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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation
FHWA Highway Safety Programs

1. Road Diet Overview

What Is A Road Diet?

Diagram depicts how a road diet is configured when converting from a four-lane roadway to a three-lane roadway.
Typical Road Diet Basic Design

Four-lane undivided highways have a history of crashes as traffic volumes increase due to the inside lane being shared by higher-speed through vehicles and left-turning vehicles. One option for addressing this concern is a Road Diet.

A typical Road Diet is the conversion of an undivided four-lane roadway to a three-lane undivided roadway made up of two through lanes and a center two-way left-turn lane (TWLTL). The reduction of lanes allows the roadway cross section to be reallocated for other uses such as bike lanes, pedestrian refuge islands, transit uses, and/or parking.

History of Road Diets

The focus of roadway projects during the 1950s and 1960s was on system capacity expansion. Whenever and wherever traffic volumes on a section of road outgrew what a two-lane road could accommodate efficiently, the next step in roadway design in most cases was to increase the cross-section to four lanes. No engineering guidance during that period encouraged consideration of a three-lane alternative. Consequently, four-lane roadways became the norm throughout the country.