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Criteria and Guidelines for Three-Lane Road Design and Operation

Status:  Complete
Report Date:  03/13/2023


A 4-3 conversion, also known as a “road diet,” reconfigures a four-lane undivided roadway into a three-lane road. Typically, one lane for each direction is separated by a bidirectional turning lane. The extra space gained may be used for a bike, bus or parking lane or a shoulder; to widen sidewalks; to add safety features such as curb extensions and medians; or to add green infrastructure. While this configuration can have safety benefits, losing two motor vehicle travel lanes could result in a more congested road or a decreased motor vehicle level of service during certain hours of the day. New guidance will help local transportation agencies predict when the impacts to level of service may be unacceptable, and balance this data point with others in decision-making. This project focused on average daily traffic (ADT) for a road recommended for conversion to determine whether the conversion would adversely impact the motor vehicle capacity along the road. Running numerous computer simulations on a variety of road, intersection and traffic scenarios, researchers found that in addition to the ADT of the road before conversion, the capacity of the crossroads and controls at intersections may impact whether a conversion could result in a reduction in motor vehicle level of service. Guidance is available for local engineers to make initial assessments about the feasibility of a 4-3 conversion.

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