NMA E-Newsletter #210: Some Myths Just Won’t Die

Every few months or so someone (usually a non-member) sends us an email that goes something like this:

How can your organization support 65 mph speed limits or even higher? Don’t you know that there’s only so much oil in the ground and that the best way to conserve fuel is to go back to a 55 mph speed limit across the board? Cars traveling at slower speeds use less fuel. It’s a simple matter of physics. Boy, are you guys dumb or what! 

It seems that certain misguided notions won’t give up the ghost. So, let’s take a look at the assumptions here.

It is true that a car traveling at a higher speed encounters greater wind resistance (drag) than the same car traveling at a lower speed and that this does decrease fuel efficiency. But that’s only part of the story.

Most experts agree that the optimum vehicle speed for highest fuel efficiency is somewhere between 50 and 65 mph, with most vehicles peaking in the 55 to 60 mph range. The variance is caused by differing engine efficiency and drag coefficients.

When the 55 mph National Maximum Speed Limit (NMSL) was enacted in 1973, federal officials predicted a 2.2 percent savings in gasoline consumption. Instead, the U.S. Dept. of Transportation found the fuel savings to be one percent, and some independent studies determined the savings to be a much lower 0.5 percent. We could achieve those kinds of savings simply by keeping our tires properly inflated.

Further proof that the 55 NMSL had practically no impact on fuel consumption comes from the U.S. Energy Information Agency. In the three years leading up to the passage of the NMSL, U.S. fleet fuel economy averaged 12.0 miles per gallon. The average for the three years following the law’s enactment was basically flat at 12.2 miles per gallon. Only with the onset of Corporate Average Fuel Economy standards in the late 1970s did fuel economy begin to take off.

So, why didn’t the lower posted speed limit produce the promised results? Part of the answer is because posted speeds have little impact on the speeds people actually drive, especially when those limits are set artificially low. (Learn more about how speed limits should be set.) The driving public didn’t modify its behavior enough to make much of a dent in fuel consumption.

In addition, artificially low posted speed limits impede the efficient flow of traffic and further offset any fuel savings. Properly set speed limits based on actual travel speeds promote smoother traffic flow. This minimizes congestion, lane changing, and sudden braking and acceleration, which has a positive impact on fuel consumption (and highway safety).

Steady-state travel on expressways will normally yield better fuel economy than varied speeds on surface roadways. To the extent that a more realistic limit on the expressway draws traffic from the nearby surface highways, the change should yield a small net gain in fuel economy.

The return to 65+ mph highway speeds has yielded a host of additional benefits related to highway safety, economic productivity and time-savings. For example, highway fatality rates have been steadily declining since the repeal of the NMSL in 1995.

For more on the NMSL failure and some practical ways to reduce fuel consumption, check out Jim Baxter’s July 2009 column in U.S. News & World Report.

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