Republican Senators Rick Jones and Tom Casperson are proposing raising speed limits throughout Michigan, claiming that the current law only encourages communities to set-up speed traps for the purpose of revenue enhancement. While such abuses are certainly commonplace, using that fact to leverage the argument that speeds should be raised across the board is irresponsible.
The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic is exceeding) and in the US is typically set 8 to 12 mph below that speed. Thus, if the 85th percentile operating speed as measured by a Traffic and Engineering Survey exceeds the design speed, compulsory legal protection is given to that speed–even if it is unsafe. This speed creep tends to continue until the 85th percentile operating speed is comparable to speed psychologically perceived as uncomfortably hazardous. The theory behind the 85th percentile rules is, that as a policy, most of the electorate should be seen as lawful, and limits must be practical to enforce. However, there are some circumstances where motorists do not tend to process all the risks involved, and as a mass choose a poor 85th percentile speed. This rule in substance is a process for voting the speed limit by driving; and in contrast to delegating the speed limit to an engineering expert.
Basically, what that means is that the speed limit will be set at what motorists generally perceive to be a safe speed. Proponents of this rule often go even further to claim that faster speeds actually increase road safety.
The real science begs to differ.
A 2009 study published in the American Journal of Public Health found that fatalities increase dramatically as speeds go up.
Results. We found a 3.2% increase in road fatalities attributable to the raised speed limits on all road types in the United States. The highest increases were on rural interstates (9.1%) and urban interstates (4.0%). We estimated that 12 545 deaths (95% confidence interval [CI] = 8739, 16 352) and 36 583 injuries in fatal crashes (95% CI = 29 322, 43 844) were attributable to increases in speed limits across the United States.
Conclusions. Reduced speed limits and improved enforcement with speed camera networks could immediately reduce speeds and save lives, in addition to reducing gas consumption, cutting emissions of air pollutants, saving valuable years of productivity, and reducing the cost of motor vehicle crashes.
Yet, the Institute of Transportation Engineers endorse the 85th Percentile Rule citing that people will drive whatever speed they want anyhow and posting unrealistic speed limits not only encourages lawbreaking but increases accidents due to driver-to-driver variances in speed. Both points have some validity, but the ITE goes on to buttress their argument with a graph showing that interstate highways with faster speeds are safer, by the mile, than other types of roads. Well, duh — those roads lack driving distractions such as intersections, sharp curves, traffic signals — they are designed to accommodate high speeds.
The ITE puts it this way “The 85th Percentile is how drivers vote with their feet”. These engineers seem to be myopically focused only on motor vehicles though. Bicycle and pedestrian safety advocates take issue with this majority rule notion. They refute saying:
[T]he main argument [is]to build more highways and freeways with faster speeds where the ends justify the means. Even if the means ignore vulnerable groups such as pedestrians and cyclists. Even if the study is now also used to serve the automobile in densely-populated urban areas, far from any freeway. They forgot to mention that, when it comes to establishing speed limits in cities, pedestrians and cyclists are excluded from this election. They don’t even get the chance to go to the polls.
Senator Casperson has a long legislative history of ignoring science over his preferred Tea Party fairy dust. This speed limit proposal looks to be more of the same.
Amy Kerr Hardin