Good Congestion and Bad

The phrase "traffic congestion" can be misleading; some reductions in traffic speed are less harmful than others.

3 minute read

October 13, 2015, 6:00 AM PDT

By Michael Lewyn @mlewyn


Traffic Light

David Lofink / Flickr

There has been much debate in planning circles (including Planetizen's own blogs) about how to measure traffic congestion. The most widely cited measurement, the Texas Transportation Institute's Urban Mobility Scorecard, compares automobile traffic speeds to "free flow" speeds (that is, auto speeds in the absence of significant traffic). By this definition, a road is congested if people drive 40 miles per hour on a road where they could be driving 50 miles per hour. 

But not all "congestion" is equally noxious. For example, suppose people drive 20 mph on a two-lane urban street where the free-flow speed is 40 mph. In such a situation, drivers lose a few seconds here and there, but traffic might flow fairly evenly. Here, lower speeds actually benefit the public in a variety of ways. Slower speeds mean that accidents (both between cars and between cars and pedestrians) are less likely to lead to death or significant bodily injury. A pedestrian hit by a car going 40 mph has more than an 80 percent chance of death, while one hit by a car going 20 mph has about a 5 percent chance of death. Similarly, streets designed to accommodate slower traffic tend to be less dangerous for drivers, because it is more dangerous, other things being equal, for a car to be hit by a fast-moving object than by a slow-moving object. Moreover, drivers are only slightly inconvenienced by slower speeds where the lower-speed street is part of an urban grid; in such places, the congestion is easy to avoid, because a driver can easily switch to another street.

By contrast, imagine the stereotypical traffic jam: bumper-to-bumper traffic on an interstate highway. This form of congestion is more irritating to drivers for two reasons. First, drivers have no alternative route, and thus feel more helpless. Second, "stop and go" traffic is less predictable than slow, even traffic; if I am stuck on I-75 going 5 mph, I have no idea when I will reach my destination. Finally, highway congestion lacks the public benefit caused by "congestion" on a traditional street: since there are generally no pedestrians on interstate highways, slow speeds on such highways do nothing to protect pedestrians. Even on an arterial street with sidewalks, a traffic jam creates only small safety benefits because traffic can rapidly go from 5 mph to 50 mph and back again.

In sum, stop-and-go interstate highway congestion is the worst form of congestion, low speeds on an urban grid actually create public benefits. So to measure congestion appropriately, we should focus on the former. 

A postscript: other than criticizing how congestion is measured, does any of this have any practical application for government policymakers?  It seems to me that my ideas cut against the common idea that "congestion" justifies turning neighborhood streets into speedways.  


Michael Lewyn

Michael Lewyn is a professor at Touro University, Jacob D. Fuchsberg Law Center, in Long Island. His scholarship can be found at http://works.bepress.com/lewyn.

portrait of professional woman

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching. Mary G., Urban Planner

I love the variety of courses, many practical, and all richly illustrated. They have inspired many ideas that I've applied in practice, and in my own teaching.

Mary G., Urban Planner

Use Code 25for25 at checkout for 25% off an annual plan!

Redlining map of Oakland and Berkeley.

Rethinking Redlining

For decades we have blamed 100-year-old maps for the patterns of spatial racial inequity that persist in American cities today. An esteemed researcher says: we’ve got it all wrong.

May 15, 2025 - Alan Mallach

Logo for Planetizen Federal Action Tracker with black and white image of U.S. Capitol with water ripple overlay.

Planetizen Federal Action Tracker

A weekly monitor of how Trump’s orders and actions are impacting planners and planning in America.

May 14, 2025 - Diana Ionescu

Rendering of California High-Speed Rail station with bullet train.

California High-Speed Rail's Plan to Right Itself

The railroad's new CEO thinks he can get the project back on track. The stars will need to align this summer.

May 19, 2025 - Benjamin Schneider

Cobblestone street with vintage street lamps in Savannah, Georgia.

Savannah Reduces Speed Limits on Almost 100 City Streets

The historic Georgia city is lowering speed limits in an effort to reduce road fatalities.

3 hours ago - WJCL

Sign for Loma Alta Park in Altadena, Los Angeles County.

A Park Reborn: Resilience and Renewal in Fire-Stricken Altadena

Rebuilt in just two months after the devastating Eaton Fire, Loma Alta Park now stands as a symbol of community resilience and renewal, even as some residents hope recovery efforts will continue to support housing stability and long-term equity.

4 hours ago - Pasadena NOw

Colorful historic homes in Madrid, Spain.

Spain Moves to Ban 66,000 Airbnbs

The national government is requiring the short-term rental operator to remove thousands of illegal listings from its site as part of an effort to stem a growing housing crisis.

6 hours ago - The New York Times

Urban Design for Planners 1: Software Tools

This six-course series explores essential urban design concepts using open source software and equips planners with the tools they need to participate fully in the urban design process.

Planning for Universal Design

Learn the tools for implementing Universal Design in planning regulations.

Write for Planetizen