Study Finds Many US Drivers Don’t See Need to Heed Speed Limits
10 November 2008
Research suggests US motorists are growing increasingly cynical about the relevance of speed limits, and a new study by Professor Fred Mannering at Purdue University indicates many motorists are more likely to think they can drive safely while speeding as long as they won’t get caught.
In recent decades, it has become more common for speed limits to be set for political reasons rather than for safety reasons. As a consequence, the motoring public seems to have increasingly begun questioning the rationality of speed limits. This is evident in observed speed data that show that the majority of drivers routinely exceed posted speed limits. A key motivating factor in drivers’ tendency to exceed the speed limit is that they believe that the excess speed does not threaten safety.
...Estimation findings show that drivers’ perception of the speed above the speed limit at which they will receive a speeding ticket is a critical determinant of what they believe is a safe speed—suggesting that enforcement plays an important role in safety perceptions.
—Mannering 2008
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. 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.
—Fred Mannering
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. 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.
—Fred Mannering
Of the 988 drivers in the survey, 21% thought it was safe to drive up to 5 mph over the speed limit, 43% thought it was safe to drive up to 10 mph over and 36% 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. 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.
—Fred Mannering
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% 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% 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. They might become slightly more conservative, but it doesn’t slow them down to the level of people who are inherently more conservative.
—Fred Mannering
The findings also showed that people get progressively more conservative about speeding as they age. A 25 year-old driver is 75% more likely to think it is safe to drive up to 20 mph over the speed limit than a 50 year-old driver.
Resources
Fred Mannering (2008) An empirical analysis of driver perceptions of the relationship between speed limits and safety. Transportation Research Part F: Traffic Psychology and Behaviour doi: 10.1016/j.trf.2008.08.004
No breakdown from this article between highway speeding and local roads speeding.
Going 65 mph in a 55 mph on an Interstate in Delaware is generally safer than going 35 mph in a 25 mph in a school zone. Context matters.
That written, the solution?
1. Raise the price of a speeding ticket 50%.
2. Parlay the extra money to hire more cops.
3. Post those cops in more areas to write more tickets.
My parents live about a mile from the local high school. Local high school has gas pumps. Local police fill their cruisers there, and are therefore on the road a lot. Speed limit is 25 mph [school zone]. Nobody breaks 30 on that road, because they know the odds that a cop sees them is high. The expected value of a speeding offense [Pr[ticket] * cost of ticket] is high, so people don't speed on that road. Make more roads like that one and speeding will be reduced.
Posted by: stomv | 10 November 2008 at 04:34 AM
The part I love is when I see the same people who fly down the highway at 80mph (60 limit within the city limits) also slow down to 50mph on the slightest curve in the highway. Hmmm, if you need to slow down that much, what happens when you need to make an emergency manuever at 80mph on the straights?
Posted by: | 10 November 2008 at 07:49 AM
I'm sure our high automobile safety standards are part of the reason. Reduce those, and not only would people slow down a little, but we might also get some of those hyper-efficient cars they sell in Europe but not in the USA due to their lower safety standards.
Posted by: Derek | 10 November 2008 at 09:04 AM
I echo the need to see the difference between local roads and interstate. also want to see the difference between good/bad weather.
wrt speed limits, local road limits are often political/revenue driven and not based on road safety. Personally I think speeding tickets do not modify behavior because it they did, more people would not speed.
Posted by: countrymouse | 10 November 2008 at 09:58 AM
The Maryland State Police routinely drive at 80 mph. The de facto speed limit for mere citizens is 10mph over the posted 65mph limit on the highways. In West Virginia, on the other hand, the speed limit is 70mph, and the State Patrol drives at 75mph (5 over the limit) and that seems to be the de facto limit for citizens, too.
IMHO, WV's policy is reasonable even though the result is the same in both states.
Posted by: Joe in MD | 10 November 2008 at 11:17 AM
What bothers me about speed limits is that they don't account for what type of car is being driven; if a MINI stops faster and handles better than a Suburban, then how can the maximum 'safe' speed for both of these vehicles be the same?
Large, clumsy SUVs should be required to travel at lower speeds than smaller, more agile vehicles. This would not only make us safer, but it would also discourage people from buying larger and (generally) less efficient vehicles.
Posted by: mitch | 10 November 2008 at 11:37 AM
Derek, what on earth are you talking about? Highway speeds in Europe are generally higher than they are here. Furthermore, we do get their cars here, the Mini Cooper, the Smart, etc. What we're missing out on is their Diesels.
stomv, you take as a given that making people slow down is a good thing. No thanks. I'd rather drive a fuel-efficient car at high speed than slow down to save fuel in an SUV. As for the highway/surface road issue, I agree; driving in excess of the speed limits on many highways is generally not unsafe because the speed limit has been set arbitrarily low. This is exactly why we don't need more cops on these roads to make us slow down, give us speeding tickets, impose fines, take away our licenses and jack up our insurance rates. The whole thing is a scam, why would we want more of it?
Posted by: Peter | 10 November 2008 at 11:40 AM
Every state in the US and almost every polititian talk about renewable energy but fails to support economical driving speeds. There is not one state in the US that had fewer traffic deaths this year than were killed at Chernobyl. More people were killed at a single automobile race accident than at Chernobyl. ..HG..
Posted by: Henry Gibson | 10 November 2008 at 02:14 PM
If you are tired of the "scam" and "revenue driven" policies try driving the speed limit. That would certainly deprive them of their revenues!!!
By the way - mixing speeds on one road without having clear divisions for driving speeds isn't going to make anybody safer. In states with lower speed limits for trucks, they also force them to stick to the right lanes (from what I have seen). If you have more than one speed limit for different vehicles (which in itself is already not a very smart thing to do as people have enough trouble paying attention to what laws apply to them as it is) you better make clear lane divisions for each speed as well.
I love it when people claim red light cameras or "speed traps" are 100% revenue driven and should be removed. I say those idiots should try obeying the law and they would deprive the county of said revenues. Hit them where they hurt the most - follow the rules of the road.
Posted by: | 10 November 2008 at 02:16 PM
Good logic there, anon. The laws are senseless and revenue-driven; therefore I should obey them. I don't think we should have different speed limits for different cars based on their fuel efficiency. I think the gas guzzlers should be taken off the roads altogether. What I am suggesting is that driving a more fuel efficient car is a better way of saving fuel than driving more slowly.
Henry, why should I care if more people die driving than died at Chernobyl? We could reduce traffic fatalities to zero and that wouldn't necessarily save the environment. I'm all for reducing traffic fatalities, but I'm not convinced that lowering the speed limit will save the environment or reduce traffic fatalities. At least not without something more. Better public transit and high speed trains, for example, would probably do both.
Posted by: Peter | 10 November 2008 at 04:38 PM
So, you are saying that you are not convinced that having less energy involved in a collision would result in fewer traffic fatalities? Talk about good logic...
You are disobeying laws because YOU FEEL (and it is PURELY YOUR OPINION) that they are senseless and revenue driven? Great, so I think letting handicap people have special parking is senseless so I should take their spots whenever I feel like it and then sue parking enforcement for littering when they give me a ticket?
I would say it is odd that you would choose to support the counties by becoming a source of revenue when you could obey the "senseless laws" with virtually no real impact on your life to remove any chance of them gaining revenues but then again you feel physics somehow has no impact on injuries despite the kinetic energy involved in an accident increasing at the rate of the square of the differences in velocities (thus a head on with two vehicles driving in opposite directions has much greater total energy to dissipate with each travelling at 75mph (150mph delta) versus each travelling at 55mph (110mph delta)).
Well, that explains quite a bit. So tell me, when you drive 30 miles through congestion where you may have max 5 miles to travel at 75mph, 20 miles at an average rate of 40mph, and 5 miles at an average rate of 10mph are you really saving that much time versus driving 60mph for the 5 miles where you had an opportunity to drive without being impeded by traffic? Does that extra 1 minute of time help you to avoid some larger traffic jam or get you to work in time so you are not late? (if so, hit the road one minute earlier). This anecdotal example is taken from what I have experienced driving the Seatle Metro area during typical rush hour.
Posted by: | 12 November 2008 at 10:51 AM