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| 03/29/17

Public transit projects take longer to reduce emissions than expected


Once the full life-cycle emissions of construction and operation are taken into account, the greenhouse gas emissions savings of a recent subway line extension in Toronto were much less than anticipated, according to new research.

It may take as long as 33 years before the building of Sheppard Line 4 within the city’s subway system turns a ‘climate profit’.

A common perception is that as soon as a new subway, elevated rail or light rail system is constructed, the commuter shift from automobiles delivers an instant, and sharp reduction in emissions. But many previous studies have only looked at emissions once such a system is built.

The study, led by University of Toronto engineering researcher Shoshanna Saxe, used data from the Toronto Transit Commission to estimate the full scale of emissions from concrete and rebar use during construction, and from the extraction, processing and transport of raw materials.

The source of energy the new system ran on was also a factor in the mitigation achievement. While Ontario as a whole now has a very low-carbon, nuclear and hydro-dominated electricity system, when the line was finished in 2002, coal was still part of the mix.

In addition, the energy used per kilometre travelled is substantial because trains are heavier than buses or cars. To reduce the emissions per kilometre, the ridership of each train must be quite high.

This meant that for the first six years of its operation, for each kilometre travelled, the 5.5km Sheppard line actually resulted in an increase in emissions when travellers shifted from buses to the subway.

This problem eased off once the electricity system decarbonised, but the researchers concluded that Line 4 will not offset the emissions produced during construction until somewhere between 11 and 33 years after construction.

The variable behind this 22-year difference between these best-and-worst-case scenarios relates to car traffic. The reduced automobile traffic on the roads resulting from drivers shifting to the subway can, counterintuitively, can make driving a car appear even more attractive. This results in motorists who otherwise wouldn’t drive actually being induced to drive more often.

The authors are keen to stress that none of this should mean that the building of public transit systems is not good climate mitigation policies. Their findings should instead be used to encourage more rigorous assessments to limit the emissions profiles of transit systems – information useful to cities such as Vancouver and Victoria with new public transit proposals on the books.

The authors suggest that use of carbon-intensive concrete and steel should be minimised via smaller stations and if possible, building above ground rather than beneath, which requires more concrete and steel.

In addition, increasing housing density along the systems route encourages greater ridership and reduces the emissions per kilometre travelled.


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If riders shift from buses to subway lines, emissions can go up, as subway construction is more carbon-intensive

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