Comment 101733

By notlloyd (registered) - website | Posted May 28, 2014 at 17:25:53 in reply to Comment 101726

The 85% rule was based on scientific studies. They were not conducted by motorist associations.(It may be that motorist associations have picked it up but the initial studies were blind studies designed to determine if reducing speed limits increased safety.) What the studies found was that if you have existing infrastructure, reducing speed limits did not increase safety. It says nothing about design. What the studies found was that if you have existing infrastructure, manipulating speed limits has limited impact on the rate or volume of traffic accidents. IOW given a specific road design, reducing speed limits below the 85th percentile will not reduce collisions.

I looked at the data. It is all collisions including pedestrian and cyclist collisions. They observed every type and they are all lumped together. So you are correct, it says nothing specifically about pedestrians.

But there are many factors in road design. Time, and costs are two factors that get marginal attention as people would say just slow down to save lives. The inadequacy of that position is that it ignores the conclusion that slowing down will not reduce death rates - depending upon the specific road in question. I found one study that said that after 10mph, road death rates are not impacted by velocity limits at all. (I will find that study.)

Studies show that danger to any specific pedestrian decreases substantially under 50kmph. So if you take a model human and hit him at 50kph, the model is more likely to die than if you struck him at 30 kph. But an important factor is the Rate. While it appears to be common sense that you should drive more slowly, the fact that higher speeds kill more efficiently says nothing about the rate. So, you need to find out how many pedestrians are killed by drivers driving over 30 kph. Most pedestrians are killed at intersections where driver inattention or pedestrian error are the cause, and not the velocity of the vehicle per se.

If you want to design streets to a maximum speed, say in high density residential areas so that vehicles will not exceed some specific velocity deemed harmful to pedestrians, then you might be correct that the right answer is to design the street so that drivers are uncomfortable driving above that limit.

However, there is no point slowing everyone down if it will not save lives. You can do it for other reasons for sure. But to simply argue that it is to save lives is a bit disingenuous.

A good example is flying in an aircraft. In any individual air crash of a large airliner, the morbidity is likely 100%. It is super dangerous to be in an air crash. But flying is way safer than driving a car based on rates - traveler miles per death. I wager that impairment, driver inattention, distracted driving, jaywalking, dimentia, old age, etc. are far more significant factors in pedestrian fatalities than velocity.

Comment edited by notlloyd on 2014-05-28 17:40:01

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