Skip to Content
Streetsblog Los Angeles home
Streetsblog Los Angeles home
Log In
Streetsblog.net

The Critical Difference Between 30 MPH and 20 MPH

It might seem like a small thing: reducing motor vehicle speeds 10 miles per hour. But that 10 mph can make an enormous difference for the safety of a street and how comfortable people feel walking or biking there.

This diagram shows how a drive's visual field is impacted by the speed they are traveling. Image via Streets.mn
A driver's visual field shrinks as speed increases. Image via Streets.mn
false

Bill Lindeke does a great job explaining what he calls "the critical 10" in a recent post at Streets.mn:

If you look at the average speed of traffic on urban commercial streets, there are a lot of things that begin to change when you slow down cars from the 30 to 35 mile per hour range into the 20 to 25 mile per hour range. Most importantly, perception, reaction time, and crash outcomes are far better at 20 than at 30 mph, while traffic flow doesn’t seem to change very much.

The perception angle is perhaps the most interesting. Driving speed has a dramatic effect on the driver’s “cone of vision.” You can see a lot more detail at 20: people on the sidewalk, a bicyclist in the periphery, or the ‘open’ sign on a storefront. At 30 mph, the window shrinks dramatically.

The same is true for what you might call ‘reaction time. I’ll often talk to drivers about urban bicycling, and they’ll respond with a terrified story about the time that they “almost hit” a bicyclist that “jumped out” at them. And “I didn’t see them” is a common refrain heard by any police officer investigating a crash. The problem is that once you hit 30+ speeds, it’s a lot more difficult to stop in time to make any difference on a potential crash.

These three factors are the big reasons that crash outcomes vary so dramatically on either side of “the critical ten.” It’s no exaggeration to say that lives depend on getting speeds right.

Elsewhere on the Network today: The State Smart Transportation Initiative shares new USDA findings about the relationship between transportation and access to healthy food for low-income people. And Carfree Austin says the Texas capital has work to do stitching its street network back together for better walkability.

Stay in touch

Sign up for our free newsletter

More from Streetsblog Los Angeles

Measure HLA Is Now Officially Law for L.A. City

Check the city maps to find what bus, bike, and walk improvements are coming to streets in your neighborhood

April 12, 2024

Metro Releases Final Environmental Documents for Southeast Gateway Line

The new Southeast Gateway Line EIS/EIR doesn't have major changes compared to the draft EIS/EIR released in 2021

April 10, 2024

Open Streets Events Coming this Month: Mission-to-Mission and Venice Blvd

Enjoy CicLAvia on Venice Boulevard on April 21 and Active Streets' Mission-to-Mission on April 28

April 9, 2024
See all posts