From the Autopia Unintended Consequences Department comes this dispatch from Tippecanoe County, Indiana, where researchers at Purdue University say the majority of drivers have no problem going 5, 10 or even 20 mph over the speed limit and see no risk in doing so.

The study of 988 drivers in that county, where Purdue is located, found few people have any respect for speed limits, which they consider nothing more than vague guidelines they can ignore. Civil engineering and economics professor Fred Mannering says that means highway officials set artificially low speed limits because they know people will ignore it. He attributes our blatant disregard for the rule of the road to the much-maligned Emergency Highway Conservation Act of 1974 that established the 55-mph speed limit for political, not safety, reasons. "It decoupled the speed-safety association," Mannering told Wired.com. "Now, there are some roads where the speed limit should be posted as 45 but they end up getting posted at 35 because they expect people to go faster," he said.

Congress adopted the nationwide 55-mph speed limit during the oil embargo era and threatened to withhold highway funding from any state that didn't comply. It repealed the law 13 years ago, when oil was cheap and gas plentiful. Mounting concern about global warming and the summer's spike in gas prices brought renewed calls to roll the limit back to 55. Mannering's study suggests it won't change anything.

His study of 988 drivers, published in next month's Transportation Research Part F (subscription), found 21 percent of them think it's perfectly safe to exceed the speed limit by 5 mph. Forty-three percent saw no risk in going 10 mph over and 36

percent say there's no harm driving 20 mph over the speed limit.

What makes that especially dangerous, Mannering said, is when the speed limit actually reflects the safest traveling speed and people still exceed it. That, he said, creates a dangerous situation where some people are following the speed limit and others are zipping past them when they absolutely shouldn't be. As anyone who has ever watched a cement truck merge in front of a speeding sports car can tell you, having two vehicles traveling at wildly different speeds on the same road can be quite risky.

The study found that people will, not surprisingly, obey the speed limit if they feel there is a risk they'd get caught breaking it. But they'll also speed up if they think there's little risk of getting busted.

The phrase "speed limit" is something of a misnomer. Before 1974, the rule of thumb was to set speed limits at the 85th percentile: 85 percent of the cars should be traveling at or below the limit, while 15 percent of cars could be exceeding it. Speed limits could be used to accurately judge how safe it was to travel on any particular roadway.

Now, that black-and-white sign rarely tells you the maximum speed you can safely travel without wrapping your car around a tree or unintentionally modifying a guardrail. It factors in fuel efficiency, pedestrian safety and the concerns of those who live in the area.

Drivers who get used to these artificially low speed limits begin to ignore them and end up routinely driving 5 to 10 mph faster than the number on the sign. Drivers also disregard speed limits when the police fail to enforce them.

Mannering thinks there are safer ways of encouraging fuel efficiency than setting speed limits at 55 mph, such as raising fuel prices. "If you want to save fuel — and I'm wearing my economics hat — do it with price," he said. "You could go to 25 or 35 mph and save fuel, but people would die of boredom." For straightaway roads that beg for a lead foot but cut through busy residential neighborhoods, Mannering recommends stricter policing. "If you have a road that's designed for 50 mph and you have kids playing in the street, put up signs and enforce it," he said.

POST UPDATED 11:25 a.m. PST.

Photo by Vlastula/Flickr