In the upcoming legislative session, the Bicycle Alliance of Minnesota is pushing for a reduction of speed limits on urban streets to 25 mph. They are currently set at 30 mph. The contention here is that a lower speed limit will make it safer for people biking and thus more people will bike.

As reported in the Star Tribune:

A limit of 25 miles per hour has long been a goal for Minneapolis, St. Paul and other communities. Supporters argue a lower limit would reduce the crash danger for pedestrians and cyclists and get more people out exercising.

This entire line of thinking is interesting to me because there are a number of assumptions in the worldview of the participants -- essentially bike advocates on one side and design professionals on the other -- that make them talk past each other.

For example, there is the pesky fact that most drivers don't pay a lot of attention to speed limits. Most people generally drive the speed they feel comfortable driving. When you lower the speed by five miles an hour, it doesn't really change behavior. This was noted in the article:

Moreover, the one test of a lower speed limit on a bike route in Minneapolis produced negligible change in traffic speeds, according to city measurements. Tests in 2012 and 2013 involved a 25 mph limit on 15th Avenue SE. near the University of Minnesota.

“It basically tells me that a sign doesn’t change behavior,” said Jon Wertjes, the city’s director of traffic and parking services.

You also have the strange pragmatism of a 25 mph goal. The studies this lobbying group and others cite show that we really need to get speeds down to 20 mph to significantly impact safety. In the current context, that doesn't seem doable politically and so a more pragmatic -- but far less effective -- goal is established. Again, from the article:

...pedestrians and cyclists have a 90 percent survival rate if hit at 20 mph or below, but that rate drops to 50 percent at 30 mph or above.

Engineers know these facts, which is why many tend to react with some bewilderment to the advocates: You want me to superficially lower speed limits -- an act that will make streets more dangerous for drivers -- to a level that won't really increase the odds of a cyclist or pedestrian surviving a collision? Huh?

While I think there are some -- okay...way too many -- cycling advocates that really do believe that speed limits and enforcement are the answer (they clearly are not), I think a great many have a deeper understanding of what is going on. For these advocates, the idea of a lower speed limit comes with some assumptions that are far outside of the toolbox of the typical design professional. From the article:

It [a report] said that traffic-calming measures, which involve such techniques as speed rises or raised pedestrian crossings, could help compliance by drivers.

Many bike advocates have come to the correct realization that changing speed limits alone will not make things safer. To improve safety, we actually need to change the design of the street. Unfortunately, changing the design of the street to affect driver behavior is currently far outside the main stream thought of design professionals. These two groups are talking past each other.

I advocate for a 20 mph speed limit, a number not based on pragmatic lobbying but on empirical evidence. I believe that, where such a speed limit is called for -- anywhere where we anticipate automobiles will be in the proximity of humans outside of a vehicle -- it is the responsibility of the engineer to design the street so that only a handful of deviants exceed the desired speed, a proportion of the population that can be dealt with through law enforcement.

How do engineers do this? They do it by making the street feel unsafe for the typical driver once their automobile exceeds the desired speed limit. Only when drivers feel as vulnerable as people walking and biking will our streets actually be safe.

Won't that destroy mobility? Not at all. It is our stroads that destroy mobility along with our balance sheets. It we are trying to squeeze a few seconds out of our commutes, do it on the stroads where it will help our tax base and not on our streets where it is killing our friends and neighbors.

#slowthecars

(All photos by Adam Coppola Photography)