Why Wider Roads Make Traffic Worse: Jane Jacobs’ Warning Proved Right
Imagine spending $2.8 billion to widen a freeway, only to find traffic worse than before. This isn’t a fluke. It’s a predictable outcome traffic engineers keep ignoring.
The last book Jane Jacobs completed before she died was Dark Age Ahead. It’s a rant against the evils (real and imagined) that she felt were crippling society. Meandering and pessimistic, it lacks the eureka moments of Death and Life of Great American Cities, but it makes some interesting points about transportation policy. One of the more memorable parts of the book was her diatribe against traffic modeling. Her arguments centered on the unique characteristics of human behavior which often don’t fit current traffic models. Close a road, and instead of an equal number of cars re-routed, total traffic volumes will be reduced instead.
The tried and true theories of fluid dynamics cannot be applied to traffic models since cars are driven by people, and predicting human behavior is more complex than predicting how water flows through a pipe. As Wes Marshall mentioned in Killed by a Traffic Engineer, an engineer who over-designs, say, a stormwater system is practicing good engineering. He’s preparing for the worst-case scenario by designing the network to carry as much flood water as possible. If meeting the minimum standard is good, exceeding the minimum standard should be better. But bigger pipes will not create more water. The “over-designing is better” idea has been erroneously applied by traffic engineers to our road network, however. Designing a street that should be 2 lanes into a 4 or 6 lane road will, in fact, create more traffic, as well as higher speeds, more crashes, and less access for pedestrians and cyclists. Over-designing in the traffic engineering world has enormous negative externalities.
This is similar to Anthony Downs’ Triple Convergence theory, which says that expanding roadway capacity will eventually lead to more drivers using the improved roads, negating whatever capacity improvements have been built. New traffic will be caused by:
Drivers who formerly used alternative routes during peak hours switch to the improved expressway (spatial convergence)
Drivers who formerly traveled just before or after the peak hours start traveling during those hours (time convergence)
Some commuters who used to take public transportation during peak hours now switch to driving, since it has become faster (modal convergence). (Stuck in Traffic, 1992)
There’s a common theme that all of these authors touch on: Closing or narrowing roads reduces traffic, and widening roads increases traffic. The solution to congestion (road building/widening) creates the symptoms (traffic) which are used to further justify more road building. This never ending cycle has been going on for decades, especially on suburban commercial strips where stroads and 8-10 lane arterials have increased traffic volumes and crashes while nearly eliminating all other mode choices.
While America has great infrastructure needs, including delayed bridge maintenance, transit and high-speed rail, it seems there are always funds available for road widening despite evidence that induced demand rapidly depletes additional capacity.
When Texas expanded the Katy Freeway in Houston a few years back, the expectation was that making the massive road even wider would relieve traffic. Some $2.8 billion later, the 26-lane interstate laid claim to being the “world’s widest freeway”—but the drivers who commuted along it every day were no better off. More lanes simply invited more cars, and by 2014, morning and evening travel times had increased by 30 and 55 percent, respectively, over 2011. (Citylab, 2016)
In trying to predict where road widening makes the best use of public funds, induced demand may actually be preferable to the opposite outcome: Expanding a roadway and having nobody drive on it. There are many 6 and 8 lane arterial roads in American cities that have traffic volumes that could easily be accommodated by a 4 lane road. There are many fast 4 lane roads that could be accommodated by a 2 or 3 lane road. These are roads with flat or even declining historic traffic volumes over the past 10 years, but which were projected to service a 2-3% annual traffic growth rate. Is maintaining over-engineered streets a good use of public funds?
As Jane Jacobs predicted, removing lanes has the opposite result that many models forecast. In case studies of 25 complete streets projects by the University of Oregon, many road diet projects saw traffic volumes decrease after construction while increasing capacity for pedestrians, transit and cyclists. New York City is notable for putting these ideas into action. The Madison Square Pedestrian Project, the Broadway Blvd Midtown Improvements, and the 34th St. Midtown Pedestrian Mall show that eliminating full traffic lanes on busy streets in order to create outdoor living rooms, cafe space and transit/bike lanes is not only technically possible, but has enormous quality of life benefits. Baltimore has also removed a full lane of traffic on Pratt St. (one of the busiest streets in the city) with little impact on congestion and replaced it with a dedicated bike/bus lane.
New York City is unique, but if these projects can work in a city with some of the highest traffic volumes in the nation, they can also work in your city. When it comes to looking at traffic volumes, and the entire traffic engineering profession from a different perspective, Jane Jacobs and Anthony Downs have been consistently proven right. Policy makers, engineers and planners should take note of their theories on traffic and human behavior when interpreting traffic model outputs. Maybe making a knee-jerk decision to add lanes to a street when the traffic model predicts excess traffic isn’t such a good idea. A more wholistic approach to transportation planning is necessary to improve traffic safety, accommodate new transportation modes, and meet sustainability goals for the 21st century.
What streets in your city are excessively wide? Does your city have examples of road diets? I’d love to hear about them in the comments.
Mark, that is very important topic. Road diet is similar to healthy food. It has so many benefits for cities. But eventually, pioneers of the medical profession will accept only chemical drugs and surgical operations. No one will talk about the benefits of diet. Also, radical road designers everywhere will ignore such theory and will always talk about Road Standards and specefications.
Hey Mark, great article! I see you attracted the usual comments.
One piece of food for thought:
I’ve decided I don’t like “road diet” as an expression. While it makes sense from a certain point of view, I also think it provokes a defensive reaction, because it strongly implies that roads are “bad” in the same way that overeating is bad. And perhaps you or I feel that way, but, a lot of people knee-jerk and stop listening.
So, other things I’ve thought about are “safety improvements” or “right sizing” or “tailoring,” phrases like that. There probably isn’t a one-size fits all catch phrase that will work, because as soon as it catches on with urbanists it’ll polarize and about half the country will learn to dismiss it as culture war bait.
But you yourself pointed out the nuance, which is so important. We don’t need to be anti-car or opposed to driving to want safety and cost effectiveness. Those are universal priorities. And when I focus on those aspects I find its more persuasive to more people.
Thanks for writing!