Why Are You Speeding?

When driving along a Provo street, how do you determine your speed?

For me, the decision is mostly an intuitive one. Near my house, for example, I assume the speed limit is 25 or 30 miles an hour because that feels natural. The lesson is that street design itself suggests a speed limit; on my street I’m not sure if there are even any signs stating the legal maximum.

A street in the Joaquin neighborhood.

A street in the Joaquin neighborhood.

Relatedly, I recently ran across this post from the blog Captain Transit Rides Again that explains how many speed limits are actually set:

1. Design the road to permit the highest speeds that the budget and the landowners will let you.

2. Measure the speed of drivers and find the 85th percentile – the speed where 85% of drivers are going at that speed or slower. Round to the nearest multiple of 5. That’s the speed limit that you put on your signs.

I don’t know if this method — called the 85th Percentile Rule and mentioned again in this Wikipedia entry — has been specifically used to set speed limits in Provo, but I wouldn’t be surprised if it was. Freedom Blvd, for example, lacks identity or the ability to sustain business in part because it has too-high speed limits (which most people break).

Freedom Blvd

Freedom Blvd

The same is true for 300 South, which is more or less a stroad cutting through a residential area. Unsurprisingly, 300 South is not a prime place to live and property values appear to be depressed.

Because the post is aimed at finding a less “crazy” way to set speed limits it suggests setting speed limits based on the desired cyclist and pedestrian environment. In addition, streets should be designed to encourage those speeds. So for example reducing the speed limit to 15 mph on Freedom would by itself be ineffective.

Though most streets in Provo fall short of this approach — which I like, by the way — Center Street is actually a decent example. There, the 15 mph speed limit is more manageable for pedestrians and cyclists and the islands, diagonal parking, trees, etc. encourage drivers to not go too fast. Or said another way, design and speed limits converge.

Center Street isn't perfect, but it does have slow speed limits and design elements that reinforce those limits.

Center Street isn’t perfect, but it does have slow speed limits and design elements that reinforce those limits.

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