The Boy Who Cried 55: Drivers Ignore Too-Low Speed Limits

From the Autopia Unintended Consequences Department comes this dispatch from Tippecanoe County, Indiana: Drivers don't pay attention to speed limits that they think are set too low.

Fred Mannering, a Professor of Civil Engineering and Economics at Purdue
University, attributes this disregard for speed limits to the
much-maligned 1974 Emergency Highway Energy Conservation Act that set
speed limits to 55 mph on all interstate highways. "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.

Mannering's study of 988 drivers, published in next month's
Transportation Research Part F(subscription), finds that 21 percent of survey respondents considered it safe to drive up to 5 mph over the speed
limit, 43 percent thought it's safe to drive up to 10 mph over and 36
percent said they're doing no harm driving up to 20 mph over the speed limit.

"What makes it dangerous is when roads are set accurately," Mannering told Wired.com, and drivers still continue to exceed the limit by 20 mph. As anyone who has ever watched a cement truck merge in front of a speeding Integra can tell you, it's also extremely dangerous to have two vehicles travelling at widely different speeds on the same road.

The phrase "speed limit" has become 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 around a tree or unintentionally modifying a guardrail. It
factors in fuel efficiency, pedestrian safety, and concerns of neighbors.
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 told Wired.com. "You could go to 25 or 35 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.

Photo by flickr user
pathawks
.