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'Road diets' will make future traffic congestion worse

Studies show that roadways with fewer than 20,000 vehicles a day do not suffer, but an arterial like Nickerson is projected to be at 29,456 by 2030. We're trading away future capacity.
Plans for "dieting" Nickerson St.

Plans for "dieting" Nickerson St. City of Seattle


Seattle officials are quick to say "road diets" will maintain the car-carrying capacity on the roads in which they are applied. However, these bike-friendly officials are much slower to admit that these diets do not improve the car-carrying capacity. Road diets are essentially exchanging the future capacity needs of the roadway for other uses, in this case, bicycle traffic.

Road diets generally do not cause congestion on corridors that carry fewer than 20,000 vehicles per day. According to this report from the Federal Highway Administration on the effectiveness of adding bike lanes:

Under most average daily traffic (ADT) conditions tested, road diets have minimal effects on vehicle capacity, because left-turning vehicles are moved into a common two-way left-turn lane. However, for road diets with ADTs above approximately 20,000 vehicles, there is a greater likelihood that traffic congestion will increase to the point of diverting traffic to alternate routes.

In other words, as traffic volumes increase above 20,000 cars per day, throughput deteriorates. The traffic volumes on Nickerson were already slightly higher than 20,000 trips per day (20,300) in 2007. So traffic congestion is likely already worse than it was before the road capacity was reduced.

The real concern is the traffic outlook for the future. According to Seattle’s traffic analysis, Nickerson's traffic volumes will grow about 1 percent per year, with an additional 3,680 from a planned development. This means Nickerson will have about 29,456 daily trips by 2030, which is nearly 50 percent more than what the Federal Highway Administration says is the tipping point for the road diet to cause higher traffic congestion.

Because of the significant up-front financial costs, we generally build transportation infrastructure to accommodate future growth. Seattle officials are doing precisely the opposite.

Seattle is already the most congested city in America. Whether you are a parent trying to make your kid’s soccer game on time or a small business trying to deliver manufactured parts, you should be concerned about trading roads for bike lanes.

Michael Ennis is the director of the Center for Transportation at the Washington Policy Center. He is the author of the in-depth Washington Policy Center study, "Vanpools in the Puget Sound Region: The Case for Expanding Vanpool Programs to Move the Most People for the Least Cost."


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