Road Diets: How and Why | FHWA

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Four-lane undivided highways have a history of relatively high crash rates as traffic volumes increase and as the inside lane is shared by higher-speed through traffic and left-turning vehicles.

One option for addressing this safety concern is a “Road Diet.” A Road Diet involves converting an existing four-lane undivided roadway segment to a three-lane segment consisting 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 stops, or parking

Benefits of Road Diet installations may include:

• An overall crash reduction of 19 to 47 percent.

• Reduction of rear-end and left-turn crashes through the use of a dedicated left-turn lane.

• Fewer lanes for pedestrians to cross and an opportunity to install pedestrian refuge islands.

• The opportunity to install bicycle lanes when the cross-section width is reallocated.

• Reduced right-angle crashes as side street motorists must cross only three lanes of traffic instead of four.

• Traffic calming and reduced speed differential, which can decrease the number of crashes and reduce the severity of crashes
if they occur.

• The opportunity to allocate the “leftover” roadway width for other purposes, such as on-street parking or transit stops.

• Encouraging a more community-focused, “Complete Streets” environment.

• Simplifying road scanning and gap selection for motorists (especially older and younger drivers) making left turns from or
onto the mainline.

A Road Diet can be a low-cost safety solution, particularly in cases where only pavement marking modifications are required
to make the traffic control change. In other cases, the Road Diet may be planned in conjunction with reconstruction or simple
overlay projects, and the change in cross section allocation can be incorporated at no additional cost.

Link: 

Road Diets: How and Why | FHWA