To identify and assess potential mobility benefits from alternative design and control strategies at freeway merge and diverge locations, researchers conducted microsimulation studies of freeway locations from different states in 2017-2018. Strategies assessed included both geometric modifications as well as simulated applications of the following control strategies:
- Mainline Metering - dynamic signal control of mainline traffic under congested low speed operations
- Coordinated Ramp Metering - feedback-based ramp metering control using both upstream and downstream data, such as the Heuristic Ramp Meeting Coordination (HERO) algorithm
- Speed Optimization via dynamic traffic calming devices - dynamic variable speed limit signs or other measures to achieve medium free-flow speeds (e.g., 30 miles per hour) near ramp locations
Several sites experiencing freeway merge / diverge deficiencies and speed-change areas resulting in capacity reduction were selected for analysis of these control strategies, including:
- I–35, KS (Mainline Metering)
- I–15, CA (Coordinated Ramp Metering)
- I–394, MN (Speed Optimization)
The research team used relatively simple networks and ran a large number of scenarios based on factorial design of system parameters such as demand volume and distance. After calibrating datasets to existing conditions, simulation data were then analyzed using advanced statistical methods to understand the effects of each scenario. Key output measures included throughput, average traffic delay, total travel time, average segment speed and average network-wide speed.
- Mainline Metering: With a mainline metering design that alternately stops mainline and ramp traffic, the simulation analyses showed delay reductions up to 21 percent. However, this potential mainline metering strategy would need further investigation for practicality and cost-effectiveness.
- Coordinated Ramp Metering: The HERO coordinated adaptive ramp metering approach tested in this study achieved an improvement of two percent in mainline travel time over the conventional ramp metering algorithm and five percent over no ramp metering. As measured by the average weighted harmonic speed on mainline and ramps, coordinated ramp metering increased speeds by one percent over conventional ramp metering and two percent over no ramp metering. Researchers noted that adequate calibration was important to attain good results.
- Speed Optimization: Speed optimization benefits were observed for some weaving and merge conditions if there is a real-time ability to detect zipper conditions near capacity when speed optimization benefits are greatest. In one weaving section simulation, average upstream vehicle speeds increased from 20.4 to 47.3 miles per hour (mph) when free-flow speed was reduced from 65 to 50 mph. In a merge area simulation, reducing free-flow speed from 65 to 45 miles per hour resulted in improvements in average upstream vehicle speed from 7.5 to 37.1 mph. However, the benefits of speed harmonization were weaker for diverge conditions, on the order of 2 to 6 percent.