Reconfiguring roadways, also referred to as a “road diet,ˮ can be a low-cost safety solution that makes space for different travel modes in addition to motor vehicle traffic. New guidance gives local engineers an initial indication of whether a road conversion would result in a loss of capacity given the roadway motor vehicle traffic demand.
Typically a four-lane, undivided roadway is converted to a three-lane road (a 4-3 conversion), with two directional motor vehicle lanes separated by one bidirectional turning lane. The extra space gained may be used for a bike, bus or parking lane or a shoulder; to widen sidewalks; to add safety features such as curb extensions and medians; or to add green infrastructure.
Increasing in popularity and a proven safety countermeasure, the 4-3 configuration has a crash reduction rate of up to 47%, according to the Federal Highway Administration, resulting from a reduction in conflict points and motor vehicle speeds.
“While a detailed evaluation of design elements should precede a 4-3 road conversion, the guidance tables we produced allow engineers to make an initial assessment of the potential impacts to level of service,” said Gary Davis, professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
Some agencies may be hesitant to convert roads with an average daily traffic (ADT) volume of more than 15,000 vehicles because of unknowns about the impacts to motor vehicle level of service and driver delay, and the resulting behavior on a three-lane road.
The focus of road conversion research has been both on safety benefits and ADT volume. Recommendations range from an ADT of 10,000 vehicles (to avoid declines in motor vehicle capacity) to 25,000 vehicles. The Local Road Research Board (LRRB) wanted a better understanding of road conversions and the impact on motor vehicle capacity.
What Was Our Goal?
The project’s goal was to develop guidance for determining when a 4-3 conversion might decrease the roadway’s level of service for drivers.
What Did We Do?
An extensive literature review identified the operational effects and gaps in knowledge of 4-3 conversions. Researchers, in consultation with the Technical Advisory Panel (TAP), reviewed numerous design features considered when converting roads, such as pedestrian refuge islands, existing road width and intersection control. From 36 design features, the TAP chose mainline motor vehicle traffic volume as the top priority for further investigation, focusing on the impacts to motor vehicle level of service of a 4-3 conversion when ADT is higher than 15,000 motor vehicles.
Three roads in Minnesota served as the examples for a factorial simulation study: one previously converted and two candidates for conversion. These roads had motor vehicle traffic volumes between 10,000 and 25,500 ADT and a variety of intersection configurations. Four levels of motor vehicle traffic volume on the mainline, three levels of crossroad motor vehicle traffic volume at signalized intersections and three at unsignalized intersections created 36 demand combinations. Assumptions of the percentage of vehicles that would turn at signalized and unsignalized intersections factored into the analysis, as well as peak hour motor vehicle traffic levels. For all the simulations, average motor vehicle delays at intersections and average motor vehicle speeds in the corridors were computed.
The simulation results were summarized in tables showing level of service ratings, classified as A through F or X (both assumed unacceptable), for intersection delays and speed in different demand combinations. While level of service may have been unacceptable in industry terms, other factors such as safety, multimodal mobility, and political or community support may have played a role in decision-making to convert from a four-lane to a three-lane road.
Finally, investigators developed eight intersection configurations to represent the study’s results and serve as example intersections with identified acceptable or unacceptable motor vehicle level of service changes for different motor vehicle traffic volumes.
What Did We Learn?
In general, guidelines based solely on ADT can be misleading. ADT must be considered in tandem with motor vehicle traffic demand on the crossroads, hourly volumes and other factors. Signalized intersections appeared to be the main constraint if the minor road approaches had insufficient capacity at the intersections.
For all three roads, simulations showed that for mainline motor vehicle traffic at both signalized and unsignalized intersections, the reduction in motor vehicle level of service after a conversion was acceptable at motor vehicle volumes corresponding to 15,000 ADT or greater.
“The motor vehicle traffic volume thresholds identified in this research will make the 4-3 conversion decision-making process more efficient by reducing the need for traffic analysis at low ADT thresholds. The guidance will be particularly helpful for agencies that may not have the resources to do a more in-depth traffic analysis,” said KC Atkins, senior transportation engineer, Hennepin County Public Works.
For minor road motor vehicle traffic at unsignalized intersections, however, the loss of motor vehicle level of service was unacceptable as the major road ADT increased from 15,000 to 18,000. Regarding the minor road approaches in two of the locations, substantial deterioration in motor vehicle level of service tended to occur when mainline motor vehicle traffic ADT exceeded 15,000. In one location, however, acceptable levels of service were possible up to 18,000 ADT as long as the minor road ADT didn’t exceed 20% of the mainline motor vehicle volumes. The third location showed acceptable motor vehicle level of service on minor roads at higher demands, likely due to a larger capacity on the minor roads at the intersections. All of the scenarios were modeled using peak hour motor vehicle volumes. Motor vehicle level of service may be better and/or vary at other times of the day.
The eight example intersection configurations include tables rating the initial indications of feasibility based on peak motor vehicle traffic flow. Configurations vary by signalized or two-way stop signs, number of through lanes on the mainline and crossroads, presence or absence of separate turn lanes, and intersection geometry.
What’s Next?
When considering road conversions, local agencies will continue to consider many factors in addition to the primary goal of safety. The guidance produced in this project will provide an initial indication of the impacts of a 4-3 conversion related to driver delay given ADTs on the mainline and crossroads. Additional factors, such as crash reduction, Complete and Green street goals, comfort for all users, stakeholder input and impacts to driver delay, will all play a role in 4-3 conversion decision-making.
Crossroads 
The improved safety and multi-modal use should win out against ADT issues.