Comment 101726

By kevlahan (registered) | Posted May 28, 2014 at 16:14:38 in reply to Comment 101718

I'm not sure quite what you are arguing here.

The 85% rule is based on the principle that rules should be aligned as much as possible with actual behaviour. I agree that it is not a good idea to engineer roads for 70km/h and then impose a 50km/h limit. But the 85% rule has nothing to do with safety! The 85% rule is promoted by motorist associations as a way to increase speed limits and avoid traffic tickets, http://www.sense.bc.ca. Note that the belief is that drivers naturally know what speed is safe and so we should just base the limit on their collective wisdom.

But even if this is true (which it is not, the average speed is based on what feels comfortable or what feels safe, not necessarily what speed is actually safe), it ignores the safety of other vulnerable road users such as pedestrians and cyclists. Very few drivers govern their speed by anything other than what feels comfortable for themselves and the American DOT guidelines on the 85 percentile rule makes no reference at all to pedestrian safety (it is also based on some pretty debatable assumptions about human behaviour and risk): http://onlinemanuals.txdot.gov/txdotmanu...

The same argument could have been made 40 years ago to claim that the legal blood alcohol limit should be based on the 85 percentile of what drinking drivers actually measure, because people are good judges about what their own safe impairment level is!

Slower traffic speeds definitely lead to fewer injuries and deaths, both because collisions are easier to avoid because reaction times are longer and because any collisions that do occur are much less damaging due to the far lower energy (increases like the square of speed). This is why, for example, school zones have reduced speeds.

Lower speed limits do not mean less traffic (they could even mean denser traffic), but they will tend to increase the number of pedestrians as the street becomes more comfortable to walk along.

The fact the "roads are there" does not mean the road design can't change. In the 50s and 60s there was a massive campaign of road widening ... they can be narrowed even more easily by adding parking or barriers.

Even in Edinburgh and Paris some major arterial roads will continue have a 50km/h limit, the change is to make 30km/h the default speed unless otherwise justified, rather than 50km/h.

Comment edited by kevlahan on 2014-05-28 16:36:32

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