Sunday, March 3, 2013

Light rail and traffic

Streetcars on Brighton Avenue 1940 
A note on Brad Plumer's article Do light-rail systems help cut down on traffic?. Anyone familiar with the Downs-Thomson paradox and related concepts would understand that light rail or any public transit system is not a "congestion reliever" in the simple sense that most people use. That's because in most congested contexts, any driver who decides to leave the car at home and switch over to transit will likely be replaced by another car on the road. This is not a hopeless outcome however--where before there was one person able to travel, there are now two. So the overall level of transportation activity has increased even though road congestion has remained the same, in this scenario.

Public transit in dedicated lanes can enable more people to travel despite congestion on the roads. If you want to actually tackle the congestion itself, then as Brad points out, road pricing seems to be the only option that has been proven to work. But I want to get to his last point which is this:
The other point is that mass-transit systems can lessen road traffic, but only if they’re part of a broader shift by a city to move to denser development. Researchers have found that people living in more compact cities with better transit options do drive less overall. But that shift doesn't happen overnight. And not all light-rail systems are even built with this sort of development in mind.
Boston happens to be one of those places with "light rail" and denser development built up along those corridors, thanks to the fact that most of it was built before zoning became a weapon used against cities. So if you take a look a Commonwealth Avenue you find something interesting. Despite the fact that the road has 6 lanes of travel (4 inner, 2 outer), MassDOT records very low traffic levels on the order of 12,000 AADT. By comparison, Washington Street in Brighton Center carries 14,500 AADT with only 2 lanes, and Harvard Avenue is all the way up at 18,100 AADT with only 2 lanes as well. Why isn't Commonwealth Avenue more like Brighton Avenue which has 4 lanes and sees volumes of 23,000 to 28,000 AADT?

First, for a variety of reasons, Commonwealth Avenue (in Allston/Brighton) is not a heavily used through-route, unlike Harvard Avenue or even Washington Street, so the traffic on it is largely coming from or going to nearby places. Second, although the development along the avenue is at city levels of density (~80-120 dwelling units/acre), because it grew up around the streetcar, there's not as much parking as such quantities of units would bring under current zoning codes. So trips are much less likely to be made by automobile. This seems borne out by the fact that the current "B" line is the busiest branch (~30,000 boardings/weekday) and that just over 50% of residents who live along this corridor get by without access to a car.

So in congruence with Brad's point, I think that you have here an example of how a century of streetcar service, combined with the long-ago shift to denser development, has lessened road traffic in the present day. Now the question for many developing light rail systems is: can you also achieve that, and perhaps on a shorter timescale?

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