A road diet, also called a lane reduction or road rechannelization, is a technique in transportation planning whereby a road is reduced in number of travel lanes and/or effective width 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 users in the context of two-way streets with 2 lanes in each direction. The road diet reduces this to 1 travel lane in each direction. The freed-up space is then used to provide any or several of the following features:
- (Wider) footpaths/sidewalks
- (Wider) landscaping strips
- Cycle lanes, on one or both sides of the road
- Wider lane widths on remaining traffic lanes (if previously unsafely narrow to allow four lanes)
- A two-way turn lane / flush traffic median for turning traffic
- A reversible centre lane
If properly designed, traffic does not divert to other streets road after a road diet, because the road previously provided excessive capacity. In other scenarios, reduction of traffic (either local traffic or overall traffic) are 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.
Lane diets 
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. National lane width guidelines are offered as a range (within the United States), and lane diets fall within this range.
Arguments for and against 
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.
United States 
There are perhaps over 20,000 road diets in the United States, with another 500-1,000 being conducted each year.
The city in North America with the greatest number of road diets (over 50) is San Francisco, with 4 to 6 road diets added each year. The city with the greatest number of road diets, per capita, is Hartford, Connecticut (12). Retail merchants in Seattle are now some of the strongest proponents for these projects, since reduced travel speeds allow for easier and safer parking, improve store access and boost overall walking and livability conditions in neighborhoods, all of which lead to improved commerce.
Palo Alto, California has studied reducing the number of vehicle travel lanes to reduce traffic impacts on some of its busiest streets since adopting a new Comprehensive Plan in 1998. Design plans were made to reduce the total number of travel lanes from four to two on Embarcadero Rd and Middlefield Rd in the early 2000s, but were never brought to the city council for approval. Lane reductions were approved and then implemented on Charleston Rd in 2006, Arastadero Rd in 2010, and Deer Creek Rd in 2011.
The city of Seattle has implemented Road Diets over the past few years, and was able to boost its ranking from 6th to 2nd highest traffic rates in the US.
- Evaluation of Lane Reduction "Road Diet" Measures on Crashes, Turner-Fairbank Highway Research Center, June 2010, http://www.fhwa.dot.gov/publications/research/safety/10053/index.cfm
- Burden, Dan and Peter Lagerwey, "Road Diets: Fixing the Big Roads", Walkable Communities, March 1999, http://www.walkable.org/assets/downloads/roaddiets.pdf
- Evaluation of Lane Reduction "Road Diet" Measures and Their Effects on Crashes and Injuries, Turner-Fairbank Highway Research Center, http://www.tfhrc.gov/safety/hsis/pubs/04082/index.htm
- Becky Trout (May 14, 2008). "Charleston Road safer, better after lane changes, council agrees". Palo Alto Weekly. Retrieved October 8, 2011.
- "City of Palo Alto – Community Meeting Notice, Arastradero Road Restriping – Trial Project". October 14, 2010. Retrieved October 8, 2011.
- Gennady Sheyner (July 29, 2011). "Palo Alto speeds ahead with traffic-calming projects". Palo Alto Weekly. Retrieved October 8, 2011.
- 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