Traffic report

Marianne Hatzopoulou, an assistant professor in McGill’s Department of Civil Engineering, and her research team have created a sophisticated computer system for modeling urban traffic. They start with satellite imagery of streets, then add a second layer showing all the intersections, with details about each traffic light’s timing. A third layer adds hard data about pedestrians, cyclists and vehicles. And that’s when the fun begins.

In collaboration with the Société de transport de Montréal (STM), the researchers recently used this tool to do an in-depth carbon emissions analysis of the 165 bus route, a 14 kilometre loop serving the city’s fourth highest average weekday ridership; more than 29,000 passengers ride the 165 between downtown and the Côte-des-Neiges neighbourhood. The researchers fed their computer model with GPS data gathered from the buses, as well as detailed STM data of passenger activity and geographic details about the street grade. “Using all that data, we can simulate a second-by-second bus speed profile throughout the entire corridor, for all times of day,” explains Hatzopoulou. That profile can be used to determine the actual engine emissions of those buses; the researchers can then tweak the model to see how emissions would change if those buses were to run on different fuels.

Greenlighting clean air

“Overall carbon emissions are initially lower with alternative fuel technologies, such as biodiesel or natural gas,” she says, “but what we observed was that a very important determinant of transit emissions is traffic flow. Congestion increases engine idling and acceleration/deceleration, which together dramatically increase engine emissions.”

One way to improve flow is a technology called Transit Signal Priority (TSP). In Montreal, as with most Canadian cities, buses stop “upstream” of an intersection. This leads to a bus often having to stop twice: Once to load and unload passengers, closely followed by a second stop to wait out a missed light. With TSP, the bus communicates with the traffic light system while loading; the system adjusts traffic signal durations in order to eliminate that second stop. Hatzopoulou and her team used their computer model to predict the impact of implementing TSP on Côte-des-Neiges intersections, along with creating Queue Jumper Lanes to give the virtual 165 bus a head-start over other vehicles. The results of these small changes, which would be relatively inexpensive to implement in reality, were significant.

“We found that, just by switching to TSP and Queue Jumper Lanes—and keeping the current fuel technology—we could achieve more than 15 per cent lower carbon emissions in a single route,” says Hatzopoulou.

Rolling forward

In 2013, the researchers will begin to build a similar model, at not quite the same level of second-to-second detail, for the entire STM bus system. “Let’s say the STM were to invest in 20 new buses that run on an alternative technology,” says Hatzopoulou. “What would be the best routes to deploy those buses in order to achieve the lowest carbon emissions? A downtown route that is busy all the time? A suburban route that is busy in rush hour but empty during the day? Those answers are still not clear, so that’s our next step.”