U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Contact: Rebecca Crowe at email@example.com, 202-507-3699
Operational Benefits for Motor Vehicles
Pedestrian and Bicyclist Benefits
Synergies and Trade-offs
The Road Diet Desk Reference is a resource to assist transportation agencies during their decision-making process in regards to considering, implementing, and evaluating Road Diet conversions. The information in the document is derived from the Road Diet Informational Guide.1
Four-lane undivided highways have a history of crashes as traffic volumes increase due to the inside lane being shared by higher-speed through vehicles and left-turning vehicles. One option for addressing this concern is a Road Diet.
A typical Road Diet is the conversion of an undivided four-lane roadway to a three-lane undivided roadway made up 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 uses, and/or parking.
The focus of roadway projects during the 1950s and 1960s was on system capacity expansion. Whenever and wherever traffic volumes on a section of road outgrew what a two-lane road could accommodate efficiently, the next step in roadway design in most cases was to increase the cross-section to four lanes. No engineering guidance during that period encouraged consideration of a three-lane alternative. Consequently, four-lane roadways became the norm throughout the country.
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
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.
A Road Diet can provide the following operational benefits:
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.
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.
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 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.
|Road Diet Feature||Primary/Intended Impacts||Secondary/Unintended Impacts|
||Increased property values||Could reduce parking, depending on design|
|Fewer Travel Lanes||Reallocate space for other uses||
|Two-Way Left Turn Lane||Remove left-turning traffic from through lane||Makes efficient use of limited roadway area||Could be difficult for drivers to access left turn lane if demand for left turns is too high|
|Pedestrian Refuge Island||Increased mobility and safety for pedestrians||Prevents illegal use of the TWLTL to pass slower traffic or access and upstream turn lane||May create issues with snow removal|
|Buffers (grass, concrete median, delineators)||Provide barriers and space between travel modes||
||Grass and delineator buffers will necessitate ongoing maintenance|
While Road Diets can improve safety and accommodate motorized and non-motorized transportation modes along a corridor, they may not be appropriate or feasible in all locations. There are many factors to consider before implementing a Road Diet. Agencies should consider the objective of the Road Diet, which could be one or more of the following:
Identifying the objective(s) will help determine whether the Road Diet is an appropriate alternative for the corridor that is being evaluated. Some example evaluative questions to answer when considering a Road Diet are shown:
|Roadway Function and Environment||
|Crash Types and Patterns||
|Level of Service||
|Pedestrian and Bike Activity||
|Frequent-Stop and/or Slow Moving Vehicles||
|Traffic Volumes and Patterns||
|Table adapted from Knapp, Welch, and Witmer, 19996|
According to the Delaware Valley Regional Planning Commission's Regional Road Diet Analysis Feasibility Assessment, "Education and outreach play a critical role in the success of a Road Diet. Many projects have demonstrated that public opposition can be strong in the early stages of a project. However, with committed stakeholders and an organized education and outreach program, the public can be better informed about the advantages and disadvantages of Road Diets."7
Agencies can also use the trial basis approach to appeal to communities where Road Diets may be feasible but are not embraced locally. During the trial basis time period, a series of before-and-after operational studies can be completed; some preliminary crash analysis can be performed; and surveys can be conducted among adjacent land owners, first responders, etc. If the trial yields positive results, consider implementing a more permanent Road Diet conversion. The trial basis approach is an effective way to demonstrate the safety countermeasure to a community.
As with any project development process, practitioners designing a Road Diet should take into account the principles and practices that guide design decisions, including geometric design and operational design. Common geometric and operational features or characteristics that should be considered during Road Diet design are:
On 55th Street in Chicago, the Road Diet design included parking-protected bike lanes and a shared lane at intersections for transit and bicycles
Once implemented, it is important to evaluate the effectiveness of the Road Diet. This typically occurs through studying pre- and post-installation crash data, operating speeds, and operational levels of service.
In addition to the basic vehicular operational and safety studies, other conversion impacts an agency may consider evaluating include:
Additional information on Road Diets is available on FHWA Office of Safety website: http://safety.fhwa.dot.gov/road_diets.
1. FHWA "Road Diet Informational Guide." November 2014.
2. FHWA "Evaluation of Lane Reduction 'Road Diet' Measures on Crashes." FHWA Report No. FHWA-HRT-10-053. (Washington, D.C: 2010)
3. Stout, Thomas B., Before and After Study of Some Impacts of 4-Lane to 3-Lane Roadway Conversions. March 2005.
4. Welch, T. The Conversion of Four Lane Undivided Urban Roadways to Three Lane Facilities. 1999.
5. FHWA Office of Planning, Environment, and Realty. "Livability Initiative" web page. Available at: https://www.fhwa.dot.gov/livability/
6. Knapp, K., T. Welch, J. Witmer. Converting Four-Lane Undivided Roadways to a Three-Lane Cross Section: Factors to Consider.
7. Delaware Valley Regional Planning Commission. Regional Road Diet Analysis Feasibility Assessment. 2008.