A road diet, also called a lane reduction or road rechannelization, is a technique in transportation planning whereby the number of travel lanes and/or effective width of the road is reduced in order to achieve systemic improvements.
A typical road diet technique is to reduce the number of lanes on a roadway cross-section. One of the most common applications of a road diet is to improve safety or provide space for other modes of travel. For example, a two-way, four lane road might be reduced to one travel lane in each direction. The freed-up space is then used to provide or enhance some of the following features:
- Adding or widening of footpaths/sidewalks
- Adding or widening of boulevards (landscaping strips)
- Adding cycle lanes on one or both sides of the road
- Adding reserved tramtracks, usually in the middle of the road
- Widening remaining traffic lanes (if previously unsafely narrow to allow four lanes)
- Adding a center turn lane / flush traffic median for turning traffic
- Adding a reversible center lane
- Conversion of the rightmost or leftmost travel lane to a breakdown lane (The Lodge Freeway in metro Detroit is an example of this after I-96, the Jeffries freeway was built.)
If properly designed, traffic does not divert to other streets after a road diet, because the road previously provided excessive capacity. In other scenarios, reduction of traffic (either local traffic or overall traffic) is intended in the scheme. Road diets are usually successful on roads carrying fewer than 19,000 vehicles per day. Road diets can succeed at volumes up to about 23,000 vehicles per day. However, more extensive reconstruction is needed. Examples include replacing signals with roundabouts, traffic calming on parallel streets to discourage traffic from diverting away from the main road, and other means to keep traffic moving smoothly and uniformly.
In a lane diet, the width of a lane is decreased to reduce vehicle speeds and yield space for other use. Typically vehicular travel lane widths are narrowed to no more than ten feet, and left turn (in countries where drivers use the right-hand side of the road) storage lanes to nine or ten feet. Resulting space can be applied to pedestrian refuges, medians, sidewalks, shoulders, parking, or bike lanes. Lane width guidelines in the United States are offered as a range and lane diets fall within this range.
In many cases of 4-lane roadways with double-yellow, this can present a hazard for oncoming traffic in the "passing lane", as well as left-turners. Sometimes 4-lane with double-yellow is upgraded to 5-lane with center turn lane since that practice is more of an upgrade than a "diet" like 3-lane with center turn lane is.
Proponents of road diets generally believe key benefits include lower vehicular speeds, reduced crash rates, and improved pedestrian safety. Other benefits of road diets include promoting better land use, reducing induced traffic, promoting greater driving attentiveness, and promoting cycling through the addition of bicycle lanes. Providing dedicated left turn (in countries that drive on the right-hand side of the road) lanes at intersections can improve vehicular safety and can enable efficiency gains along the roadway.
Researchers have found that road diets can be expected to reduce overall crash frequency by 19% to 43%, with the higher crash reductions occurring in small urban areas than in metropolitan areas.
A leading proponent of road diets is former Florida Bicycle and Pedestrian Coordinator Dan Burden, who now helms the Walkable and Livable Communities Institute. Burden and Peter Lagerwey published an article on the topic in 1999 noting that in two cases, 95% of residents were initially opposed to roadway constriction. Additional studies have shown that road diets often achieve these positive effects without reducing traffic volumes.
Among studies now showing that there are safety improvements to driving when lane widths are reduced include a recent report by the National Cooperative Highway Research Program (NCHRP) and work analyzing traffic safety for 14 years in all 50 states by Robert B. Noland.
Road diets can negatively affect the speed and reliability of transit service operating on the roadway, particularly if bus stops are located in pullouts and traffic queues delay buses attempting to re-enter traffic. Constructing bus bulbs can mitigate these effects though this feature results in delays for other vehicles.
Not all multi-lane arterials are good candidates for road constriction. Added congestion can outweigh benefits if vehicle traffic volumes exceed the capacity of the three-lane roadway. This threshold is approximately 20,000 vehicles per day. Burden notes additional characteristics of better candidates for vehicle lane removal.
Other concerns regard public safety; police, fire and ambulances may be slowed and if an evacuation is ordered, the evacuation will be slower.
Among American cities, San Francisco has completed the most road diet projects, over 40 since the late 1970s. Valencia Street, which was reduced from four to two travel lanes with a center turn lane and bike lanes added in March 1999, has become a national model for traffic engineers of the common "4-to-3 lane" road diet type.
San Jose, California has implemented several road diets since November 2011, when the City Council unanimously adopted its "Envision 2040" General Plan, which calls for road diets on streets with excess vehicle capacity "to provide wider sidewalks, bicycle lanes, transit amenities, and/or landscaping". Road diets were completed on 3rd, 4th, 10th, and 11th streets in August 2012, and on Hedding Street in July 2013.
Palo Alto, California has studied reducing the number of travel lanes to improve safety on some of its busiest streets since adopting a new Comprehensive Plan in 1998. Design plans were made for road diets on Embarcadero Road and Middlefield Road in the early 2000s, but were never brought to the city council for approval. 75-yr-old local resident Ming Yuan Zuo was killed by a pickup truck driver while walking across Embarcadero Road in January 2013. Lane reductions were approved and then implemented on Charleston Road in 2006, Arastadero Road in 2010, and Deer Creek Road in 2011.
- Turner-Fairbank Highway Research Center (June 2010). "Evaluation of Lane Reduction 'Road Diet' Measures on Crashes". Federal Highway Administration. FHWA-HRT-10-053.
- Burden, Dan & Lagerwey, Peter (March 1999). "Road Diets: Fixing the Big Roads" (PDF). Walkable Communities.
- Turner-Fairbank Highway Research Center (March 2004). "Evaluation of Lane Reduction 'Road Diet' Measures and Their Effects on Crashes and Injuries". Federal Highway Administration. FHWA-HRT-04-082.
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- Trout, Becky (14 May 2008). "Charleston Road Safer, Better after Lane Changes, Council Agrees". Palo Alto Weekly. Retrieved 8 October 2011.
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- Project for Public Spaces: Rightsizing Streets Guide
- Summary Report: Evaluation of Lane Reduction "Road Diet" Measures and Their Effects on Crashes and Injuries
- Applying the Road Diet for Livable Communities (2005 slide presentation with photos)
- The Safety and Operational Effects of "Road Diet" Conversions in Minnesota (2007)
- Streetscape Improvements: Enhancing Urban Roadway Design (updated 2007)
- Road Space Reallocation Roadway Design and Management To Support Transportation Alternatives (updated 2007)
- Interview with Dan Burden (2005)
- Seattle's Stone Way Report (2010)
- Walkable 101: Road Diets
- StreetFilms: Road Diet
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