Experts say America's bullet trains will need to carry 10 million passengers to offset the environmental impact of construction

There's a lot of talk right now about the capital costs of high-speed rail - the planned Los Angeles-San Francisco line, which would be the model for America, may eventually cost some $98 billion (or about $75 billion in 2010 money) - but for the most part its environmental benefits are taken for granted. Rail transport tends to be greener than car and air travel, so it stands to reason that as high-speed rail attracts people off the roads and runways, net carbon emissions will fall.

Often that comparison overlooks one critical detail: the environmental damage caused by building high-speed rail lines in the first place. Unless high-speed rail travel reduces emissions by more than what it generates during construction, the project may not be worthwhile from an environmental perspective. Indeed, some researchers have their doubts. A recent British study suggests that high-speed construction emissions may be significant enough to call entire projects into question, writes Eric Morris, who described the work a couple years back at the Freakonomics blog:

When the emissions spewed by all those earth movers, tunnel boring machines, bulldozers, trucks, cranes, etc. are taken into account, the carbon advantage for HSR vis a vis air travel largely evaporates.

Largely, but emerging work shows, not entirely. A new study by Swedish researchers Jonas Westina and Per Kagesona of the Royal Institute of Technology concludes that high-speed rail can offset the emissions created during construction if it attracts enough riders from air travel. The $98 billion question, of course, is just how many riders is enough?

Westina and Kagesona modeled the environmental damage generated during the construction of a hypothetical 500 kilometer double-tracked high-speed rail line that travels 10 percent of its route through tunnels. Then they calculated the change in carbon emissions that would occur for a person who shifts to high-speed rail travel from another mode - namely, flying or driving.

The goal was to find the point at which savings from these shifts balance out the environmental costs of constructing and operating the high-speed line. In the January 2012 issue of Transportation Research Part D: Transport and Environment, Westina and Kagesona conclude that to balance construction emissions, high-speed rail traffic volumes "need to be large, and the diverted traffic should primarily come from aviation." If high-speed rail were to attract a high rate of passengers who used to travel by airplane, Westina and Kagesona estimate that a line needs to average 10 million annual one-way trips "to compensate for the annualized construction emissions."

The finding seems in line with previous research from Kagesona, who has questioned whether high-speed rail mitigates climate change as much as some advocates suggest. As he wrote in a previous paper, published in late 2009 [PDF]:

There is no cause to prohibit investment in high speed rail on environmental grounds so long as the carbon gains made in traffic balances the emissions caused during construction. However, marketing high speed rail as a part of the solution to climate change is clearly wrong. ... The principal benefits of high speed rail are time savings, additional capacity and generated traffic, not a reduction of greenhouse gases.

At first glance, 10 million passengers could be a hard figure for American high-speed rail lines to reach. About 10 million passengers rode Amtrak in the Northeast Corridor in 2010, but that's the most heavily trafficked rail artery in the country. That figure also combines both regional rail and the Acela train; taken by itself, Acela, which is the closest thing America has to a high-speed train, carried roughly 7 million riders.

Still Acela is not true high-speed rail, and global passengers figures suggest that legitimate high-speed systems have no trouble exceeding a 10 million passenger threshold. While California officials anticipate only upwards of 11 million riders by 2025, the annual ridership figure by 2040 will range between around 30 and 44 million passengers [PDF], after the completion of Phase 1, which connects Los Angeles and San Francisco. According to the rail authority's new business plan, those figures are entirely reasonable given the populations along the California line [PDF]:

As shown, the Spanish HSR system serves cities with a combined population of 7.9 million people and has annual ridership of 10 million; the French system serves a combined 15.1 million people and generates 31 million annual riders. California’s system will serve a population base projected to be over 49 million in Full Phase 1.

In short, it seems entirely reasonable to conclude that the California line will eventually offset the environmental damage caused by its construction, and then some. Still it's fair to wonder whether or not the environmental benefits of high-speed rail are worth its considerable cost. In a 2009 post for the New York Times Economix blog, Ed Glaeser concluded that while these benefits exist, they are "quite small relative to the cost of the system." Certainly California will help its case by keeping its project as cost-effective as possible. But it's important to keep in mind that high-speed rail systems aren't constructed in a vacuum. In reaching their 10 million-rider figure, Westina and Kagesona don't appear to consider the environmental impact that would occur if California ditches its rail plan and instead constructs enough roads and railways to accommodate its current culture of air and highway travel. That impact would no doubt be considerable: at its current pace, the state's department of transportation will spend around $286 billion, largely on road projects, over the same period it plans to build the high-speed rail line. 

Photo credit: Jason Lee/Reuters

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