Setting speed limits
How fast, or slow, should we go? Determining the “right” roadway speed in America has been a subject of discussion for decades. Perpetual debate over it implies it’s a difficult endeavor.
Freeway speeds across America range from 60 to 80 mph (even 85 in parts of West Texas) — speeds that many are still not content with. Some drivers want to raise the limits and others wish to lower them.
There is endless justification on both sides of the issue, along with a new call to lower the national limits in the interest of saving fuel. This outcry may come from the same contingent that makes regular appeals for dropping speeds in the interest of safety — but it’s now more topical to reference potential fuel savings.
However, it has never been proved that raising limits increases accident rates, or that lowering limits reduces them. Additionally, the speed for optimum economy varies among vehicle types. Those factors and more make finding an ideal speed limit difficult.
One study found that raising the speed limit from 65 to 70 mph on Interstate 65 in Indiana has not increased the probability of fatalities or severe injuries. “These findings are important because the influence of speed limits on roadway safety has been a subject of continuous debate in the state of Indiana and nationwide,” said researcher Fred Mannering, a professor of civil engineering at Purdue University. He added, “Everybody expects that when you increase the speed limit, injuries and the severity of injuries are going to increase, but that hasn’t happened on the interstate highway system in Indiana.”
Nor has it happened elsewhere, but as usual, one can find statistics to support virtually any stance. One study showed that a speed limit increase from 55 to 65 mph resulted in roughly a 3 percent increase in the accident rate and a 24 percent increase in the probability of a fatality once an accident occurred. But other studies have contended that legislation-enabled speed limit increases have actually saved lives, arguing that increasing from 55 to 65 mph did so because it reduced how often drivers vary speed and curtails speed differential among vehicles.
So the issue is complex. If Indiana found no accident rise by raising limits to 70 mph, then what about 75 mph? At what point does an increase impact safety? Various studies both support and dispel preconceptions of the interrelatedness of speed and fatalities.
And the new cry to lower speeds for saving fuel sparks additional debate, as finding the most fuel-efficient speed is not an exact science either.
It is estimated that truckers would realize a 27 percent fuel savings by lowering the speed limit 10 mph (75 to 65). Cars would not benefit as greatly though, and in some cases not at all. As is typical, it’s impossible to please everyone — with differing weights, aerodynamics, gear ratios, horsepower, camshaft grinds, fuels, fuel delivery systems and computer engine management, optimum speeds vary with each vehicle.
Washington freeways were once marked 70 mph for cars, and 60 mph for trucks. What’s wrong with that? Many truckers lobbied for higher speeds because “time is money.”
Of course, whatever the limit, drivers will adhere to it in varying degrees depending upon enforcement and regional attitudes.
Definitive answers to finding the right speed are elusive, at best.
Readers may contact Bill Love via e-mail at precisiondriving@spokesman.com.
