Mitigating Oversaturation With Cooperative Automated Driving Systems: Applying Bundled Speed Harmonization, Cooperative Adaptive Cruise Control, and Cooperative Merging Applications to Managed Lane Facilities
Cooperative automated driving systems use radar and other communication systems to control the throttle and brake systems of vehicles to operate more effectively, with shorter headways and improved response to other vehicles. Federal Highway Administration (FHWA) researchers conducted a study to evaluate the potential of cooperative automated driving systems in reducing traffic congestion and increasing roadway capacity on freeways. The proof-of-concept tests were conducted using both microscopic traffic simulation with a hypothetical managed lane freeway system and field testing on the I-95 Express Lanes in Virginia. The study focused primarily on Society of Automation Engineers (SAE) Level 1 driving automation concepts that require driver engagement and steering at all times.
A microscopic simulation model was first used to assess how the Integrated Highway Prototype (IHP) systems might improve traffic performance on a simple freeway network, where vehicles enter a one-lane managed lane facility, additional traffic enters from an entrance ramp, and the combined traffic proceeds to the end of the managed lane facility. The IHP systems integrate three automation applications:
- speed harmonization
- vehicle platooning
- cooperative merge
The simulations included varying percentages of vehicles equipped with IHP systems. Field tests were then conducted using FHWA research vehicles on the I-95 Express Lanes in Virginia. The field tests were initially conducted with the roadway closed to traffic and then repeated in light traffic conditions. Since the testing was conducted primarily while the road was closed to other traffic, the lead vehicle followed speed profiles derived from the simulation study, so that the effects of other traffic could be reflected in the tests. Both a baseline scenario with normal traffic conditions and an experimental scenario with traffic incorporating the IHP applications were modeled in the simulation models and field tests. More than 20 runs were made in baseline and experimental scenarios.
- Simulation results and field testing confirmed that the vehicles achieved much closer following distances and higher speeds in the IHP case than in the baseline case.
- With all vehicles equipped with the IHP systems, the capacity of the managed lane was increased over 50 percent. Capacity in the experimental scenario was 3,190 vehicles per hour per lane (vphpl) as compared to the baseline capacity of 2,073 vphpl.
- When the level of vehicle demand was set to the capacity enabled by the IHP systems, the average delay with IHP systems was lower by more than 80 percent.
- Field tests on an actual managed lane facility showed that a platoon of closely spaced vehicles was able to maintain a throughput equivalent to more than 3,600 vphpl over an extended freeway section.