Proposed Speed Limit Increase Based on Pseudoscience

images[8]Republican Senators Rick Jones and Tom Casperson are proposing raising speed limits throughout Michigan, claiming that the current law only encourages communities to set-up speed traps for the purpose of revenue enhancement. While such abuses are certainly commonplace, using that fact to leverage the argument that speeds should be raised across the board is irresponsible.

The Senators employ something called the 85th Percentile Rule — based on a traffic study conducted 50 years ago, and not updated since. The rule goes like this:

The speed limit is commonly set at or below the 85th percentile operating speed (being the speed which no more than 15% of traffic is exceeding) and in the US is typically set 8 to 12 mph below that speed. Thus, if the 85th percentile operating speed as measured by a Traffic and Engineering Survey exceeds the design speed, compulsory legal protection is given to that speed–even if it is unsafe. This speed creep tends to continue until the 85th percentile operating speed is comparable to speed psychologically perceived as uncomfortably hazardous. The theory behind the 85th percentile rules is, that as a policy, most of the electorate should be seen as lawful, and limits must be practical to enforce. However, there are some circumstances where motorists do not tend to process all the risks involved, and as a mass choose a poor 85th percentile speed. This rule in substance is a process for voting the speed limit by driving; and in contrast to delegating the speed limit to an engineering expert.

Basically, what that means is that the speed limit will be set at what motorists generally perceive to be a safe speed. Proponents of this rule often go even further to claim that faster speeds actually increase road safety.

The real science begs to differ.

A 2009 study published in the American Journal of Public Health found that fatalities increase dramatically as speeds go up.

Results. We found a 3.2% increase in road fatalities attributable to the raised speed limits on all road types in the United States. The highest increases were on rural interstates (9.1%) and urban interstates (4.0%). We estimated that 12 545 deaths (95% confidence interval [CI] = 8739, 16 352) and 36 583 injuries in fatal crashes (95% CI = 29 322, 43 844) were attributable to increases in speed limits across the United States.

Conclusions. Reduced speed limits and improved enforcement with speed camera networks could immediately reduce speeds and save lives, in addition to reducing gas consumption, cutting emissions of air pollutants, saving valuable years of productivity, and reducing the cost of motor vehicle crashes.

Yet, the Institute of Transportation Engineers endorse the 85th Percentile Rule citing that people will drive whatever speed they want anyhow and posting unrealistic speed limits not only encourages lawbreaking but increases accidents due to driver-to-driver variances in speed. Both points have some validity, but the ITE goes on to buttress their argument with a graph showing that interstate highways with faster speeds are safer, by the mile, than other types of roads. Well, duh — those roads lack driving distractions such as intersections, sharp curves, traffic signals — they are designed to accommodate high speeds.

The ITE puts it this way “The 85th Percentile is how drivers vote with their feet”. These engineers seem to be myopically focused only on motor vehicles though. Bicycle and pedestrian safety advocates take issue with this majority rule notion. They refute saying:

[T]he main argument [is]to build more highways and freeways with faster speeds where the ends justify the means. Even if the means ignore vulnerable groups such as pedestrians and cyclists. Even if the study is now also used to serve the automobile in densely-populated urban areas, far from any freeway. They forgot to mention that, when it comes to establishing speed limits in cities, pedestrians and cyclists are excluded from this election. They don’t even get the chance to go to the polls.

Senator Casperson has a long legislative history of ignoring science over his preferred Tea Party fairy dust. This speed limit proposal looks to be more of the same.

Amy Kerr Hardin

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3 Responses to Proposed Speed Limit Increase Based on Pseudoscience

  1. Suzi says:

    Passing seems to be one of the more dangerous maneuvers so perhaps it would be more wise to design the roads better – two lanes in each direction, a turning lane, and with a real bicycle/pedestrian lane on both sides. Bikes, for the most part, have no business on the roads since most have little to no room for them and cars moving over into the opposing lane is no longer an easy option since most of our roads are well-traveled. With better roads, higher speeds can be done without increasing accidents. Also, it would be great to spend money on infrastructure and provide good jobs for a large number of citizens.

  2. Cory Johnston says:

    There is just so much wrong with this idea that I don’t know where to start. Our roads are generally designed for faster than posted speeds so there is already an encouragement to drive faster under the notion of “voting with your feet.” Second is that the 85th percentile rule takes little account for location and conditions. I know far too many residential school zones that have a posted speed limit of 45 to 55 m.p.h. because the roads are straight with good visibility. Yes the road itself may be capable of these speeds but the conditions are not and it is unsafe for everyone. Then there is the notion that driving is safer if everyone is going the same speed. Since the assumption of this proposal is that everyone is speeding, they must assume that everyone, and every vehicle, is capable of safely driving in excess of 70 or 80 m.p.h. I don’t think it takes a scientific study to know that is not true. The state cannot afford to fix the roads and bridges we have now, even as plans are in place to build more and widen what we have. But someone thinks it would be a good idea to drive faster on these same roads. Mileage will go down, accidents will certainly be worse when they occur, wear and tear on the roads will increase, and why even talk about merging traffic issues. I do have a solution however. Let’s change the Michigan no-fault law to make these lawmakers, and the drivers that exceed a reasonable speed limit, responsible for any damage and loss of life they cause.

  3. John Baxter says:

    The 85% rule is actually good science. Most drivers know how fast they can drive safely UNLESS there are invisible or unknown hazards ahead. The 85% rule should be followed except where experience has shown that unusual hazards require a lower limit.
    Just because a lower posted limit reduces crashes does NOT mean people driving at a given limit are going too fast. A mere 3.2% improvement could easily be explained as intimidation of the very fastest drivers, the relative few without good judgment. If we lowered the limit on Interstates to 15 mph, all fatalities would surely be eliminated. But, does that really make sense?
    Since so few people keep a sensible following distance, isn’t it likely that lowering the limit simply adapts the overall speed to the distance so many drivers follow at? How about a well-rounded safety program that 1 teaches people to follow 3 seconds behind and 2 exposes all licensed drivers to repeated training regarding the actual causes of accidents, which so often are far more complicated than simply going too fast. How about an enforcement program that gets the police out from behind radar guns and has them on the road, and watching all the aspects of bad driving that cause accidents, whether it be inattentiveness, tailgating at higher speeds, changing lanes without signaling and ensuring the lane is unoccupied, etc. We need a well-rounded safety program that is based on a sophisticated understanding of all the errors that cause accidents and teaches people how to drive at highway speeds rather than quixotically attempting to oversimplify the problem with the unscientific idea that speeding is just about the only cause of crashes.

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