Commonway
Version V. 2026.09.03

Four-to-three lane road diet

RFT-CDR-01 Corridor

Four-to-three lane road diet

Converts an existing four-lane undivided roadway (two through lanes each direction) into three lanes: one through lane each direction plus a center two-way left-turn lane, reclaiming the freed width for other uses.

#Overview

This is the classic road diet. A road built with more through capacity than its traffic actually needs gets restriped, sometimes with minimal construction, to remove one lane of excess capacity and separate through traffic from left-turning traffic, which today shares a lane and creates rear-end and sideswipe conflicts. The reclaimed width typically becomes a bike lane, wider sidewalk, on-street parking, or landscaping.

#Where this leads: Target patterns

Default (Standard Collector) (COL-DEF-01), Default (No Parking, No Direct Access) (ART-DEF-01), Downtown Thoroughfare (COL-DTH-07)

#Applicability and thresholds

Most effective below roughly 19,000 vehicles per day; can succeed up to about 23,000 to 25,000 with careful signal coordination, though more extensive redesign is typically needed above the lower threshold. Corridors with closely spaced signals need timing coordination to avoid queuing impacts at adjacent intersections.

#Evidence and effectiveness

FHWA reports an overall crash reduction of 19 to 47 percent for four-to-three lane road diets, driven by separating through and turning movements and reducing the speed variance between lanes that a four-lane cross-section otherwise creates.12

#Getting it built

Can move through the full construction spectrum. A road diet is frequently implemented as a low-cost restriping project, sometimes opportunistically bundled with routine repaving, making it one of the more common demonstration- and pilot-stage strategies before becoming permanent. See Construction Methodology & The Build Spectrum.

An Intersection Control Evaluation is not typically required unless the road diet is paired with a change in intersection control along the corridor. See Decision & Evaluation Processes.

#Who can move this

Usually a city or county public works or transportation department decision, often achievable without new legislation since it can be executed as a maintenance restriping project. See Legal & Planning Mechanisms and Funding & Financing Mechanisms.

Five-plus lane reallocation (RFT-CDR-02) (the equivalent approach for wider roadways), Lane diet (RFT-CDR-03) (a lighter-touch alternative that narrows lanes without removing one)

#References

  1. Legal / regulatory Federal Highway Administration, "Road Diets (Roadway Reconfiguration)," Proven Safety Countermeasures, FHWA-SA-12-013.
  2. Evidence-based Federal Highway Administration, Highway Safety Information System, "Evaluation of Lane Reduction 'Road Diet' Measures and Their Effects on Crashes and Injuries," 2010.
  3. Precedent-based NACTO Urban Street Design Guide, "Neighborhood Main Street."

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