Does Building Transit Reduce Traffic Congestion?

A Strong Towns member and local transportation and urbanism advocate recently posed a question for discussion in our Slack community:

Am I doing it wrong if I have my City look at traffic time savings when it comes to assessing value of transit spending?

This is a question I've seen come up over and over for years, in part because one of the most tempting ways, politically, to "sell" transit in a community in which most people drive their own cars is to promise those drivers that it will improve traffic conditions for them.

The question is, will it?

The Simple Answer: Don't Count On It

The evidence is mixed on whether transit actually alleviates road congestion, and a superficial traffic-time savings analysis likely won't say much of value.

The problem is that the principle of induced demand applies to all travel, just as it does with new road capacity. If you get a bunch of people to switch to transit, freeing up space on a congested stroad or highway, it’s likely that new drivers will quickly fill that space. It's the flip side of the observation made by economist Anthony Downs in his seminal 1992 book Stuck in Traffic:

Visualize a major commuting freeway so heavily congested each morning that traffic crawls for at least thirty minutes. If that freeway were magically doubled in capacity overnight, the next day traffic would flow rapidly because the same number of drivers would have twice as much road space.

But very soon word would get around that this road was uncongested. Drivers who had formerly traveled before or after the peak hour to avoid congestion would shift back into that peak period. Drivers who had been using alternative routes would shift onto this now convenient freeway. Some commuters who had been using transit would start driving on this road during peak periods. Within a short time, this triple convergence upon the expanded road during peak hours would make the road as congested as before its expansion.

In our case, the added capacity is on a public transit line that can substitute, but the effect on the parallel road is the same: as soon as it becomes uncongested, it becomes clear that there was latent demand among drivers who had been avoiding the congested road, and they show up to fill the gap.

The More Complicated Answer: Sometimes

Transit isn't going to eliminate road congestion. But there is evidence that where high-quality transit directly parallels a severely congested road, it provides a viable alternative to driving. And that alternative can put something of a ceiling on how bad congestion can get.

Think of it like a pressure-release valve of sorts: the road commute might be slow, but it will only get so slow, because at the point at which taking transit for a given trip becomes faster than driving in absolute terms, more and more people will just switch to transit. If it's available, that is. CityLab describes one such study, involving a Los Angeles transit strike.

The Best Answer: Don't Base Your Case on Congestion Relief, but Build Transit Anyway

Ultimately, the best reason for transit simply isn’t to make life easier for drivers. And it probably doesn’t do so. And if you build transit on the promise that it will reduce congestion, and it doesn't do that, you have handed transit opponents their best talking point.

Just witness Nashville voters' defeat of a massive light-rail plan for the traffic-clogged region in 2018. Said Wired in its postmortem:

Nashville’s plan would have offered an alternative to people who would gladly give up driving if there were another way to get around. Its opponents landed their best blow by arguing all these bus lines and light rail wouldn’t actually cut traffic for everyone who stayed behind the wheel. And they were likely right.

“We have a lot of experience showing you can throw billions into public transit, and for the most part it doesn’t put a dent into car trips,” says Robert Cervero, a transportation expert at UC Berkeley. Blame induced demand: Whatever slack is left from people abandoning their cars for transit is eventually taken up by new drivers, enticed by any empty capacity on the roads.

There are a ton of good reasons to invest in transit, but the key is to frame it as an alternative way of getting around that has benefits in its own right, not as something that will help drivers.

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Transit helps people move between places they want to be, and it does so at a far lower overall cost than driving when you consider all of the social costs—environmental effects of cars, for example, or above all, car infrastructure's wasteful use of a city's most valuable and finite resource: land.

High-quality public transit is arguably essential to a strong city or region, connecting compact, financially productive, walkable/bikeable places to each other across distances too great to walk or bike. From a Strong Towns perspective, the automobile-oriented development pattern is financially ruinous to cities. And transit is necessary to make alternatives to that development pattern viable.

So find the people in your city or region who are already using transit, and the places where transit is already viable. Identify the most pressing needs among that population, and move quickly and incrementally to meet them. Iterate on that approach, and you'll begin to actually free more and more people from car dependence—not just shorten their car commutes by a minute or two.

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