Skip to Content
Streetsblog USA home
Streetsblog USA home
Log In
Streetsblog.net

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

false

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:

It's a method that cities all over the planet use to determine speed limits. It's the standard. Nobody questions it. Certainly not the engineers and planners who, for decades, have been served it up and who have swallowed it whole during their studies. Which reminds us of the old traffic engineer joke: Why did the engineer cross the road? Because that's what they did last year.

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:

Imagine a street where the average speed is 50 km/h. If the speed limit is reduced by 5 km/h then, according to this archaic model, the drivers are allegedly exposed to a higher risk. What is most shocking is that this entire concept completely ignores pedestrians and cyclists. Another horrific conclusion from this graph is that when you increase the speed limit, the crash risk is alleged to be less than for slow speeds.

The Institute of Traffic Engineers wrote: “The 85th Percentile is how drivers vote with their feet”.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.

All this right now in 2012. In your street. With your tax money.

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.

Stay in touch

Sign up for our free newsletter

More from Streetsblog USA

Friday Video: The London Neighborhood Where Bikes Outnumber Cars

...and how they got to that impressive milestone.

July 11, 2025

Friday’s Headlines Battle Galactus

Like the Marvel supervillain, U.S. interstate highway system seems to eat up everything in his path. A new book explores how to stop it.

July 11, 2025

New Report Shows Pedestrian Fatalities Drop — But Experts Say Not Enough

The Governors Highway Safety Association report showed a 4 percent drop in the number of pedestrian deaths last year, putting a slow on a dangerous trend — but advocates say the drop isn't nearly big enough.

July 11, 2025

Talking Headways Podcast: Localities Subsidize the State DOT

Adie Tomer of Brookings on how to improve regional coordination around infrastructure.

July 10, 2025

Five of the Ugliest Transportation Policies In the ‘Big, Beautiful’ Bill

Here's a rundown of some of the transportation provisions in the Republicans' reconciliation package, and what they might mean for your community.

July 10, 2025

Viva La Thursday’s Headlines

Why is French transit ridership up 10 percent since before the pandemic, while American transit ridership is down 23 percent?

July 10, 2025
See all posts