Inadequately timed traffic signals at intersections are a major contributor to traffic congestion and increased travel times. Adaptive signal timing can detect and respond to real-time vehicle queues, resulting in more efficient vehicle movement through a corridor than traditional traffic signals. A revised max-pressure traffic signal controller could decrease delays and increase vehicle throughput at intersections.
While some Minnesota cities and counties have been updating signal controllers with adaptive technology, most use a combination of signal controller types, including pretimed signal control with fixed phase sequence and timing. Adaptive traffic-actuated signals, on the other hand, respond to the vehicles present, but may not allow for control of other variables to ensure optimal throughput in the intersection and minimum delay for drivers.
Max-pressure traffic signal controllers detect real-time vehicle queue length—vehicles entering and exiting the intersection—to optimize signal timing. This method uses a mathematically proven model to calculate a pressure value for each phase, activating the one with the greatest pressure. The algorithm dynamically adjusts phase sequence within seconds based on real-time traffic measurements.
“The mathematically proven algorithm for max-pressure traffic control used in simulations on seven county intersections showed significant benefits to drivers. Travel delays decreased and vehicle throughput increased with adaptive signal control strategy,” said Ben Hao, traffic operations engineer, Hennepin County.
While promising, the method is largely theoretical and has not been implemented for a number of practical reasons. The current model assumes separate lanes for vehicles turning left through an intersection, which may be inaccurate. Also, the acyclic system determines each time whether to switch to the next phase, resulting in no maximum waiting times for short vehicle queues and pedestrians.
Hennepin County operates over 450 traffic signals, all of which are traffic actuated. This Local Road Research Board project examined the known issues with max-pressure control to optimize signal timing for the greatest performance possible.
What Was Our Goal?
The goal of this project was to investigate and resolve issues with max-pressure traffic control to prepare for a pilot study of intersections in Hennepin County.
What Did We Do?
Researchers leveraged past work on the max-pressure traffic signal control method to tailor it for potential implementation in Hennepin County. The existing model was modified to a mixed queue system to accommodate vehicles turning left from an intersection’s through lane. The acyclic nature of the previous model was remedied by modifying the model to allow selection of phases according to a predefined order and by creating a maximum waiting time constraint while retaining the maximum possible vehicle throughput.
Two corridors—one with three intersections and the other with four intersections—were chosen for simulations of the revised max-pressure controller. These intersections have moderate to high, but variable, demand.
Using past data from the intersections and defining trips for each vehicle, including its origin, departure time and destination, researchers compared performance among the modified max-pressure controller, the county’s current actuated signal controllers and the initial max-pressure system. Calculations at each intersection included delays for the worst lanes, overall delays, number of vehicles moving through each hour and number of phase changes.
Lastly, the research team explored how to implement max-pressure signal control in Hennepin County with currently available traffic signal hardware and software.
What Did We Learn?
The simulations of the max-pressure traffic control algorithm showed significantly reduced overall average and worst lane delays in most intersections compared to current signal controls for almost all off-peak and on-peak periods. Decreases in vehicle throughput at a few intersections were outweighed by the reductions in delays.
“This was the first effort to produce a practical, detailed and calibrated simulation of max-pressure traffic control. It was based on real data and multiple time periods and sets a sound stage for next steps,” said Michael Levin, assistant professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
While coordination among signals along a corridor was not explicitly studied, max-pressure control implicitly results in signal coordination by responding to queue lengths as vehicle platoons move through intersections. Traffic engineers can predefine an ordered set of phases, which are selected in mathematical order and based on real-time conditions.
The potential of integrating max-pressure control with Hennepin County’s existing systems was demonstrated. More traffic sensors would be needed to define the real-time queue length. Other inputs not traditionally collected, such as average turning proportions, are needed. Max-pressure control requires significant processing power, however, which may be addressed through add-on software.
Finally, recommendations for a small-scale study and larger scale implementation were provided.
Phase 2 of this project is already underway. Hardware and software necessary for max-pressure traffic control are being lab-tested in simulations using data from one of the same seven intersections. If Phase 2 proves successful, Hennepin County will consider implementing the technology within its advanced traffic management system in one or more of the simulated intersections. These project results will benefit other cities and counties transitioning to adaptive traffic signal control and exploring max-pressure.