WEST LAFAYETTE, Ind. — Research suggests U.S. motorists are growing increasingly cynical about the relevance of speed limits, and a new study indicates many motorists are more likely to think they can drive safely while speeding as long as they won’t get caught.

“So the faster you think you can go before getting a ticket, the more likely you are to think safety’s not compromised at higher speeds,” said Fred Mannering, a professor of civil engineering at Purdue University.

Mannering used a series of mathematical equations in “multinomial logit models” to calculate probabilities based on data from a survey of 988 motorists in Tippecanoe County, Ind., where Purdue is located.

Findings generally agree with other data taken in recent years.

“For whatever reason, respect for speed limits seems to have deteriorated,” Mannering said. “A 2002 survey indicated two-thirds of all drivers reported they exceeded the posted speed limit, and roughly one-third reported driving 10 mph faster than most other vehicles. These figures are even more disturbing when you consider that they’re self-reported and likely to be understating the degree of speeding problems.”

The Indiana survey participants were asked: “At what point do you feel speeding becomes a threat to the personal safety of you and your family?” The motorists were given three choices: 5 mph, 10 mph or 20 mph over the speed limit.

The survey was taken before and after a 2004 media campaign launched in the county stressing the dangers of speeding that included radio and newspaper messages.

Using survey data, Mannering applied a series of mathematical equations in a model to estimate the probabilities of speed and safety viewpoints for drivers in various categories.

Findings are detailed in a research paper appearing in the journal Transportation Research Part F: Traffic Psychology and Behavior, available both online and in an upcoming print edition of the journal.

“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.”

Of the 988 drivers in the survey, 21 percent thought it was safe to drive up to 5 mph over the speed limit, 43 percent thought it was safe to drive up to 10 mph over and 36 percent thought it was safe to drive up to 20 mph over the speed limit.

“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.”

The research showed the media campaign relating to the dangers of speeding had no statistically significant impact on drivers’ views on speeding and safety.

Other findings showed that women who have never been stopped for speeding are 68 percent more likely to think that it’s only safe to drive 5 mph over the speed limit compared to all men and other women who have been stopped for speeding. Both men and women drivers who have been stopped for speeding in the last year are about 25 percent more likely to believe that it is safe to drive up to 20 mph over the speed limit than those who have not.

“This is probably because people who habitually speed are not significantly deterred by being stopped for speeding,” Mannering said. “They might become slightly more conservative, but it doesn’t slow them down to the level of people who are inherently more conservative.”

The findings also showed that people get progressively more conservative about speeding as they age. A 25 year-old driver is 75 percent more likely to think it is safe to drive up to 20 mph over the speed limit than a 50 year-old driver.

WEST LAFAYETTE, Ind. — Researchers at Purdue University have determined that raising the speed limit from 65 to 70 on Interstate 65 in Indiana has not increased the probability of fatalities or severe injuries.

“These findings are important because the influence of speed limits on roadway safety has been a subject of continuous debate in the state of Indiana and nationwide,” said Fred Mannering, a professor of civil engineering. “Indiana highway-related accidents result in about 900 fatalities and 40,000 injuries annually and place an incredible social and economic burden on the state.”

The findings add new fuel to an ongoing debate, with some studies indicating that the benefits of raising the speed limit outweigh potential safety hazards while others suggest just the opposite.

“The safety of raising the speed limit has been a matter of considerable concern in Indiana since the state raised its speed limits on rural interstates and selected multilane highways on July 1, 2005,” Mannering said. “Everybody expects that when you increase the speed limit, injuries and the severity of injuries are going to increase, but that hasn’t happened on the interstate highway system in Indiana.”

Findings are detailed in a research paper presented earlier this year at a meeting of the Transportation Research Board of the National Academies of science and engineering. The paper, appearing in an upcoming issue of the Transportation Research Record, was written by Mannering and research assistant Nataliya V. Malyshkina.

The researchers used a series of mathematical equations in “multinomial logit models” to calculate accident probabilities based on motor vehicle accident data from 2004 and 2006, before and after the speed limit increased. The models showed that the increased speed limit did not affect the probability of suffering a severe injury in an accident.

Understanding the magnitude of the safety impact of increasing speed limits, or whether safety is improved or compromised, remains a contentious subject, Mannering said.

That’s because research has not been able to convincingly unravel the effects of speed limit changes from factors such as speed enforcement; vehicle miles traveled; vehicle occupancy; seat belt usage; alcohol use; proportions of passenger cars, minivans, pickup trucks and sport utility vehicles; and vehicle safety features, including air bags and antilock brakes.

The models used in the research calculate “estimated probabilities” based on accident data. Of the 204,382 accidents on state highways in 2004, 21 percent resulted in injuries and about 0.4 percent were fatal. The same percentages were seen in the 182,922 accidents in 2006. Comparing only highways where the speed limit rose to 70 showed virtually no change from 2004 to 2006.

The speed-limit increase made Indiana the 30th state to raise interstate speed limits up to 70 mph on rural interstates.

Findings by other researchers suggest even higher speed limits on interstate highways might also result in no rise in the probability of severe injuries.

“If going from 65 to 70 doesn’t have a significant effect on the severity of accidents, you have to ask yourself, what about 70 to 75?” Mannering said. “At what point does it begin to impact safety?”

The change has highlighted the tradeoff between speed and safety, a debate that began more than three decades ago since passage of the Emergency Highway Energy Conservation Act in 1974, which mandated a 55 mph national maximum speed limit on interstate highways in the United States, Mannering said.

State and federal speed-limit policy changes have been driven by various research findings and subsequent legislation, including the National Highway System Designation Act of 1995, which gave states freedom to set interstate speed limits.

Most research efforts have concluded that the 1974-mandated 55 mph interstate speed limit saved lives, a conclusion that has been confirmed by some studies looking at recent speed limit increases on interstates.

“For example, one study found that a speed limit increase from 55 to 65 resulted in roughly a 3 percent increase in the accident rate and a 24 percent increase in the probability of a fatality once an accident occurred,” Mannering said. “But then other studies have contended that legislation-enabled speed-limit increases have actually saved lives. One study argued that increasing from 55 to 65 saved lives because of shifts in law enforcement resources, the ability of higher speed limit interstates to attract riskier drivers away from inherently more dangerous non-interstate highways and reducing how often drivers speed up and slow down.”

The state also increased the speed limit from 55 to 60 on non-interstate multilane roads, but findings were inconclusive regarding the impact on those roadways, Mannering said.

However, the researchers cautioned that future speed limit increases should be carefully assessed for those roads on a case-by-case basis. The potential for accidents is higher on such roadways because of factors including stop-and-go traffic and vehicles entering the road from various businesses and residential areas.