Road diet

A road diet is a technique in transportation planning whereby a road is reduced in number of travel lanes and/or effective width in order to achieve systemic improvements.

Techniques

A typical road diet technique is to reduce the number of lanes on a roadway cross-section. One of the most common applications of a road diet is to improve safety in the context of two-way streets with 4-lane sections. In this case, two travel lanes in each direction are converted into a 3-lane section with one travel lane in each direction, optional bicycle lanes, and a two-way turn lane in the middle. The two-way turn lane can be transitioned into dedicated left turn lanes at intersections. The additional space that is freed up by removing a vehicular travel lane can be converted into a bicycle lanes on either side of the roadway.

Road diets do not displace traffic, unless they have exotically high numbers. Road diet ranges typically start at 8,000 vehicles per day, and climb to 19,000 vehicles per day. At 20,000 vehicles-per-day the diet is called a "Super Road Diet." These diets range from 19,000 on up to about 23,000 vehicles per day. They are undertaken by replacing signals with roundabouts, and other means to keep traffic moving smoothly and uniformly.

Lane diets

In a lane diet, the width of a lane is decreased in order to achieve reduced overall roadway width or other goals. Typical lane diet techniques include narrowing vehicular travel lane widths to no more than ten feet, and left turn storage lanes to nine or ten feet. It is suggested that resulting space created by reducing lane widths be applied as bike lanes, or at least a wide shoulder. This gives added sight lines, turning radius and other benefits to all size vehicles. National lane width guidelines are offered as a range, and lane diets fall within this range.

Arguments for and against

Proponents of road diets generally believe a key benefit is lower vehicular speeds and/or improved pedestrian safety. Other benefits of road diets include promoting better land use, reducing induced traffic, promoting greater driving attentiveness, and promoting cycling through the addition of bicycle lanes. Providing dedicated left turn lanes at intersections can improve vehicular safety and can enable efficiency gains along the roadway.

A leading proponent of road diets is former Florida Bicycle and Pedestrian Coordinator Dan Burden; Burden and Pete Lagerwey published a definitive article on the topic in 1999. Additional studies have since been published, showing that road diets achieve these positive effects -- often without reducing traffic volumes. Among studies now showing that there are safety improvements to driving when lane widths are reduced include a recent report by the National Cooperative Highway Safety Research Program (NCHRP) and work analyzing traffic safety for 14 years in all 50 states by Robert B. Noland.

Road diets can be controversial -- In some cases, road diets will reduce road capacity and limit the usefulness of the road to the transportation network. ^{[citation needed]} Lane diets can be controversial for similar reasons.

Road diets can negatively affect the speed and reliability of transit service operating on the roadway, particularly if bus stops are located in pullouts and traffic queues delay buses attempting to re-enter traffic. Constructing bus bulbs can mitigate these effects though this feature results in delays for other vehicles.

Popularity

There are perhaps over 20,000 road diets in the United States, with another 500-1,000 being conducted each year.

The city in North America with the greatest number of road diets (29) is San Francisco. The city with the greatest number of road diets, per capita, is Hartford, Connecticut (12). One or two new road diets are added to each of these cities annually. Retail merchants in Seattle are now some of the strongest proponents for these projects, since reduced travel speeds allow for easier and safer parking, improve store access and boost overall walking and livability conditions in neighborhoods ... all of which leads to improved commerce.^{[citation needed]}

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