The environmental catch-22 of public transit construction
It may take decades for subways to ‘pay back’ emissions produced during their making, study says
Public transit is commonly considered among the most powerful weapons in any city’s arsenal in the fight against climate change, with a robust network of transit lines seen as a direct route to lowering greenhouse gas emissions.
But according to new research from the University of Toronto, it can take decades for a major transit project like a subway to offset the carbon emissions generated by its construction. The study’s lead author argues that cities won’t fully realize the environmental benefits of transit unless policy-makers pay closer attention to how it’s built.
Shoshanna Saxe, an associate professor at the university’s department of civil engineering and one of the study’s authors, said that while there was already research about the pollution-saving effects of public transit, most previous studies only looked at transit once it was built, and didn’t account for the significant emissions produced during construction.
Saxe, who used to work as a geotechnical engineer designing TTC subway stations, said that approach misses the bigger picture.
“We should be thinking about greenhouse gas emissions and the impact at all the stages of design,” she said.
“We should be thinking about them during construction, we should be thinking about them as we do our transport and land-use planning. And this study puts the numbers underneath it to help us think about it in a constructive way.”
The study, which was published this month in the journal Transportation Research, sought to measure the “greenhouse gas payback period” of Toronto’s newest subway line, Line 4 (Sheppard).
It found that in the worst-case scenario Line 4 won’t offset the emissions generated to construct it until 2035, or 33 years after its completion in 2002. Using more optimistic models, the line may have paid back its emissions 18 or, less likely, 11years after construction.
“I think it’s probably somewhere in between,” Saxe said.
Building even a relatively small subway like the 5.5-km Sheppard line is a massive, pollution-intensive project.
Using data provided by the TTC, Saxe estimated the line required 358,851 cubic metres of concrete and 40,000 tons of rebar to build.
Producing the construction materials generated significant pollution, including at the mineral extraction, mining and processing stages.
Emissions were also generated by the energy consumed at the subway construction sites and the movement of people and materials to and from construction locations.
In total, Saxe estimated that Line 4’s eight-year construction generated almost 168,000 metric tons of carbon dioxide equivalent.
She also determined that operating the subway sucks up about 22,940,000 kilowatt-hours of electricity a year.
The pollution generated by building and operating the line is offset by people taking the subway instead of cars or buses, and the increase in residential density around the line’s five stations.
Density is a significant factor in pollution because denser housing requires less energy than stand-alone homes.
The variable that led to the 22-year difference between Saxe’s best- and worst-case outcomes was related to car traffic. While the subway could be expected to reduce traffic on Sheppard, data showed no decline in the number of motor vehicles on the road after the subway was built.
Saxe said one likely cause was a phenomenon known as “induced demand,” which dictates when road space is freed up car travel becomes more attractive, and the available space is soon filled by more drivers.
The level of induced demand associated with the subway is difficult to determine, Saxe said, which is why her research provided such a wide range of outcomes.
Saxe stressed that her study shouldn’t undermine the environmental case for building public transit because even in the worst-case scenario Line 4 is expected to be in operation decades after it reaches its carbon tipping point and will “over its lifetime have important meaningful contributions.”
It’s also likely that busier subways would have an easier time erasing their carbon footprint. Line 4 is the TTC’s least-used subway, and with only about 50,000 daily riders carries fewer people than the 504 King streetcar.
But to Saxe the pressing question is “how can we better support our infrastructure so that it pays back faster?”
She pointed out that the federal government has set a target of reducing greenhouse gas emissions 80 per cent below 2005 levels by 2050. The goal is already considered a long shot, but being able to reap the environmental rewards of transit projects sooner would help.
The study’s recommendations include cutting back on construction materials by designing smaller, leaner stations and, where possible, lay- ing track above ground instead of in tunnels.
It also recommends implementing zoning policies and tax incentives that encourage density along subway lines and mitigating induced demand by making driving less attractive through measures such as lower speed limits and eliminating car lanes.
Lindsay Wiginton, a transportation analyst with the Pembina Institute, predicted that research like Saxe’s that quantifies the life cycle pollution costs of transit projects will be “increasingly important in guiding infrastructure investment decisions at all levels of government.”
She said that the transportation sector is responsible for 35 per cent of Ontario’s emissions, and 23 per cent of Canada’s.
“Addressing emissions from the transportation sector broadly is going to be crucial in bending down our emissions curve.”