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Friday, December 23, 2011

Transportation More Roads Create More Traffic

Posted by Dominic Holden on Fri, Dec 23, 2011 at 12:23 PM

I missed this last week. Governor Chris Gregoire's announcement that we need a $21 billion transportation package dedicated largely to more roads (because it will relieve congestion and end gridlock) coincided with the Sightline Institute digging into a report on US road construction for the last 175 years. Here's Clark Williams-Derry on the findings:

Co-authored by researchers Gilles Duranton and Matthew A. Turner from the University of Toronto, it’s a careful and remarkably thorough analysis of the relationship between urban highway space and traffic volumes in the US. And its key finding is straightforward:

For interstate highways in the densest parts of metropolitan areas we find that vkt [Vehicle Kilometers Traveled] increases in exact proportion to highways

In short, the authors find that building new urban highways simply increases traffic volumes—not in some general, intuitive sense, but in the sense that, all else being equal, a one percent increase in urban highway space increases urban road travel by precisely one percent.

To be fair, Duranton and Turner also find that boosting transit doesn't reduce congestion, either. Any additional road space—created by new roads or by drivers switching to transit—is simply consumed by vehicles that fill up the empty lanes. The full report is here.

Comments (8) RSS

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Aren't endless roads packages the reason that Los Angeles has the nation's fastest traffic? Oh. Whoops.

Posted by Original Andrew on December 23, 2011 at 12:37 PM

I think it's important to note that the reason why boosting transit capacity doesn't reduce congestion is because for every one person who leaves their car in the garage to take the bus, another person takes their place on the road. This happens either because, say, a person who'd been leaving five minutes earlier to get a head start on congestion now starts leaving at the later time, or someone who'd been taking an alternate route takes the more direct one, or someone who'd been carpooling with an asshole decided to ditch the asshole and drive alone.

But if you were to eliminate transit? Jesus fuck.

Posted by TVDinner

on December 23, 2011 at 12:39 PM

SIghtline seems to be confusing congestion due simply to bad planning with congestion due to people swarming to use transportation that serves their needs, especially in cities. It might be good to be a bit clearer about this if you want to attack the transpo package. It suggests better planning to get more people where they want to go by whatever mode. It leans on roads because it's a statewide survey, and most of the state isn't in one city or another.

But in the city we must remember bustling transportation corridors are vital, so long as they're not clogged because of poor condition or planning, as is today the case so often. Because planners wish to reduce unneeded congestion does not mean, as Sightline seems to be saying, that busy roads are not a good thing. A city whose users and residents flock to every mode of transport is a healthy one. Streets not jammed would be a very bad sign. London, Paris, New York, LA - all with fiercely bustling sidewalks, subways, busways, roads and highways. If we want density to flourish here, really flourish, we have to recognize that it means not being bothered by crowdedness.

Posted by gloomy gus on December 23, 2011 at 1:11 PM

@2: To put it another way, transit increases mobility at a given level of road capacity. (And, in dense urban areas, transit spending increases mobility more effectively than the equivalent amount of highway spending.)

Posted by aleks on December 23, 2011 at 1:28 PM

@4 gets it.

Transit adds capacity without building new roads. More capacity means more people using the same urban space. Investment in more and better transit is more efficient, with both dollars and geography, than new roads.

Posted by Moag on December 23, 2011 at 1:32 PM

Do you know what does reduce congestion? Congestion pricing, i.e. tolls. If more routes were tolled, poor people would stay away and rich people would enjoy uncongested traffic.

The system works so well for parking spaces that whenever I drive to shop within Seattle, I hope that it is during pay-to-park hours; that way, I know I will be able to quickly find street parking.

Posted by David Wright on December 23, 2011 at 2:13 PM

How do you know that increased traffic volume isn't causing additional road space?

Posted by venomlash on December 23, 2011 at 7:04 PM

Remember also that its VKT (vehicle kilometers traveled) that increases with increased capacity. Which means either (or both) of the the two factors --V or KT-- can be rising.

Just as reasonable an offering is that with greater road capacity the urban center is able to draw in commuters from increasingly longer distances, enlarging both the pool of commuters available to work and the average distance traveled per trip.

Posted by Timrrr on December 24, 2011 at 12:03 AM