Building more roads doesn’t work. A comprehensive examination of all American cities shows that the amount of driving increases in lock–step with the expansion of the road system.
University of Toronto economics professor Matthew Turner told a conference last week his stunningly consistent findings suggest cities can’t build their way out of congestion and that building more road capacity simply results in more driving. The Solving Gridlock Forum organized by Transport Futures heard from experts from across Canada and the US and also hosted a Toronto mayoralty debate on public transit.
Turner’s team examined data over several decades for every American metropolitan area over 50,000. It first looked at length of interstate highways within those cities and found a consistent relationship with the amount of driving on that system.
“If you have a city with 10 per cent more roads, there’s 10 per cent more driving,” Turner told the conference. “If you take two cities which otherwise are about the same, one has 10 per cent more interstate lane kilometres, you will see that there is about 10 per cent more driving on those interstate lane kilometres.”
He found the same relationship “for other classes of roads within those cities”. And the pattern also held when changes to road capacity took place. For this part of the analysis the team compared data from 1980, 1990 and 2000.
“If you add 10 per cent lane kilometres to any given city then on average the amount of driving increases by 10 per cent,” he reported. “So both of those things line up in a way that you would expect if roads were causing driving.”
A third test looking just at cities over 5 million got the same results as did a replication using data from Japanese cities.
“This is as good as it gets in social sciences,” Turner said. “This is as close as you’re going to get to a fact.”
The source of the additional traffic Turner said is more difficult to determine with existing data, but most of it comes from changes in individual driving behaviour.
“When you add roads you get more traffic primarily because households change their individual behaviour: they drive more. So they will live a little further from work; they will drive further to get to a restaurant; they will be more likely to go out for shopping trips.”
He also found about a quarter of increased traffic on the interstate highway system came from long–haul trucking – echoing a complaint by Hamilton councillors over use of the Red Hill Parkway.
“Long haul truck traffic seems to be very opportunistic. If you put more capacity through your city, the long–haul truckers are there to use it.”
Turner’s studies also found that when you compare road capacity to time spent driving a doubling of road space generally resulted in a 10 per cent decrease in average speed of travel.
“So if you build roads in response to traffic congestion, you will elicit this extra demand, you will suck more users into their cars and scale up your city and things will get slower,” he reports. “So you can’t build your way out of traffic congestion and this says you can’t even stay even.”
Turner cautioned that his findings indicate more transit is unlikely to reduce congestion because any resulting road capacity will quickly fill up. This isn’t a reason to not expand transit services, but he argues that should be done where the cost is justified by the transit use. Turner suggests that road pricing is a good option because so much of driver behaviour is optional or possible to shift to less busy periods of the day. V