveling much faster or slower than average. The occurrence of a large number of crashes involving turning maneuver partly explains the increased risk for motorists traveling slower than average and confirms the importance of safety programs involving turn lanes, access control, grade separation, and other measures to reduce conflicts resulting from large differences in travel speeds.
      When the consequences of crashes are taken into account, the risk of being involved in an injury crash is lowest for vehicles that travel near the median speed or slower and increases exponentially for motorists traveling much faster. One of the major concerns in all of the studies is the travel speed before the crash. Emerging technology used in mayday, vehicle tracking, and adaptive speed control systems provide the opportunity to accurately and continuously capture travel speed. This technology should be applied in improving our understanding of the relationship between speed, speed variation, and safety.
      When a crash occurs, its severity depends on the change in speed of the vehicle at impact. The fatality risk increases with the change in speed to the fourth power. International research indicates the change in injury crashes will be twice the percentage change in speed squared, and fatal crashes will be four times the percentage change in speed. These relationships are based mainly on speed limit and speed changes on high–speed roads. More research is needed to assess their applicability to low–speed urban roads.
      In general, changing speed limits on low and moderate speed roads appears to have little or no effect on speed and thus little or no effect on crashes. This suggests that drivers travel at speeds they feel are reasonable and safe for the road and traffic regardless of the posted limit. However, on freeways and other high–speed roads, speed limit increases generally lead to higher speeds and crashes. The change in speed is roughly one–fourth the change in speed limit. Results from international studies suggest that for every 1 mi/h change in speed, injury accidents will change by 5 percent (3 percent for every 1km/h). However there is limited evidence that suggests the net effect of speed limits may be positive on a system wide basis. More research is needed to evaluate the net safety effect of speed limit changes.
      Most of the speed related crashes involve speed too fast for conditions. This would suggest that variable speed limits that adjust with traffic and environmental conditions could provide potential benefits.
      Despite the large number of references concerning traffic calming, very few reports include results of a systematic evaluation. In many cases traffic volumes as well as speed are reduced. As a result of the traffic diversion, crashes may be migrating to other roads. More research is needed to assess the system wide impacts and permit comparisons to be made among individual as well as combinations of traffic calming measures.