The Fuse

Would Raising the Speed Limit Affect Fuel Demand?

by R. Kress | July 15, 2015

Summer driving season is when the country’s love affair with automobiles steps up a notch. But when motorists are cruising down the highway, the speed limit is a constant reminder that the freedom of the open road isn’t without restrictions. The debate over the speed limits has reemerged, with some transportation experts arguing that raising the speed limit may have significant safety benefits for drivers. However, faster speeds bring more fuel consumption, and increased speed limits could have real ramifications for fuel consumption nationwide.

A recent op-ed from Stephen Boyles, Assistant Professor of Transportation Engineering at the University of Texas at Austin, calling for higher speed limits has ignited conversation about the issue of speed limits online. As Boyles puts it, roads are safer than ever and are engineered for faster driving. In fact, he calls for the average driving speed on the road to dictate the speed limit—instead of the other way around.

“Current best practices in transportation engineering—supported by extensive research by organizations such as the Federal Highway Administration, the National Cooperative Highway Research Program and the Institute of Transportation Engineers—is that speed limits should be set at the 85th percentile of traffic speed,” Boyles wrote in the op-ed. “That is, only about one out of seven cars should be driving faster than the speed limit. Any more than that and the speed limit should be raised.”

Mixed evidence on safety

Based on Boyles’ findings, it is the discrepancy between drivers who choose to obey the speed limit and those who do not, rather than higher speeds overall, which contributes to many accidents on the highway.

Boyles’ argument is worthy of consideration: A higher speed limit would improve relations between drivers and law enforcement and, based on his research, prevent accidents caused by motorists who suddenly brake upon spotting a speed trap. Furthermore, the higher speed limit would cut back on the speed discrepancy between the drivers who choose to obey the speed limit and those who do not. Based on his findings it is this discrepancy, rather than higher speeds overall, which contributes to many accidents on the highway.

Over the past several years, a number of states have moved to raise their speed limits. When Utah increased its speed limit in 2013, the state conducted research finding that drivers were averaging between 82 and 84 mph—regardless of whether or not the speed limit read 75 or 80 mph. This spring, the Utah Department of Transportation is proposing to further expand the regional scope of the 80 mph speed limits—even though roadway deaths have jumped by around a third since 2014. But local highway patrol says the uptick in fatalities is not attributed to speed but to distracted drivers and failure to buckle up.

Driving up fuel consumption

While safety is always a primary concern, what would higher speed limits mean for fuel consumption? Boyles calls the traditional 55 mph speed limit a “well intentioned” relic of the 1970s, when oil shortages dictated a speed that could reduce oil demand as much as possible. A national speed limit of 55 mph was repealed back in 1995—to mixed results.

“Travelers are free to make their own choices about whether, when, where and how to drive,” Boyles told the Fuse this week in an interview via email. “This means that a lot of well-intentioned ideas are actually counterproductive, including lowering speed limits to improve safety or decrease fuel consumption.”

There’s no question that driving at a lower speed lowers oil use. As a result, an increased speed limit could have an impact on how much fuel the country consumes all together. In 2008, the U.S. Government Accountability Office (GAO) examined this relationship, finding that once a vehicle is going above 35 to 45 mph—depending on its particular physical size and characteristics—reducing speed by 5 mph can increase its fuel economy by 5 to 10 percent. Furthermore, if there were a national speed limit of 55 mph—that drivers actually followed, of course—the country could save some 175,000 to 275,000 barrels of oil every day.

A 2010 study from independent environmental research firm CE Delft reiterated that, indeed, slower is better in terms of optimizing fuel use. The study used CO2 emissions as its metric in this determination, but points out that fuel consumption and CO2 emissions are directly correlated. According to CE Delft’s data, lowering the speed limit for cars to around 50 mph would cut CO2 emissions on highways by some 30 percent over the long term.

In another a study published earlier this year, economist Arthur van Benthem examined the cost-benefit analysis of raising the speed limit. He shows that there is, in fact, an optimal speed for fuel consumption and carbon emissions—and once motorists exceed 50 mph, emissions and fuel usage rise (see graphic).


Van Benthem 2015

From a policy perspective, raising the speed limit would counteract national efforts to improve fuel economy nationwide. At worst, such a policy might reverse some of the progress already being made.

With this evidence in mind, from a policy perspective, raising the speed limit would counteract national efforts to improve fuel economy nationwide. At worst, such a policy might reverse some of the progress already being made. U.S. fuel consumption has flatlined in the past half-decade, and stands to fall further as fuel efficiency standards are working their way into the country’s automotive fleet.

As Boyles sees it, the speed limit—whether high or low—will not have much of an effect on how people actually drive. While he believes that energy conservation is a national priority, he argues that the number on the highway sign seems to have little effect on real-world human behavior.

“I don’t believe that speed limits are an effective way to make people drive slower—drivers tend to set their speed based on what they perceive to be safe. Posted speed limits have surprisingly little impact on the speeds drivers actually choose,” Boyles explains to the Fuse in response to the data showing the correlation between increased speed and lower gas mileage. “If we want to reduce CO2 emissions, we have to find a more intelligent way to slow down drivers than simply posting a lower limit.”

Since faster highway speeds mean more fuel consumption, Boyles argues that public transportation is a better solution—he himself takes the bus to work every day.

“The most important things we can do to increase transit ridership are… promoting denser development and discouraging suburban sprawl; improving reliability… and changing the cultural view that public transit is only for those who can’t afford cars,” Boyles says.

Similarly, the research from CE Delft points to public transportation as the solution to the inherent disconnect between drivers’ actual speed and the limit posted on signs. As the study puts it: “Because of the longer travel times resulting, lowering motorway speed limits will also lead to less car-kilometers being driven and a certain shift from private car to public transport.”

In other words, lowering the speed limit would, in fact, deter people from driving altogether by making it a more tedious task. As a result, more people would seek out public transportation as it becomes a speedier and more viable option—essentially lowering fuel consumption all around.

Changing the speed limit doesn’t have much to offer politicians or motorists.

Either way, changing the speed limit doesn’t have much to offer politicians or motorists. In theory, a higher speed limit may improve highway safety, but more research is needed given the outcome in Utah. Lowering the speed limit would push down gasoline consumption and save drivers money, and could encourage use of public transportation. However, drivers will find the lower speeds inconvenient and are unlikely to adhere to them.

The most appropriate course of action probably needs to come from motorists themselves. Were highway drivers to adhere to the speed limit, they would improve safety for themselves and others, save money on gasoline, and—of course—be that much less likely to get a speeding ticket.