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

Sweeping view of Portland, Oregon with Mt. Hood in background against sunset sky.

Oregon Passes Exemption to Urban Growth Boundary

Cities have a one-time chance to acquire new land for development in a bid to increase housing supply and affordability.

March 12, 2024 - Housing Wire

Aerial view of green roofs with plants in Sydney, Australia.

Where Urban Design Is Headed in 2024

A forecast of likely trends in urban design and architecture.

March 10, 2024 - Daily Journal of Commerce

Cobblestone street with streetcar line, row of vintage streetlights on left, and colorful restaurant and shop awnings on right on River Street in Savannah, Georgia.

Savannah: A City of Planning Contrasts

From a human-scales, plaza-anchored grid to suburban sprawl, the oldest planned city in the United States has seen wildly different development patterns.

March 12, 2024 - Strong Towns

Aerial View of Chuckanut Drive and the Blanchard Bridge in the Skagit Valley.

Washington Tribes Receive Resilience Funding

The 28 grants support projects including relocation efforts as coastal communities face the growing impacts of climate change.

March 18 - The Seattle Times

Historic buildings in downtown Los Angeles with large "Pan American Lofts" sign on side of building.

Adaptive Reuse Bills Introduced in California Assembly

The legislation would expand eligibility for economic incentives and let cities loosen regulations to allow for more building conversions.

March 18 - Beverly Press

View from above of swan-shaped paddleboats with lights on around artesian fountain in Echo Park Lake with downtown Los Angeles skylien in background at twilight.

LA's Top Parks, Ranked

TimeOut just released its list of the top 26 parks in the L.A. area, which is home to some of the best green spaces around.

March 18 - TimeOut

News from HUD User

HUD's Office of Policy Development and Research

Call for Speakers

Mpact Transit + Community

New Updates on PD&R Edge

HUD's Office of Policy Development and Research

Write for Planetizen

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.