Congestion and Level of Service

By Dom Nozzi


“Induced traffic” represents new car trips created by, for example, a road widening that would have not occurred had we not widened the road. The “triple convergence,” as described by Anthony Downs, occurs when, for example, a road is widened, which attracts new car trips to the widened road that were previously using other routes, attracts car trips during rush hour that were previously avoiding rush hour, and/or were non-car trips that are now car trips due to the widening.

Both induced traffic and the triple convergence inform us that many travelers opt not to travel certain routes, opt not to travel at rush hour, or opt not to drive a car if a route is congested.

If the route is less congested (because of a recent widening, or because some motorists have become transit users, pedestrians, or transit users, for example), those discouraged travelers “converge” back on the route, on rush hour and on car travel. The road congests again. And rather quickly. Unless the community is losing population.

Another way of putting this is that in our world, there will pretty much always be a latent demand for more driving. Much of that demand is discouraged or diverted by congestion. Much of the discouragement goes away when the road is less congested. Roads are not like pipes carrying water. They are more like pipes carrying gas. Expand the pipe and the gas expands to fill the larger pipe. We cannot loosen our belts to avoid obesity. We cannot widen our way (or shift to non-car travel) out of congestion.

Many claim that less congestion results in less auto emissions (less air pollution). However, Jeff Kenworthy and Peter Newman convincingly showed about 20 years ago that congestion reduces emissions and gas consumption on a regional basis, despite what we’ve always believed (one of the great many benefits of urban congestion). Why? Because as implied above, congestion imposes what Ian Lockwood calls a “time tax.” And “low-value” car trips (driving across town to rent a video at rush hour on a major arterial, for example) decline.

Again, the key is not to reduce congestion. Congestion is a sign of city vitality. A healthy city cannot (nor should it) reduce congestion – at least not by using the conventional tactics such as road widening or synchronized traffic signalization. A healthy, sustainable, livable city must, instead, provide alternatives to congestion: convenient bicycling/walking/transit, compact development, pricing roads/parking, etc. And all of these healthy alternatives are much more likely, politically, when there is a lot of congestion. It is no coincidence that those cities with the worst congestion have the best transit.

Congestion, in cities, is our friend. When we make it our “enemy,” we unintentionally join forces with the sprawl/road/car lobby, since the default solution for reducing congestion is road widening.

One reason that congestion in cities is our friend is that the most essential and effective way to reduce excessive car dependence (and promote walking/bicycling/transit) is to inconvenience cars. The most feasible way to inconvenience cars is to “let it be” when it comes to congestion. To not bankrupt ourselves and destroy our communities by widening roads/parking lots to reduce traffic/parking congestion.

Increasing the number of bicycle or pedestrian trips not only will not reduce congestion. Such claims that increasing bicycling/walking will reduce congestion also perpetuates the downwardly spiraling, counterproductive efforts to try to reduce congestion. Those seeking a better community must end their (unintended) alliance with the sprawl lobby. Doing that means letting go of efforts to promote “congestion reduction.”

And embracing efforts to provide ways to avoid the (inevitable) congestion.

Level of Service

When we take actions to ease car travel, there is no win-win. Providing for cars is a zero-sum game. That is, each time we make car travel easier, we make travel more difficult for bicyclists, pedestrians and transit users. Providing for cars is also a recipe for downwardly spiraling quality of life for the community.

Transportation “level of serivce” (LOS) conventionally measures how “free-flowing” the car traffic happens to be on a road. Sometimes, this is measured by how many traffic light phases a car must wait for before passing through an intersection (the degree of delay, in other words), or a measure of the total number of cars that use a road each day compared to the size/capacity of the road.

Using transportation LOS as our measure implicitly assumes that congestion is an accurate assessment of quality of life. But using LOS as a yardstick perpetuates this ruinous assumption that free-flowing traffic and quality of life are one in the same.

In fact, when one observes which cities have the worst congestion, it would seem that the reverse is the case. That higher congestion levels commonly means a more impressive, attractive community.

We need to ask other, more appropriate questions to measure quality of life: How healthy is the retail? The downtown? Are large numbers of tourists interested in visiting? Are there lots of bicyclists? Transit users? Pedestrians? How expensive is downtown housing compared to similarly-sized cities? (the higher the expense of downtown housing, the higher the downtown quality of life). Are residents proud and protective of their city?

In my view, asking about LOS is nearly irrelevant to the question of healthy transportation and quality of life. Indeed, a good argument can be made that there is a negative correlation between using LOS as a measure and the quality of the transportation and community.

An unintended consequence of using transportation LOS is, as I mention above, perpetuating the asking of the wrong question. Asking about LOS distracts us from asking better questions along the lines of those questions I suggest above.

Asking the right question is often the crucial first step in taking beneficial actions (or, in science, solving puzzles in the field of research). Long ago, we didn’t reduce the cholera epidemic by measuring how many prayers were said. We learned through science that it was better to ask how we could reduce contamination by bacteria.


Visit my urban design website read more about what I have to say on those topics. You can also schedule me to give a speech in your community about transportation and congestion, land use development and sprawl, and improving quality of life.


Or email me at: dom[AT]

50 Years Memoir CoverMy memoir can be purchased here: Paperback = Hardcover =

My book, The Car is the Enemy of the City (WalkableStreets, 2010), can be purchased here: is the Enemy book cover

My book, Road to Ruin, can be purchased here:

My Adventures blog

Run for Your Life! Dom’s Dangerous Opinions blog

My Town & Transportation Planning website

My Plan B blog

My Facebook profile

My YouTube video library

My Picasa Photo library

My Author spotlight





Filed under Bicycling, Urban Design, Walking

5 responses to “Congestion and Level of Service

  1. Pingback: More Roads Equals More Congestion : Great City

  2. Dom. You are of course right. LOS is a fundamentally flawed way of assessing the impacts of new developments. As a transportation planner I am often sakes to assess the traffic impacts of new developments 10 years in the future. Usually, the LOS will be E or higher and we then have to recommend road widening to ease the problem. I hate doing it because I know it’s flawed and this article sums up the three areas that this traditional assessment falls down in. 1) induced traffic 2) congestion is a sign of a healthy city and makes other modes of travel more attractive and 3) urban form and land use development ate more important and if you keep widening the road you’ll ruin the neighborhood.

  3. Pingback: The Many Transportation Reforms Needed in Boulder, Colorado | Dom's Plan B Blog

  4. Dom,
    I picked up an old book of mine, “New Urbanism and Beyond,” and I’m very glad I did. It’s brought me to be introduced to another great thinker fighting against this sprawl-inducing, life-sucking, traffic-flow obsessing, distracting BUT ALL TOO HONORED Level of Service. I’ve been seeking better ways of expressing this frustration here in central AZ, land of sprawl, and I see you are years ahead of my own discovery. I’m glad to have connected with you. Thank you for helping spread the word. I’m very dedicated to spreading this idea around until it’s recognized as the gold standard in much the same way as Shoup’s “High Cost of Free Parking” has made parking reform attractive.
    If you’re attending APA’s NPC19 @ SF this April, it’d be great to meet you. I’m helping organize a session that hopes to spread this very message of this blog post. I’m sorry I didn’t find you sooner. It’s titled “Solving Traffic: Wrong Problem, Wrong Results”

  5. Ryan, Thank you for your kind, encouraging note. Unfortunately, I won’t be attending the APA event in April. I very much support your efforts to return our society to the timeless tradition of designing for people rather than cars. As you know, you have your work cut out for you in AZ. Dom

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s