A road diet, also known as a lane reduction or rechannelization, is a transportation planning and engineering technique used to improve safety, mobility, and livability on urban and suburban roadways. It involves reconfiguring an existing road to provide fewer and/or narrower travel lanes for vehicles, reallocating the freed-up space for other modes of transportation or community uses.
Purpose and Goals
The primary objectives of a road diet typically include:
- Improving Safety: By reducing the number of through lanes and consolidating turning movements into a dedicated center turn lane, road diets can significantly reduce conflict points between vehicles, as well as between vehicles and pedestrians or cyclists. This often leads to a measurable reduction in crashes, especially head-on and turning collisions. Reduced vehicle speeds, often a byproduct of fewer lanes, also contribute to safety.
- Enhancing Multimodal Mobility: The reallocated space can be used to create or widen sidewalks, add dedicated bicycle lanes, or improve public transit facilities, thereby encouraging walking, cycling, and public transportation.
- Managing Vehicle Speeds: Reducing the perception of excessive road width or capacity can subtly encourage drivers to reduce their speed, contributing to a safer and more comfortable environment for all road users.
- Improving Community Livability and Economic Vitality: By making streets safer and more comfortable for pedestrians and cyclists, road diets can foster more vibrant public spaces, encourage local business activity, and enhance the overall quality of life in adjacent neighborhoods.
- Optimizing Roadway Efficiency: While reducing lanes, a well-designed road diet can often maintain or even improve traffic flow, especially by providing a continuous left-turn lane that removes turning vehicles from through-traffic lanes.
Common Configurations
The most common road diet conversion involves transforming a four-lane undivided roadway (two lanes in each direction) into a three-lane roadway, typically consisting of:
- One general-purpose lane in each direction.
- A continuous two-way left-turn lane (TWLTL) in the center.
The remaining space is then reallocated. Potential reallocations include:
- Dedicated bicycle lanes (one in each direction).
- Wider sidewalks or planting strips.
- On-street parking.
- Medians or other landscaping features.
Other configurations might include converting a six-lane road to five lanes, or removing one or more lanes to create a dedicated bus lane, but the four-to-three lane conversion is the most prevalent and well-studied.
Considerations and Efficacy
Road diets are most effective on urban and suburban arterial roads with moderate traffic volumes (typically up to 20,000-25,000 Average Daily Traffic, though this can vary based on specific conditions). They are generally not suitable for high-speed, high-volume freeways or major arterials where maintaining throughput for very high traffic volumes is the overriding priority.
Concerns often arise regarding potential traffic congestion. However, studies by transportation agencies like the Federal Highway Administration (FHWA) have frequently shown that well-implemented road diets often have minimal impact on overall traffic throughput for the design traffic volumes, and can even improve it by streamlining left-turn movements and reducing conflicts. The safety benefits, in particular, are well-documented, with many projects reporting significant reductions in crashes.
History
The concept of road diets gained prominence in the United States in the 1990s as part of broader movements towards traffic calming, complete streets, and urban revitalization. While similar principles may have been applied earlier, the term "road diet" became widely adopted to describe this specific rechannelization technique, emphasizing its goal of "slimming down" a road's vehicular capacity to create a more balanced and safer environment for all users.