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Do higher speed limits equal more dangerous conditions?

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As a legislative bill that proposes raising some North Carolina highway speed limits from 70 to 75 races through the Senate, you may wonder if that's such a good idea. It isn't necessarily a bad one. When the Texas Department of Transportation debated raising the speed limit between San Antonio and Austin to 85 mph — the highest speed limit in the country — Slate debated whether this increase would cause more accidents. "It’s often assumed that higher caps will make roadways more dangerous, because motorists will exceed whatever ceiling is in place," wrote Katy Waldman.

However, "[i]n a [1996 National Motorists Association] study of 22 states where speed limits were either raised or lowered by five, 10, 15, or 20 miles per hour, researchers found that cars' average velocities did change, but by less than two miles per hour." New York raised its speed limit cap to 65 mph in 1995, and the state's crash rate reportedly decreased by 4 percent. Likewise, Ohio reported a total of six deaths — the lowest fatality rate in Ohio Turnpike history — when it raised the speed limit to 70 mph in 2011. Sen. Neal Hunt proposed the bill to "move traffic along" and help prevent people from getting pulled over when there's light traffic and no reason they couldn't be driving a little faster. "You've still got to drive safely," he said.

This isn't to say that faster speed limits are always the answer, but perhaps the higher speed limits don't automatically mean less safe roadways.

About the Editors: The Shapiro Lewis Appleton & Favaloro personal injury law firm, whose attorneys work out of offices in Virginia (VA) and North Carolina (NC), edits the injury law blogs Virginia Beach Injuryboard, Norfolk Injuryboard, Eastern Shore Injuryboard, and Northeast North Carolina Injuryboard (include hyperlinks to each of those) as a pro bono service.

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  1. aloysious says:
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    [This isn’t to say that faster speed limits are always the answer, but perhaps the higher speed limits don’t automatically mean less safe roadways.]

    Ever heard of “physics”…?

    A vehicle’s kinetic energy is proportional to its velocity squared. When a crash occurs, all or part of the kinetic energy is dissipated, primarily through friction and mass deformation.

    As kinetic energy increases exponentially with speed, so does the potential for mass deformation, including humans that are inside and outside of the vehicle.

    Increased speed means increased stopping distances and less reaction time.

    I think we can easily reasonably conclude increased speeds = decreased “safety”.

    “The first thing we must recognize is that crashes are not accidents.”
    -Ricardo Martinez, M.D., NHTSA Administrator, 1997

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    Engineers have known for at least 70 years that you almost always get the smoothest and safest traffic flow with the fewest crashes when the posted limit is set at the 85th percentile speed of free flowing traffic under good conditions rounded to the nearest 5 mph interval.

    It is NOT a matter of higher or lower limits, it is a matter of the RIGHT 85th percentile limits.

    On NC rural Interstates, almost all should be at 75 mph with a few of the best places at 80 — IF safety is the true goal.

    The physics argument is nonsense. The dangers of a crash that does NOT happen are zero. The goal is to minimize crashes and 85th percentile speed limits almost always accomplish that goal.

    See the science on our website under the Speed Limits link and particularly read the Michigan State Police item in the Articles link.

    James C. Walker, Life Member-National Motorists Association

  3. Kevin Duffan says:
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    Thank you Mr. Walker for offering the reply (much more eloquently stated) that I was going to type out. It isn’t simply a matter of physics. Sure, faster speeds equal more violent collisions, but that doesn’t tell the whole story. Faster speeds don’t equal MORE collisions, and there is a point at which the injuries are going to occur no matter if the car causing the injury was going 72mph or 74mph or maybe even 79mph. It could all be the same result.

    I once read a study that unfortunately I can’t pull up right now that discussed many of the points that Walker pointed out. Decreasing accidents is more about traffic flow than speed.

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    Thanks, Mr. Duffan.

    I started studying this issue as an avocation as a freshman at the Univ. of Michigan in 1962/63. There is a massive body of literature showing that 85th percentile limits tend to be the safest. Readers can see a selection of it at our website http://www.motorists.org. And here is the history of our organization’s first 30 years, the group most responsible for getting the National Maximum Speed Limit eased in 1987 and repealed in 1995.
    http://www.motorists.org/nma-first-30

    One of the most interesting items is the study by Lave and Elias after 65 mph Interstate limits were authorized in 1987. States that adopted 65 limits had larger reductions in the fatality rates than states which retained 55 limits.

    Unfortunately, the National Highway Traffic Safety Administration, the Insurance Institute for Highway Safety, and many other groups that make money from speeding tickets have put out huge amounts of misinformation and deliberate dis-information on this issue.

    If we could get the revenue out of traffic laws and enforcement procedures, it would be a LOT easier to get things done to maximize safety.

    James C. Walker, Life Member-National Motorists Association

  5. aloysious says:
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    The question is: Do higher speed limits equal more dangerous conditions?

    It is typical of the ignorant of traffic safety to interpret that there is no danger until a crash occurs, and that a crash will not occur, and that speed is not a factor in either the likelihood of a crash occurring or of the severity of injuries sustained in those crashes.

    “Joksch (1993) found that the risk of a car driver being killed in a crash increased with the change in speed to the fourth power as shown in figure 5. The risk of a fatality begins to rise when the change in speed at moment of impact exceeds 30 mi/h (48 km/h) and is more than 50 percent likely to be fatal when the change exceeds 60 mi/h (96 km/h).

    The probability of death from an impact speed of 50 mi/h (80 km/h) is 15 times the probability of death from an impact speed of 25 mi/h (40 km/h).”
    http://www.fhwa.dot.gov/publications/research/safety/98154/speed.cfm#speedseverity

    “Another way to examine the relationship between vehicle speed and traffic safety is to measure the effects of lowering or raising speed limits on the incidence and severity of crashes.

    Table 3 summarizes the results of studies of this type conducted in several countries. The table shows that crash–incidence or crash severity, or both measures, generally decline whenever speed limits have been reduced. Conversely, the number of crashes or crash severity generally increased when speed limits were raised, especially on freeways.”
    http://www.fhwa.dot.gov/publications/research/safety/98154/speed.cfm#speedsafety

    It isn’t that motorists could not operate relatively safely at higher speeds, it’s that they refuse to exercise good judgment at every speed. They tend to measure travel time in seconds and feet, if not fractions thereof.

    The other side of the greater speed coin is “speed adaptation”; after driving faster slower speeds seem slower than they are. Speed adaptation is the suspected culprit to explain why the vast majority of multiple-vehicle crashes occur at intersections closest to limited access highways.

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    There is no doubt that crashes at 80 mph versus 70, or 40 mph versus 30, involve more energy and more danger. That is simple physics.

    aloysious and others who rely mostly on the physics arguments fail to realize:

    1) total danger is the consequences times the probability of a crash. Please note that the fatality rate per mile traveled today is less than 25% of that in the 1960s. With denser faster traffic, driving is a LOT safer than 40 or 50 years ago.

    2) posted limits have virtually no effect on actual travel speeds, particularly at the upper end. You can raise or lower posted limits by up to 15 mph and in almost all cases the 85th percentile speeds will change by a maximum of 3 mph, and usually by 1 mph or less. If the 85th percentile speed on a freeway is 81 mph and someone near the top of the normal speed range crashes, it makes absolutely no difference in the crash consequences if the numbers painted on the signs are 80, 75, 70, 65, 60, or 55.

    3) higher posted speeds on our Interstates and other high quality roads draw more traffic to those roads and off lesser roads with higher injury and fatality rates. The fatality rate per mile traveled on an Interstate is two to four times lower than a surface highway with lower actual speeds. You WANT the traffic on the best roads and realistic speed limits accomplish that. See the Lave and Elias study where states that went to 65 in 1987 had greater drops in their fatality rates than states which retained 55.

    4) The same thing happened after 1995, though to a smaller extent than in 1987. States which went to limits above 65 had a slightly better reduction in their fatality rates than states which retained 65 limits. The difference was much smaller than after 1987, but it was in the correct direction.

    5) Higher speed limits with less variance on Interstates means a bit shorter travel times for long journeys. This reduces the fatigue factor in crash probabilities. You don’t tend to change the fastest vehicles by much, you DO raise the speeds of many of the slower groups so their trips are shorter in time and smoother.

    6) If you actually drive at 65 on a road designed for 80, there is a significant boredom and lack of attention factor. The task is too easy so you lose focus – and crashes at 65 ARE dangerous. It is much better to have those drivers at 70-75 mph and paying more attention. It is also much better to have them at 73 mph when caught by one of the cars at 80, the smaller variance helps.

    7) Most of the quoted research in aloysious post are not from the USA and there are large cultural and enforcement differences. If almost everyone actually goes 55 instead of 65, you will have reduced crash consequences. But people will NOT do that on roads designed for 75 to 85 mph, so the speed variance is a worse problem than faster but smoother traffic.

    Traffic safety is a fiendishly complex matrix to manage. Focusing on one element like the physics argument will lead to wrong conclusions.

    Oh, and to give the physics argument its due, if everyone ACTUALLY drove at 20 mph or less, the fatality rate would be near zero in modern cars. And commerce would grind to a halt.

    James C. Walker, Life Member-National Motorists Association