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0. Executive Summary

About this Information

This information paper on a 4 lane to 3 lane road diet was created by the Federal Highway Administration for the purpose of reporting the findings from their research. It is copied here with minor adjustments to show photos directly relating to Alameda de las Pulgas, that short section from Santa Cruz Ave to just before Avy Ave in West Menlo Park, California.  (original FHWA document).

Road Diet Informational Guide


Executive Summary

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.

Road Diet Definition

Conversion of a four-lane undivided road to a three-lane undivided road made up of two through lanes and a center two-way left-turn lane.

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 (see Figure 1).1

Figure 1. Road Diet
Side-by-side photos depict a roadway before a road diet application and the same roadway after the road diet application, which turned it from a four-lane roadway to a three-lane roadway with a center two-way left-turn lane.
Photo Credit: Virginia Department of Transportation

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.

Geometric and operational design features should be considered during the design of a Road Diet. Intersection turn lanes, traffic volume, signing, pavement markings, driveway density, transit routes and stops, and pedestrian and bicyclist facilities should be carefully considered and appropriately applied during the reconfiguration for appropriate Road Diet implementation.2 As with any roadway treatment, determining whether a Road Diet is the most appropriate alternative in a given situation requires data analysis and engineering judgment.

Once installed, it is important to monitor the safety and operational effects of the roadway, and to make changes as necessary to maintain acceptable traffic flow and safety performance for all road users. Evaluation of Road Diets will provide practitioners the information needed to continue implementing reconfiguration projects in their jurisdictions.

Table 1. Problems Potentially Correctable by Road Diet Implementation
SafetyRear-end crashes with left-turning traffic due to speed discrepanciesRemoving stopped vehicles attempting to turn left from the through lane could reduce rear-end crashes
Sideswipe crashes due to lane changesEliminating the need to change lanes reduces sideswipe crashes
Left-turn crashes due to negative offset left turns from the inside lanes.Eliminating the negative offset between opposing left-turn vehicles and increasing available sight distance can reduce left-turn crashes
Bicycle and pedestrian crashesBicycle lanes separate bicycles from traffic; pedestrians have fewer lanes to cross and can use a refuge area, if provided
OperationalDelays associated with left-turning trafficSeparating left-turning traffic has been shown to reduce delays at signalized intersections
Side street delays at unsignalized intersectionsSide-street traffic requires shorter gaps to complete movements due to the consolidation of left turns into one lane
Bicycle operational delay due to shared lane with vehicles or sidewalk use.Potential for including a bike lane eliminates such delays
OtherBicycle and pedestrian accommodation due to lack of facilitiesOpportunity to provide appropriate or required facilities, increasing accessibility to non-motorized users
AestheticsProvisions can be made for traversable medians and other treatments
Traffic calmingPotential for more uniform speeds; opportunity to encourage pedestrian activity
Adapted from Kentucky Transportation Center's Guidelines for Road Diet Conversions3

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