Clear, accessible definitions for common urban planning terms.

What Is a Road Diet?

3 minute read

A road diet ‘trims down’ multilane roadways by reallocating street space to uses other than car traffic, improving safety for pedestrians, encouraging multimodal travel, and enhancing overall livability.

Two-lane street with pedestrian crossing island

Montgomery County Planning Commission / Road diet in Lower Merion Township, Pennsylvania

A road diet is a street reconfiguration that is designed to improve traffic safety using reduced or narrowed driving lanes, curb extensions, bulb-outs, bike lanes, pedestrian medians, landscaping, and other relatively low-cost interventions. Road diets are shown to reduce driving speeds and improve safety for pedestrians, cyclists, and other vulnerable road users. They also reclaim space for bikeways, transit lanes, and other multimodal infrastructure.

The Federal Highway Administration recommends that communities consider road diets on four-lane undivided highways that present a high safety risk. There are four common types of road diets: “three lanes to two,” “four lanes to three,” “bike lanes to cycle tracks,” and “40-footer lane insertion.” The most commonly used type of road diet in the United States involves converting a four-lane, two-way road to a three-lane road, using one lane for each direction and a shared turn lane in the center. The extra space is used for bike lanes, sidewalks, or parking.

Diagram of road diet before/after with four-lane road reduced to three-lane road and median
Source: Federal Highway Administration

Advocates see road diets as one of the most cost-effective and fast infrastructure improvements with the biggest payoff. Safe, comfortable, pleasant street spaces can encourage more people to walk, bike, and take transit, as well as engage with their neighbors and community. Road diets can provide an opportunity to add shade trees and green space, which can help limit the urban heat island effect and reduce air pollution.

While road diets are not appropriate for all roads (they tend to work best on roads with 8,000 to 20,000 daily vehicle traffic), they can lead to significant drops in crashes, deaths, and injuries when deployed properly. Even without reducing the number of lanes, narrowing roadways and adding bike lanes or traffic calming measures such as medians or curb extensions can make crossing safer for pedestrians, encourage slower speeds, and, in the case of New York City, even improve traffic. Road diets benefit drivers because left-turn lanes can reduce delays, fewer lanes can mean easier entry for side-street traffic, and a reduced speed differential between different roadways can improve safety and the flow of traffic.

Road with raised median and bus lanes
Road diet on Luten Avenue in Staten Island, New York

The AARP, which promotes the interests of senior citizens, supports road diets because fast-moving roadways pose a heightened danger to elderly people who may have mobility issues or hearing or vision impairments. Shortening the distance pedestrians need to cross improves safety, particularly for people with mobility challenges. Traffic calming and amenities such as seating, curb extensions, and landscaping can also improve the comfort and beauty of a street.

One of the first road diets in the United States was installed in 1979 in Billings, Montana. Today, road diets are part of the toolbox for the Complete Streets movement, which advocates for reimagining streets as more than just thoroughfares for car traffic. Complete Streets projects shift the focus to pedestrians, bikes, and transit, centering safety, sustainability, and equity over vehicle throughput.

Like many bike lanes or other projects that threaten to reduce street parking or driving lanes, road diets encounter opposition from various community groups. Hurdles include complaints over lost traffic lanes and parking spaces, increased property values that can lead to displacement, and maintenance costs. Research shows that road diets rarely lead to a substantial drop in car traffic or a major increase in congestion. Data show that congestion on streets with less than 20,000 daily vehicle trips is not impacted by road diets. One study of a road diet in Northeast Los Angeles revealed that even though the change did not affect congestion, there was a disconnect between the reality of the project and local perception. For example, “merchants assume more customers drive than reflected in customer survey responses.”

In some cases, gaining support for road diets requires undoing decades of transportation planning that privileges speed above all else. Getting community buy-in can involve an intensive process that requires plentiful opportunities for community members to understand the proposed changes and contribute their ideas as well as follow-up campaigns after implementation that show the outcome of the project.

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