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More Than Just Perceived Traffic Safety

Current study shows a correlation between reduced speeds and fewer traffic accidents

The goal of modern technology from VITRONIC is to increase the safety of every individual involved in road traffic. It supports the enforcement of the observance of speed limits as stipulated by the law. These laws are not put in place to deprive car drivers and motorcyclists of the joy of driving, but rather because adjusted speeds result in road safety. This isn't just a perception, it's a fact. Assertions that "people who drive faster pay more attention and therefore drive more safely," sound more like a Bro Science statement than a verified scientific finding. Facts on this topic can be found in a current study presented by the International Traffic Safety Data and Analysis Group on behalf of The International Transport Forum entitled "Speed and Crash Risk". Some of the most important findings and arguments for adjusted speed limits are presented here.

Speed and crash risk
The aim of the study is to objectively document the correlation between speed and crash risk. It examines how the implementation of speed limits or the utilization of enforcement systems influence a reduction in average speed and thus the frequency at which accidents occur, as suggested by theoretical models of the relationship between speed and crashes. The higher the absolute speed, the greater the risk of a crash. This can be explained by the fact that drivers require a certain amount of time to react to unforeseen events. The higher the driving speed, the more distance covered before the driver responds. At high speeds, there is less time available to react to changes in the environment and the driver is less likely to be able to maneuver. Moreover, the greater the difference in speed of individual vehicles, the greater the overall risk of an accident. This is because differences in speed increase the number of potentially dangerous situations. For example, the probability of a rear-end collision with a slower vehicle traveling ahead increases as does that of a head-on collision when overtaking a slower vehicle.

Empirical facts and practice
There are numerous empirical studies that have examined the degree to which a change in the average speed impacts the number and seriousness of accidents. Nilsson's Power Model states the following: an increase in average speed by one percent results in an increase in accident frequency by approximately two percent, an increase in the frequency of serious accidents by three percent, and an increase in the frequency of fatal accidents by four percent. The correlation of the effects of greater differences in speed is not as easy to determine. There are 13 studies that examine these questions. Although almost all studies conclude that a significant variance in speed increases the crash risk, there are great differences when evaluating the effects, since the numerical estimates of these vary greatly. Furthermore, different studies use different survey methods, which means a precise meta-analysis is almost impossible.

International case studies
To test theoretical considerations in practice, the study looked at a number of international case studies to examine the increase or decrease in speed and the use of automated speed enforcement measures. The results are as follows:

1.    In Hungary, the speed limit within city limits was changed on March 1, 1993 from 60 km/h to 50 km/h. This resulted in an overall reduction in the number of accident casualties by 37.7 percent on roads within these urban areas.

2.    In May 2001, speed limits outside city limits in Hungary were changed from 120 km/h to 130 km/h on highways, from 100 km/h to 110 km/h on partial highways, and from 80 km/h to 90 km/h on rural roads connecting towns and villages. In the case of the latter, the average speed increased subsequently by 2.5 percent and the number of accident casualties increased by 13 percent.

3.    Between 1997 and 2003, all Australian Local Government Areas, with the exception of the Northern Territory, changed their standard maximum speed in within city limits from 60 km/h to 50 km/h. In New South Wales, the average speed fell by 0.5 km/h while the overall number of accidents fell by 25.3 percent and the number of injured persons fell by 22.3 percent.

4.    In Denmark, the typical speed limit on highways was increased from 110 km/h to 130 km/h on April 30, 2004. After these increased speeds were implemented, the number of injuries due to accidents and the number of personal injuries on the stretches of roads with higher speed limits of 130 km/h was significantly higher than on comparable roads.

5.    In 2004, an environmental speed limit was implemented on three major roads in the city of Oslo. The speed limit was reduced during the winter seasons of 2004-2007, between the months of November and March, from 80 km/h to 60 km/h. On those roads with reduced speed limits, the average speeds fell by approximately 7.5 percent from 76 km/h to 71 km/h. The number of injuries due to traffic fell by 28.

6.    In 2008, the entire speed limit system in Sweden was reformed. On rural roads that connect towns and villages, a new set of speed limits, i.e. 80, 100, and 120 km/h, were implemented to replace the previous limits of 70, 90, and 110 km/h. As a result, speed limits on many rural roads were reduced from 90 km/h to 80 km/h and increased from 110 km/h to 120 km/h on some highways with high speed limits. On rural roads, where speed limits were reduced from 90 to 80 km/h, the average speed fell by 3.1 km/h and the number of casualties fell by 41 percent. However, the number of severely injured did not change significantly. On highways where limits were increased, the average speed limit increased by 3.4 km/h and the number of severely injured increased by 15 people per year. The number of casualties did not change significantly.

7.    In France, automatic flash units were introduced in 2003. This occurred after President Chirac decided in 2002 to make traffic safety one of his top three priorities during his time in office. Between 2003 and 2009, 1,661 stationary radar cameras were installed, supplemented by approximately 932 mobile units. Between 2002 and 2005, the average speed on secondary roads fell by 8.9 km/h and on highways with two or three lanes by 7.7 km/h. The number of casualties due to accidents in rural areas decreased by 25-35 percent, on urban freeways by 38 percent, and on city streets by 14 percent.

8. Section control was implemented in December 2005 in the Italian highway system. Analysis results show a significant decrease in average speed and also a noticeable decrease in speed variability. On the A56 express highway, the average speed of light vehicles decreased by 10 percent and the number of accidents by 32 percent.

Recommendations by the International Transport Forum
When a speed limit is increased, strict enforcement or an infrastructure upgrade is necessary to compensate for the increased crash risk caused by a higher average speed. Without such compensation measures in place, more casualties and injured parties can be expected. It is recommended to use automated speed enforcement systems to effectively reduce speed.
Experience worldwide has already proven that such solutions used to reduce speed, and thus, the frequency of traffic accidents, are effective. Section control, meaning the measurement of the average speed across a section of the road, is a relatively new measure that appears to be very effective, not only in regards to reduction of speed, but also in supporting a more homogeneous traffic flow.

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