Department of Transport and Main Roads

Title: Adaptive Real-Time Optimisation of Offsets for Traffic Signal Coordination in a Network.

Industry Partner: Department of Transport and Main Roads (DTMR), Queensland

Problem Description:

Traffic Signal Coordination has been an important area of research over the last few decades, due to its practical implications on the users of urban transport networks, in terms of the economical, health and environment impacts of congestion. Existing traffic signal coordination systems attempt to minimise delays or stops, or maximise throughput, or a combination of these objectives. However, an important part of Traffic Signal Coordination is the “subjective” quality of coordination as perceived by the traffic engineers and experienced by drivers. One measure of the quality of coordination is the number of intersections a vehicle may travel through without stopping, or “green waves”. It is usual that traffic engineers will adjust the resulting plans and timings from the current coordination systems to provide an improved perception of coordination. Another measure of the quality of the coordination is how well it embodies the policy of the organisation. For example, some offsets may passively encourage speeding if the first few vehicles arrive at the downstream intersection near the end of the green signal. There is currently no way to avoid this except by manual adjustment of the resulting plans.

The existing traffic signal coordination optimisers have a number of key variables that they can adjust to affect the coordination of traffic signals in a network. These are:

  • Cycle Times: The total time taken to complete a set of phases so that all directions of traffic at an intersection have been given a chance to progress. Phases are periods of time in which non-conflicting traffic movements are allowed to depart in a green period.
  • Phase splits: The percentage of the cycle time given to a particular phase, also known as “green splits”.
  • Offsets: The difference in time of the start of cycles at consecutive traffic signals.

The focus of this project will be on optimising offsets, and optionally the phase sequences. Alternative phase sequences, such as the Lead/Lag Right-Turn Phase, Diamond Overlap Phase or Split Phase, can significantly aid coordination, however choosing the best combination of phase sequences and offsets is not a trivial task.

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