6. Conclusion

Road Diet Informational Guide

The most common 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). Road Diets can be used to address safety concerns with four-lane, undivided highways associated with relatively high crash rates as traffic volumes increase and as the inside lane is shared by high-speed and left-turning vehicles. The reduction of lanes allows the roadway cross section to be reallocated for other uses such as bike lanes, pedestrian refuge islands, or parking.⁹³

The benefits of Road Diets include improved safety, traffic calming, and the opportunity to repurpose segments of the roadway to create on-street parking, bike lanes, or transit stops. Based on the history of safety studies presented in this guide, practitioners can expect a crash reduction of 19 to 47 percent after installing a Road Diet. Variables include pre-installation crash history, installation details, and the urban or rural nature of the corridor.

When planning for or designing a Road Diet, it is important to be aware of the opportunities and potential drawbacks that one type of treatment may have on other travel modes. When deciding whether a particular element is appropriate for an individual street, or whether a Road Diet in general is appropriate, the surrounding context should be taken into consideration, including the extended roadway network. Each decision will have to be made on a case-by-case basis and will depend on the desired operation of the street in question. Consider coordinating with non-motorized advocacy groups, transit agencies, freight stakeholders, and emergency responders as necessary to understand their needs through the design of a Road Diet. Common feasibility factors include the following:

The need for improved safety for all road users
A desire to incorporate context sensitive solutions and Complete Streets features
Operational considerations, such as:
- Whether the existing roadway operates as a de facto three-lane roadway o The need for reduced speed or traffic calming
- Average daily traffic
- Multimodal level of service
- Peak hour volumes and peak direction o Turning volumes and patterns
- The presence of slow-moving or frequently stopping vehicles, such as transit, curb-side mail delivery, and others
A desire to better accommodate bicycles, pedestrians, and transit service
Right-of-Way availability and cost
The existence of parallel roadways, parallel parking, and at-grade railroad crossings.
Public outreach, public relations, and political considerations.

Geometric and operational design features are important during the design of a Road Diet reconfiguration. Geometric design includes identifying details of the project in plan, profile, and cross-section. Important issues include overarching principles of design, design controls, design elements, cross-section design, intersection design, and consideration for all road users. The following list represents just a few of the geometric design considerations one should consider during the Road Diet design phase:

Road functional classification
Design vehicles, driver characteristics, and presence of non-motorized users
Corridor sight distance, grade, horizontal curvature, and superelevation
Cross-sectional elements, such as lane widths, cross slope, presence of curbs or shoulders, access management, and presence of on-street parking or bus turnouts
Intersection design elements, such as alignment and profile of intersection approaches and intersection sight distance.

Practitioners must make a number of operational decisions as well, including cross-section allocation, pedestrian accommodations, signalization changes, transition points, and pavement marking and signing. As with any roadway treatment, data analysis and engineering judgment are required to determine whether a Road Diet is the most appropriate alternative in a given situation.

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 level of service. Additional tools and methods, both specific and general, should be used to evaluate conversion impacts, including the following:

Safety (e.g., crash frequency/type/severity, pedestrian-vehicle conflicts)
Travel speeds (e.g., average travel time, mean/85th percentile speeds, percent of vehicles traveling at high speeds)
Arterial level of service, delay, queuing
Intersection operations (e.g., turn delays; volume/capacity ratios; signal operations)
Traffic volume, including diversion to parallel routes
Corridor operations including transit operations and similar, the two-way left-turn lane operations, and the ability to evaluate “stopped traffic” in one through lane
Pedestrian and bicycle safety and operations
Economic impact / livability.

In conclusion, a Road Diet can be a low-cost safety solution when the installation is coordinated with scheduled pavement marking modifications or planned in conjunction with reconstruction or simple overlay projects. Road Diets have the potential to solve a number of traffic operations and safety issues and to incorporate non-motorized users when applied at the most appropriate locations.

previous | next