This little-known rule causes speed limits to go up in Texas

(NEXSTAR) – When the signs changed on State Highway 130 in 2012, and the posted speed limit rose to 85 mph, the 90-mile stretch of road officially earned the title of fastest highway in the nation. But Highway 130 is far from the only Texas route where cars have been going faster over time — and are doing so legally.

Texas, like many other states around the country, sets speed limits on state roadways using the "85% rule." Basically, the Department of Transportation comes out to a highway, monitors how fast everyone is going in a speed survey, and sets the new speed limit at the 85th percentile. 

If you think about it a different way, the speed limit is essentially being set by the top 15% of fastest drivers. 

Once DOT engineers determine the speed of the 85th percentile, they'll round the number to the nearest 0 or 5, and make that the new speed limit.

“The method by which speed limits were set meant that the fastest drivers would set the speed limit since most drivers generally travel just above the posted limit,” Colin Sweeney, a Los Angeles Department of Transportation spokesperson, told Nexstar. Los Angeles and other California cities also used to rely on the 85% rule to set limits on city streets before a recent law gave municipalities more control.

Numbered state highways usually start with a baseline speed limit of 70 mph in Texas, but speed surveys and the 85% rule are how you end up with 75 mph, 80 mph and 85 mph limits.

As speeds increase, so do crash fatalities. EMC Insurance ran the physics calculations and found that your chance of dying in a crash doubles when you increase speed by 10 mph.

"In practical terms, increasing driving speed from 60 mph to 80 mph increases the risk of a fatal crash by 4 times," the company writes.

But Texas transportation officials say setting a speed limit too low can also have its dangers.

"If reasonable drivers see an unreasonably low speed limit without seeing a need to drive that slowly, they tend to ignore the signs and develop disrespect for speed limits in general," explains the Texas DOT. That means more speeding tickets for "reasonable people," the agency says.

City streets start with a baseline 30 mph speed limit under Texas law, but those can also be raised using speed surveys. Cities can also choose to lower speed limits if they believe they're too fast for the surrounding areas, for example around schools or in an area with lots of pedestrians.

In recent years, pedestrian and bicyclist deaths have shot up in Texas.

Residents can request speed limits be lowered in their neighborhoods, but the City of Grapevine says that might not even make things safer.

"Studies have shown that most people drive at the speed they are comfortable with for the given conditions regardless of the posted speed limit," the city says. "Also, safety is not improved by establishing unreasonably low speed limits, since this only encourages more variation in vehicle speeds, leading to more conflicts."

The Texas Department of Transportation, while cautioning against setting speed limits to low, acknowledges that it's sometimes worth slowing things down a notch. Under certain conditions, like in an area with a history of crashes or narrow pavement, speed limits on a state highway can be lowered up to 10 or 12 mph.