Matt Kahn discusses the lane closure on the 405 here. He refers to a very nice manuscript by Giles Duranton and Matthew Turner: the Fundamental Law of Road Congestion: Evidence from US cities who concluded that the provision of public transit is unlikely to have any impact on total vkt (vehicle kilometers of travel.)

I say this to my students and they go crazy because if there is one thing planners love, it’s transit. I love transit, too, but it is not really a way to address congestion except in very specific conditions, and then only for a short time period.

How can this possibly be?

It’s likely that vkt won’t change with transit supply for the same reasons that the road will eventually congest even if you provide additional road capacity. If there is sufficient demand for mobility on a corridor, you can serve some of that demand with alternative supply–like transit or telecommuting–and because those trips have shifted off the high-demand road, there are people who are now willing to drive on the corridor because it’s less costly in terms of time delays. The demand for road service and transit service work in tandem and people sort themselves based on preference, but it’s not like demand for the roadway goes way.

Now that doesn’t mean you have done no good. By serving more trips on transit, you’ve dampened vkt *growth*–you haven’t “eliminated car trips” the way many people suggest when they discuss transit provision. But by serving the existing demand on the corridor through parallel transit rather than new road capacity, you’ve probably (probably) made things better than they would have been had the additional supply come in the form of new road space.

So what about air quality? This is where it gets sticky. The outcomes depend on who jumps on the road when the transit passengers leave. If we take a bunch of high-income, new car owners off the road (Prius drivers even) and their trips are replaced by Silverados and POSs, then emissions can worsen from baseline conditions. Yes, I just outlined a scenario where transit provision made emissions worse. What if the new trips are made in hydbrids? Then we have probably lowered emissions.

But wait: it is also likely that even though traffic mix may change by shifting some riders to transit, emissions are probably less than if the additional trips on that corridor were served by new road capacity rather than transit.

There are other ways this scenario can play out. The key thing to keep in mind is that absent something that actively suppresses vkt or incentivizes fuel economy/emissions control, you do not lower overall vkt by providing alternative supply.

Transit supply is important even if it doesn’t always match our grand claims of “improving air quality.” It’s hard to improve air quality over baseline conditions without technology or emissions charges. Transit supply generally helps us by dampening emissions and vkt growth; which means we reduce what these might have been with road expansion rather than improve them from baseline conditions (which, when you think about it, still exist).

And shifted trips aren’t the only ones served by public transit; transit supply can draw its own demand, which increases mobility. It is value added there as well.

Not as sexy as “improves air quality,” I know. But maybe we can say “improves future air quality.”