Everyone knows easing up on the accelerator can improve your fuel economy and reduce your emissions. But what kind of impact would it have on the environment if everyone had to slow down?
A potentially big one, as it turns out.
Dutch researchers say lowering the speed limit to 80 km/h (50 mph) would cut transportation-related CO2 emissions by 30 percent. Less drastic cuts in maximum speed would yield reductions of 8 to 21 percent, according to the study by CE Delft.
How?
Beyond significantly reducing the amount of fuel vehicles burn, a strictly enforced 50 mph speed limit would increase the time required to cover a given distance. That would lead many people facing long commutes to ditch cars in favor of other modes of transport, like rail. Longer term, the impact could prompt people to move closer to urban centers.
The findings assume two things. First, there is a viable alternative to driving, such as a comprehensive rail system. And second, that people want to return to urban centers. The researchers concede these points when they note the findings apply only to the Netherlands and may vary elsewhere.
Still, the underlying point is valid -- generally speaking, cutting your speed reduces emissions. The faster you go, the more fuel you burn because wind resistance increases exponentially. Lowering your speed by 5 mph when traveling at 35 to 45 mph will boost fuel economy as much as 10 percent (.pdf), according to a report the General Accounting Office prepared in 2008.
Of course, different cars achieve their optimal fuel economy at different speeds. The GAO examined 13 vehicles produced between 1988 and 2005. The 1997 Toyota Celica achieved 52.6 mpg at 25 mph, while the 1995 Oldsmobile 88 achieved 34.1 mpg at 65 mph. Regardless of that, studies have shown efficiency falls precipitously above 65 mph. Cutting your speed from 70 mph to 55 mph reduces fuel consumption 25 percent.
The Department of Energy noted in 2008 that lowering the national speed limit to 55 mph would save 175,000 to 275,000 barrels of oil daily. The United States consumes about 19.5 million barrels per day. But the GAO is quick to note that a lower limit would have little effect in urban areas, where congestion forces people to drive more slowly than 55 anyway. It cites the Federal Highway Administration when it says fewer than one-quarter of all vehicle miles traveled would be directly affected by a 55 mph limit.
We've tried this before. The National Maximum Speed Law set a national speed limit of 55 mph in 1974. The aim was to cut national petroleum consumption by 2.2 percent, but the Department of Transportation says it saved just 1 percent. Perhaps that's because the law was widely ignored and barely enforced. The law was repealed in 1995 and the authority to set speed limits reverted to the states.
A more effective way to curb fuel consumption would be to increase the gas tax. But that's a whole 'nother story.
*Photo: Library of Congress. In Washington, D.C., speed limits went from 40 mph to 35 within 24 hours of the Baruch Rubber Investigating Committee recommending lower speeds nationwide in September, 1942. *
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