We all know that the chances a car collision will be fatal to a biker or walker rise exponentially with the speed of the car—it’s simple physics. And when fewer folks walk or bike due to cars going too fast for them to feel safe, this threatens not only human health, but climate health as well.
Local control of speed limits is lost
Due to the 85th percentile method, speed limits have been raised on state “trunkline” roads within the city limits of municipalities throughout the state such as Ypsilanti, Saline, and Ann Arbor, despite the fact that the city’s governments and residents may be opposed to the raised limits.
In the process of in-depth research for that show, including interviewing the lead investigator of the study that is used to justify the method, I learned that while the 85th percentile method may be somewhat safer for drivers, it is not at all safer for bicyclists and pedestrians.
How is the 85th determined?
When a speed study is performed to determine the 85th percentile speed, any car that is slowing down because a bicyclist or pedestrian is present IS NOT COUNTED. Only free-flowing traffic is counted. One hundred measures are taken, and then plotted. The 85th percentile is the speed FASTER THAN 85% of the other drivers. It is NOT the speed 85% of people are driving. Rather, it is the head of the pack.
The end result is that the speed limit is adjusted upwards, higher than where most people are comfortable driving. And as we know, most people routinely drive a little faster than the speed limit. So the upward creep feeds on itself.
And the faster cars are going, the fewer folks are willing to walk or bike those roadways. Not including goals for increasing biking and walking when setting speed limits does not make sense at a time when cities are striving hard to achieve climate action goals.
A point of agreement
Most folks agree that changing the condition of the road (e.g. narrowing it, adding traffic calming, etc.) is more effective in reducing speeds than is posting more signs and/or increasing enforcement. But, the money is just not there to change every road to a condition that brings speeds down to a level that is safe for walkers and bikers. If we throw up our hands and allow the fastest speeders to dictate our speed limits, we are cheating ourselves and our future.
Check out the plethora of resources about speed limits, including the transcript from a speed limit forum we hosted in 2014, on the website www.michiganspeedlimits.org.