The two main takeaways from my interview with SDOT Senior Engineer Darlene Pahlman:
- The City of Seattle’s policy is that Link trains always receive signal priority, regardless of time of day, location, or direction of travel. The objective is that trains never stop between stations, aside from stops caused by unavoidable human factors.
- The signal settings and algorithm is very complicated, and due to fine-tuning has many exceptions. It’s therefore very difficult to generalize into general principles. There are dozens of variables and thousands of permutations and it’s impossible to definitively state how the system will react without precisely defining the scenario and checking the code.
Reasons your train might stop
Ideally, as the train prepares to depart a station the operator presses a request button. This sets up a chain of requests for a green signal that enables travel at or just under 35mph, which should allow uninterrupted travel to the next station. Below is a (non-exhaustive) list of some reasons your train might have stopped, or not gotten its permission to leave the station immediately:
- The operator has delayed the train. This could be due to something on the tracks, a late runner, or merely some difficulty keeping the train at the optimal speed. This often results in a “phantom train” of requests propagating up the line for several signals.
- Headways less than four minutes. If trains in the same direction are coming less than four minutes apart, the system will deny the later train priority. Early on, revenue trains were having this problem fairly often. According to Pahlman, however, “over the last few months the headways have seen big improvements and are much more on target than during the first few months.” Today, close headways usually result from either trailing an out-of-service train or a “phantom train.”
- Pedestrian Signal in Progress. Obviously, pedestrians must be given the chance to complete their crossing.
- Skipped Pedestrians. If a pedestrian signal has been skipped in the cycle, it will not be skipped a second consecutive time, “to encourage compliance with the signal display.”
- Emergency Vehicles. Priority requests from Link and from emergency vehicles are resolved on a first-come first-served basis. If the emergency vehicle gets there before the Link request does, the emergency vehicle will go first.
SDOT Wants to Know
The process of adjusting signal timing continues. If you operate a train or automobile and notice an abnormally long wait, please document the time of day, location, direction of movement, and length of wait and email it to firstname.lastname@example.org. All that detail will allow engineers to check the logs and evaluate if changes need to be made.
SDOT doesn’t need to hear about every time a Link train stops at a light, but if you are a regular passenger on a certain trip and notice the same thing happening day after day, SDOT would like to know that as well.
In Part II, I’ll share answers to some of your specific questions.