Improved Safety

Road Diets reduce vehicle-to-vehicle conflicts that can contribute to rear-end, left-turn, and sideswipe crashes by removing the four-lane undivided inside lanes that serve both through and turning traffic. Studies indicate a 19 to 47 percent reduction in overall crashes when a Road Diet is installed on a previously four-lane undivided facility as well as a decrease in crashes involving drivers under 35 years of age and over 65 years of age. ^2,3

An analysis of 45 Road Diet sites in California, Iowa, and Washington showed a 29 percent reduction in total crashes²

(Adapted from Welch, 1999)

Mid-Block Conflict Points for Four-Lane Undivided Roadway and Three-Lane Cross Section (Adapted from Welch, 1999)⁴

Road Diets also improve safety by reducing the speed differential. On a four-lane undivided road, vehicle speeds can vary between travel lanes, and drivers frequently slow or change lanes due to slower or stopped vehicles (vehicles stopped in the left lane waiting to turn left). Drivers may also weave in and out of the traffic lanes at high speeds. In contrast, on three-lane roads with TWLTLs the vehicle speed differential is limited by the speed of the lead vehicle in the through lane, and the left-turning vehicles are separated from the through vehicles. Thus, Road Diets can reduce the vehicle speed differential and vehicle interactions, which can reduce the number and severity of vehicle-to-vehicle crashes.

Operational Benefits for Motor Vehicles

A Road Diet can provide the following operational benefits:

• Separating Left Turns. Separating left-turning traffic has been shown to reduce delays at signalized intersections.
• Side-street Traffic Crossing. Side-street traffic can more comfortably enter the mainline roadway because there are fewer lanes to cross. This can reduce side-street delay.
• Speed Differential Reductions. The reduction of speed differential due to a Road Diet provides more consistent traffic flow and less "accordion-style" slow-and-go operations along the corridor.

On some corridors the number and spacing of driveways and intersections can lead to a high number of turning movements. In these cases, four-lane undivided roads can operate as de facto three-lane roadways. The majority of the through traffic uses the outside lanes due to the high number of left-turning traffic in the inside shared through and left-turn lane, in which a conversion to a three-lane cross section may not have much effect on operations.

In Santa Monica, California, speeding, crash history, and high pedestrian activity led to a Road Diet along Ocean Park Avenue.

Pedestrian and Bicyclist Benefits

Road Diets can be of particular benefit to non-motorized road users. They reallocate space from travel lanes—space that is often converted to bike lanes or sidewalks, where these facilities were lacking previously. These new facilities can have a tremendous impact on the mobility and safety of bicyclists and pedestrians. Even the most basic Road Diet has benefits for pedestrians and bicyclists, regardless of whether specific facilities are provided for these modes. As mentioned above, the speed reductions that are associated with Road Diets lead to fewer and less severe crashes. The three-lane cross section also makes crossing the roadway easier for pedestrians, as they have fewer travel lanes to cross and are exposed to moving traffic for a shorter period of time. Incorporating a pedestrian refuge island – a raised island placed on a street to separate crossing pedestrians from motor vehicles – makes crossing the roadway even shorter and less complicated. Pedestrians only have to be concerned with one direction of travel at a time.

Livability Benefits

Added to the direct safety benefits, a Road Diet can improve the quality of life in the corridor through a combination of bicycle lanes, pedestrian improvements, and reduced speed differential, which can improve the comfort level for all users. Livability is, "about tying the quality and location of transportation facilities to broader opportunities such as access to good jobs, affordable housing, quality schools, and safer streets and roads."⁵

Synergies and Trade-offs

Synergies between improvements for one mode and their impact on another have been discovered with the implementations of Road Diets. The following table shows examples of how some primary features of Road Diet installations may have both positive and negative secondary (or unintended) impacts.