Fred Mannering, a professor of civil engineering at Purdue, analyzed data from a survey of 998 Tippecanoe County, Ind., motorists using a series of mathematical equations in "multinomial logit models" to calculate probabilities.

"The intent of the study was to statistically assess drivers' perception of the relationship between speed limits and safety," Mannering said. "In recent decades it has become more common for speed limits to be set for political reasons rather than for safety reasons. Consequently, the motoring public seems to have increasingly begun questioning the rationality of speed limits. This is evident in observed speed data that show the majority of drivers routinely exceed posted speed limits.

"The new findings show that the speed enforcement is critical to motorists' safety perceptions," Mannering said. "Let's say you think enforcement is getting lax and the speed at which you think you will get a ticket goes up from 7 mph over the speed limit to 10 mph over the speed limit. If that happens, our statistical results indicate that you would be 27 percent more likely to think you can safely drive up to 20 mph over the speed limit."