Road Diet

Road Diet

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Overview

A Road Diet is a treatment given to an urban roadway in which the number of lanes is reduced, and the freed space converted to parking, bike lanes, landscaping, walkways, or medians. Road Diets are implemented to provide additional pavement and safety for bicyclists and pedestrians, reduce speeding, and to make room for parking.

Road diets are anathema to traditional traffic engineering principles because they tend to reduce roadway capacity. However, in practice, road diets can cause vehicle speeds to readjust to a more optimal speed, increasing the throughput of vehicles per lane. For this reason, road diets sometimes reduce congestion, and generally always increase safety for all users of the roadway. Studies in Seattle found that road diets decreased the rate of crashes by 6%.

“Traffic Taming”

The need for road diets comes from the fact that multi-lane urban roads are built to handle large volumes of traffic during the morning and evening rush hours. Generally, during the other 22 hours of the day, the road is larger than necessary. This abundance of spare pavement encourages speeding, and places bicyclists and pedestrians at far higher risk than a typical two-lane road.

When the public or local merchants lining the road perceive that serving rush hour through-traffic is not worth the negative impacts of the off-peak excess capacity, a road diet may make sense. Redesigning urban arterials to increase off-peak safety is emerging as a goal – known by some as “traffic taming” – distinct from traffic calming , which applies to low-volume residential streets. Road diets are only one of a number of possible traffic taming approaches.

From Four Lanes to Three

The most frequent type of conversion is four lanes to three [1]-Burden], with the middle lane serving as a two-way turn lane (TWTL). Alternatively, the middle “lane” can be a raised median with breaks or left turn pockets for turns. Studies show that road diets involving streets serving up to 23,000 vehicles per day substantially improve safety without significantly reducing roadway capacity. Most road diet projects result in the same or greater traffic volumes, but at a slower speed.

Dan Burden, of Walkable Communities, Inc., notes that virtually every urban community in the U.S. has four lane roads that are overbuilt -- in a manner that encourages speeding, documents a number of U.S. and Canadian road diet projects in Road Diets: Fixing the Big Roads. As Burden explains, the capacity of a three-lane road is almost equivalent to that of a four-lane road, because it operates more efficiently, and because left-turning vehicles are removed from the flow of traffic, reducing delay. A well-studied conversion confirms these observations.[2]

Three-lane roads are inherently safer because the speed is set by the most prudent driver, because there is only a single lane of on-coming traffic to monitor when turning left, and because the two directions are separated by the TWTL or median.

ALSO ON THE LIVABLE STREETS NETWORK

STREETSBLOG: The Benefits of a Road Diet

REFERENCES

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[1] Burden, Dan and Lagerwey, Peter. Road Diets: Fixing the Big Roads.

[2] Welch, Thomas M. 1999. The conversion of four-lane undivided urban roadways to three-lane facilities. TRB Circular E-C019: Urban Street Symposium.

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