The crossing guard: timing LRT crossings requires a delicate balance
By Andrea Collins
Watched pots never boil, red lights never turn green and, for motorists in Edmonton, LRT crossings are always busy.
When Edmonton’s South LRT line was under development, Wai Cheung (Civil ’84) of the city’s modelling group was given a challenge: evaluate the sequencing and timing for the crossings of vehicles, pedestrians and LRT trains and co-ordinate them in the most efficient way possible.
No pressure there, right?
To get the job done, Cheung collected data he’d need and ran it through a special software package to simulate different traffic scenarios.
“We took information provided by Edmonton transit regarding the anticipated speeds and stops for the LRT trains, in conjunction with what we knew about buses, trucks, automobiles and pedestrians, and put them into our model,” says Cheung. “That data was fed into the simulation and was used to co-ordinate the traffic lights and LRT crossing gates so the new LRT line would have minimal impact on traffic flow.”
That process worked, in theory. But once the new south line opened, there was a glitch. The unexpectedly high usage of the south LRT meant that at peak times it took longer than anticipated to load and unload the train. The LRT’s speed and frequency changed, resulting in a long wait for the gates to lift for vehicular traffic—sometimes as much as 15 minutes at the 51 Avenue crossing.
“The software is exacting,” says Cheung. “But it can only model on the assumptions you enter, like anticipated speed and acceleration rates of each type of vehicle, or the frequency of LRT crossings. We were told that the trains on the south line would run five minutes apart and stop for 30 seconds at each station. When the high usage changed things, the south- and northbound trains passed within a few minutes of each other and that meant that the gates didn’t always have time to come up between trains. The quick fix was to have the traffic signal team adjust the timings and they resolved the situation quickly. However, having this problem enabled our team to learn more about transit operations and all the parameters that need to be factored in when modelling for a new line.”
Today, Cheung is working on the new LRT line that will connect downtown to NAIT, a line he expects will have fewer problems. The busiest downtown sections of the line are underground, and once the line emerges to the street level, LRT stops will be at dedicated platforms or train and transit centres built apart from roads.
“However, we’ve tried to consider every conceivable thing that could affect LRT speed and stop times,” says Cheung. “For example, we’ve looked at how long it takes each conductor to walk from one end of the train to the other when they need to reverse direction at the end of the line, and we will use a range from the slowest to the fastest turnarounds in our computations.”
As for the next big challenge, Cheung expects it to be the southeast-to-west surface line, which is currently in the planning stage. “It’s a long line with numerous surface crossings. We’re looking for the optimal system for that one; good modelling will be crucial.”
This article first appeared in U of A Engineer magazine.