The Express-News got a nice scoop on Texas considering raising the speed limit on some West Texas interstates to 80 miles per hour. But the story has a classic news-stats error. It quotes a state official saying, in effect, that people are driving that fast anyway and that the change is just reflecting reality:
[Carlos Lopez, director of traffic operations for TxDOT] said the department surveyed how fast cars were traveling on both interstates and found 85 percent of them were driving up to 79 mph.
But that means the opposite of what Lopez is trying to say. “Up to 79 mph” means only that people were driving at that speed or below. It tells you nothing about whether they’re going 78 mph or 2 mph. It’s just as factual as writing:
[Joshua Benton, proprietor of crabwalk.com] said that his blog surveyed how fast infants were crawling across the day-care center’s carpet and found 100 percent of them were crawling up to 79 mph.
What I presume Lopez meant is something like 85 percent of drivers are driving above 79 mph. Or maybe that 85 percent are driving between 75 and 79. Who knows?
I believe he meant the 85th percentile. This is the technique that civil engineers use to estimate a safe speed limit for a road during a traffic study.
The easy way to put it, then, would have been “The study found that 15 percent of motorists are already driving at 80 mph or above.”