Study finds many motorists don't see need to heed speed limits

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.

Source: Purdue University

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Nov 07, 2008
I drive safely, I never obey speed limits, they completely fail to reflect the safe speed in most cases.

Nov 07, 2008
My brother did some equation a while back where he figured out how much time he saved himself per year by driving 10 over limit minimum.

Its the people that think they are driving the Indie 500 that cause problems.

Nov 07, 2008
As automotive technology has advanced, the driver has come to be less and less in touch with the real physics of what he/she is actually doing. More horsepower, power steering, power brakes, better tires, improved road/engine/environmental isolation have made it hard for the driver to truly have any appreciation for the huge physical forces involved.

Most notably, modern brakes have made stopping quickly a common and seemingly risk-free task. And modern suspensions are very good at giving the driver a very enhanced sense of control and stability.

As an example, if you were to drop a car nose down from about thirty feet off the ground, it would hit the ground going just about 25 mph. Yet most people probably imagine a collision at that speed posing nearly negligible physical threat to the occupants. It's like jumping out of a third story window.

So it seems natural to me that drivers would find speed limits too low in most situations.

Cars will continue to advance. I think the solution is better driver education. And of couse, better science education, especially real-world physics.

Nov 07, 2008
5, 10, or 20 miles over WHAT speed limit? Unless the study specified the zone, it seems hazardous to draw any conclusion from it.

An obvious example is a school zone where children are present vs a limited access interstate. Upping your speed from 15 mph to 35 mph in the former is different than upping it from 65 to 85 on a sparsely traveled interstate. It's hard to believe that people would assess these situations identically.

Nov 07, 2008
most speed limits are arbitrary and frivolous. Extensive studies in Calif. ca. 40 yrs ago and repeated in other states confirmed that the safest speed to drive is at the 85th percentile, not at the set speed limit. It is my understanding that Calif. is required to survey all major roads every 2 yrs and set the speed limit at the 85th percentile.

Nov 08, 2008
Mayday, I loved your post, but keep in mind 30MPH is the standard speed limit for small streets.

40MPH for medium streets, and 60 to 70MPH for highways. That sortof makes your 3-story warning moot. At these speed limits it's already so much more than a 3-story building that it doesn't matter.

Speed limits are irrelevant. Most accidents would be avoided if people would just drive intelligently:

1) Show your signal so others know what you're going to do.
2) Keep in the right lane most of the time, and go into the left *only* for passing.
3) Keep at least 3 seconds between you and the car in front of you (use road markers to determine this), that will give you enough reaction time to avoid an accident if it happens in front of you. (in other words, don't tailgate!)

If drivers were to actually follow those 3 simple rules, we wouldn't need speed limits. You could drive 120 MPH safely and comfortably. In fact it would speed up traffic significantly, you'd get to where you wanted to go faster.

In other words, speed limits do not create safety. Intelligent driving does.

Nov 08, 2008
1) Show your signal so others know what you're going to do.
2) Keep in the right lane most of the time, and go into the left *only* for passing.
3) Keep at least 3 seconds between you and the car in front of you (use road markers to determine this), that will give you enough reaction time to avoid an accident if it happens in front of you. (in other words, don't tailgate!)

You forgot #4, (although implied), the major cause of accidents:

4) Pay attention at all times and don't get distracted!

Nov 08, 2008
Speed differentials are what create damage and injury. It reduces reaction times and increases forces involved. Speed limits are largely for the sake revenue in metropolitan areas. In mountainous rural roads speed limits are more realistic and informative.

Nov 08, 2008
Speed limits are set by politicians. Safe driving speeds are generally set by motorists. Most people drive a speeds they feel safe.
In Los Angeles, where the posted freeway speed limit is 65 mph, 80 is closer to the norm... at least when traffic is moving smoothly.
Another factor concerning speed limits is the use of enforcement to augement municipal income.
I agree with another poster. Speed limits are archaic when considering improvements in automobile performance and the state of modern highways.

Nov 12, 2008
This seems to me to be an unintended consequence of the stupid national 55 mph speed limit of the 70s thru today still in areas. The limited access roads were, and are, engineered for safe driving at 75 mph with a slop-factor of about 10 mph. continuing the 55 mph mandate to save gas, once the crisis ended, made much of the country into scofflaws and the police into enablers. In those circumstances this is the inevitable result.

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