One for the Dustbin: The 85th Percentile Rule in Traffic Engineering

Have you ever heard of the 85 percent rule in traffic engineering? I hadn't until I tried to get the speed limit lowered on my residential street, which is home to a K-8 elementary school in Cleveland's Detroit Shoreway neighborhood.

That's when the local traffic engineer explained to me that the 35 mph speed limit -- which, in effect, means drivers can go 45 with impunity -- was appropriate for my street because 15 percent of drivers exceed 35 mph. Outrageously, that's actually how transportation engineers in most places determine the safe traveling speed: They watch how fast cars go. While some cities have made progress with different approaches, like 20 mph zones, by and large it's the most reckless 15 percent who set the speed limit -- not the residents of a street or the parents of school children who might be walking around, crossing the streets those drivers travel on.

So that's the 85th percentile rule, and Pedro Madruga at Copenhagenize explains more thoroughly why it is badly in need of rethinking:

This whole concept is based on a model called the "Solomon Curve" that was developed in 1964. The Solomon Curve states that those who travel too slow are at greatest risk in traffic, Madruga explains:

Tell me about it!

Elsewhere on the Network today: The Architect's Newspaper writes up a greenway proposal in Milwaukee that could help connect segregated communities, and which seems to be gaining momentum. Jarrett Walker at Human Transit gives his take on why the recent Freakonomics travesty -- "Save the Earth, Drive Your Car" -- is so wrong. And Cap'n Transit gets in on the fun with his own critique.