Road Dieting

The concept of road diets is an alternate approach to dealing with road congestion that’s gained popularity in recent years. The typical solution to heavy traffic on roads is to widen them with more travel lanes. The problem is such an approach can induce demand and instead of two lanes of traffic jam, you get four lanes going nowhere.¹

Instead, with a road diet approach, you might turn a four-lane road into three lanes: two travel lanes and a turn lane in the middle.

Realizing these unintended outcomes, some localities implemented a type of road diet: reconfiguring the four lanes (two in each direction) into three (one each way plus a shared turn lane in the middle). The change dramatically reduced the number of “conflict points” on the road-places where a crash might occur. Whereas there might be six mid-block conflict points in a common four-lane arterial, between cars turning and merging, there were only two after the road diet.

Likewise, at an intersection, eight potential conflict points became four after a road diet.

The result was a much safer road. In small urban areas (say, populations around 17,000, with traffic volumes up to 12,000 cars a day), post-road diet crashes dropped about 47 percent. In larger metros (with populations around 269,000 and up to 24,000 daily cars), the crash reduction was roughly 19 percent. The combined estimate from all the best studies predicted that accidents would decline 29 percent, on average, after a four-to-three-lane road diet — DOT’s reported figure.

Pedestrian and bike usage tends to increase as well (b/c that extra street can be converted to bike lanes or sidewalks), speeding decreases, and car travel times are largely unaffected. This quick video by Jeff Speck shows four different approaches to road dieting:

Update: See also Braess’ paradox.

Braess’ paradox or Braess’s paradox is a proposed explanation for a seeming improvement to a road network being able to impede traffic through it. It was discovered in 1968 by mathematician Dietrich Braess, who noticed that adding a road to a congested road traffic network could increase overall journey time, and it has been used to explain instances of improved traffic flow when existing major roads are closed.

The paradox may have analogues in electrical power grids and biological systems. It has been suggested that in theory, the improvement of a malfunctioning network could be accomplished by removing certain parts of it.

(thx, david)

Update: A street in Oakland recently underwent a road diet: two of five lanes were converted into protected bike lanes. The result is an increase in biking and pedestrian use, a decrease in collisions, a decrease in speeding, and an increase in business along the street.

Along nine blocks of Oakland’s Telegraph Avenue, biking is up 78 percent since protected bike lanes were installed. Walking is up 100 percent - maybe because, thanks to the single lane of through traffic in each direction, the pedestrian yield rate doubled in the mornings and tripled in the afternoons.

Meanwhile, the number of traffic collisions fell 40 percent. Retail sales in a district that has sometimes struggled are up 9 percent, thanks in part to five new businesses.

And the median car speed is now the speed limit: 25 mph. As usual on such projects in urban areas, the main effect of removing a car passing lane was not to jam traffic, only to prevent irresponsible drivers from weaving between lanes in order to get to the next stoplight more quickly.

1. The concept of induced demand can be seen in other places, like New Orleans’ overcrowded jails. ↩

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