Speed Limit - Justification - Road Traffic Safety

Road Traffic Safety

According to a 2004 report from the World Health Organisation a total of 22% of all 'injury mortality' worldwide were from road traffic injuries in 2002 and without 'increased efforts and new initiatives' casualty rates would increase by 65% between 2000 and 2020. The report identified that the speed of vehicles was 'at the core of the problem and said that speed limits should be set appropriately for the road function and design along with physical measures related to the road and the vehicle and effective enforcement by the police. Road incidents are said to be the leading cause of deaths among children 10 – 19 years of age (260,000 children die a year, 10 million are injured). They are also occasionally set to reduce vehicle emissions or fuel use.

Maximum speed limits place an upper limit on speed choice and if obeyed can reduce the differences in vehicle speeds by drivers using the same road at the same time. Traffic engineers observe that the likelihood of a crash happening is significantly higher if vehicles are traveling at speeds faster or slower than the mean speed of traffic; when severity is taken into account the risk is lowest for those traveling at or below the median speed and "increases exponentially for motorists driving faster".

It is desirable to attempt to reduce the speed of road vehicles in some circumstances because the kinetic energy involved in a motor vehicle collision is proportional to the square of the speed at impact. The probability of a fatality is, for typical collision speeds, empirically correlated to the fourth power of the speed difference (depending on the type of collision, not necessarily the same as travel speed) at impact, rising much faster than kinetic energy.

The 2009 technical report An Analysis of Speeding-Related Crashes:Definitions and the Effects of Road Environments by the National Highway Traffic Safety Administration showed that about 55 percent of all speeding-related crashes in fatal crashes had "exceeding posted speed limits" among their crash factors, and 45 percent had "driving too fast for conditions" among their crash factors. However, the authors of the report did not attempt to determine whether the factors were actually a crash cause, contributor, or an unrelated factor. Furthermore, separate research finds that only 1.6% of crashes are "caused" by drivers what exceed the posted speed limit. Finally, exceeding the posted limit may not be a remarkable factor in crash analysis as there exist roadways where virtually all motorists are in technical violation of the law.

The speed limit will also take note of the speed at which the road was designed to be driven (the design speed) which is defined in the USA as "a selected speed used to determine the various geometric design features of the roadway". However traffic engineers recognize that "operating speeds and even posted speed limits can be higher than design speeds without necessarily compromising safety".

Vision Zero, which envision reducing road fatalities and serious injuries to zero by 2020, suggests the following "possible long term maximum travel speeds related to the infrastructure, given best practice in vehicle design and 100% restraint use":

"Roads with no possibility of a side impact or frontal impact" are sometimes designated as Type 1 ( motorways/freeways/Autobahns ), Type 2 ("2+2 roads") or Type 3 ("2+1 roads"). These roadways have crash barriers separating opposing traffic, limited access, grade separation and prohibitions on slower and more vulnerable road users. Undivided rural roads can be quite dangerous even with speed limits that appear low by comparison. For example, in 2008, Germany's 100 km/h (62 mph)-limited rural roads had a fatality rate of 9.5 deaths per billion travel-km, over four times higher than the autobahn rate of 2.2 deaths. Autobahns accounted for 33% of German road travel in 2008, but just 11% (495 of 4,477) of traffic deaths.