4 Comments so far

1. Sanjay May 31, 2006 1:14 pm

   Now, because I’m a just and loving commenter, I’m not going to point out what it is about the idea that most drivers can guesstimate the safest speed, and it’s the 85th percentile, which made me laugh out loud. I’ll leave that to other readers.

   But, now, let’s be clear here. The roads where they’re doing this is are roads where basically nobody’s driving most of the time. So for one thing, I’d like to see the mean and standard deviation on those numbers saying fatalities went down when they increased the speed limit! For another, it strikes me that the speed limit in that case probably wants determining essentially by automotive and civil engineers. And for yet another, claims about “going the speed of the traffic” or the “85th percentile” would appear to be 100% useless on these roads. You are by definition going the speed of the traffic.

   Now, how exactly police officers who drive for a living are going to figure out what the 85th percentile _is_ and that it’s the safest — I mean, how they collect those data — I have grave questions about. My instinct is whatever they say is wrong and, again, this is a question for the civil engineers. But again, the 85th percentile idea is useless. If you lower the speed limit to 55, or raise it to 85, I rather suspect that 85th percentile is going to change, don’t you? Surely then it’s not a very good idea to use that to determine the speed limit, nor to agrue — especially again in the isolated driver case like this — that it’s a good guide to desired speed.

2. doinkicarus May 31, 2006 3:01 pm

   I certainly understand your concerns about the mean/SD in such a study, but you leave it open with the qualifier that in such circumstances, one is most likely the only car on the road. Perhaps I understated the obvious: regarding the assertion that “the 85% is going to change” in accordance with the posted speed limit: I certainly do not doubt that it does, nor do I disagree with you. NB that the 85% is a situational figure, it was not meant to imply the 85 percentile of all drivers on all roads everywhere at all times. Perhaps I should’ve qualified it a little better.

   Point being: the relativity of the 85% does not change with regards to the posted speed limit. If the posted limit is 55, and ALL (or even the vast majority of ) drivers on the road are exceeding 80 mph (bear with me here) it is certainly imprudent to drive 55.

   Now, it’s nice to sit back and argue that every other driver ought to be driving 55 mph. But you cannot control the speed of the other drivers on the road, you can only adapt your speed to best fit your surroundings. Note that this works just as well in reverse: If the posted limit is 70, and for whatever reason (say it’s raining or its foggy or dark or etc.) and nearly everyone is going 45, it’s absolutely unsafe to be driving 70 mph.

   It is not frequently that another vehicle overtakes you with impunity - remember that a car overtaking you by 7-8mph will appear to be moving past you at a relatively fast clip. Imagine standing still and having a runner go by you at 8 mph - the effect is the same. On most roads, the great majority of drivers drive at a pace which I’d estimate to be a very narrow band, perhaps 3-5 mph. Anyways, the 85% would be approximately 1 SD from the mean, or, just faster than the average car. And given the narrow band within which most vehicles travel, 1 SD might be something like 1mph, faster than “average.” I can’t imagine a circumstance where the 85 percentile would be tremendously faster than average.

3. Sanjay May 31, 2006 4:52 pm

   You write that the relativity of the 85% doesn’t change with posted speed limit. I doubt it. In fact I rather suspect you can disprove it with data if you look hard enough.

   I got what you meant about the 85% — sure it’s situational. I just point out (1) it is totally irrelevant to the Texas case, and (2) I think your source is questionable. As for arguing that everyone ought to be doing 55 — actually that’s rather different from arguing that the speed limit ought to be 55. In fact they’re generally set with the awareness that people exceed them some.

   I suspect the band in which vehicles travel is broader than all that. For example I, well, drive kind of a fast car. But I also occasionally drive jazz musicians for a club, professionally. In that case I stick tight at the speed limit — and I notice a lot of other professional drivers do the same, essentially for liability reasons (much much better to get rear-ended at 65 than to get in an accident at 75, in that circumstance, for liability reasons). Now, on real crowded roads I imagine the band is a lot narrower, but, duh — it almost has to be for physical reasons. But on most moderately travelled highwayys it seems to me the left/right speed still spreads over 10-15 mph, with drift left being a sort of proxy for speed (as one expects). Odd exception is the DC area where I find the fools tend to pass on the right hand side, which I think should be a shooting offense.

4. doinkicarus June 1, 2006 8:54 am

   Sanjay,

   The Texas data says: 85% of drivers are +/- 2mph of average speed. This suggests a range of (approximately) +/- 1.5 SD. Normal probability distribution suggests that the 85th percentile is 1 SD greater than mean. So perhaps, the speed limit ought to be more like 78.5mph, but now we’re splitting hairs. On this particular road, 85% (the vast majority) of drivers had a speed range of (very narrow) 4 miles per hour on a road on which they were “isolated” drivers. What they seem to be saying is, look: Everyone is already driving this fast, and we don’t have an epidemic of highway injuries/deaths, certainly nothing greater than what would otherwise be expected. So let’s at least legitimize the safe speeds at which people currently travel.

   Simply, it doesn’t matter what the speed limit is, if nobody is obeying it. Whether they should obey it or not is a different story, which neither of us seem concerned with. The only thing you can do, after all the other players have made their move, is make the safest move then available to you. If you don’t believe that the situational 85% is that “safest move,” (perhaps everyone is driving 100mph on an icy road) then you should probably pull over as early as you can, and wait for the traffic to pass you by.

   Putting aside the statistics: If one is the “only car” on the road at the time - in such circumstances where collision is at best, unlikely, we must consider that it is economically inefficient to worry about enforcing speed limits in vast jurisdictions with small police forces - their efforts are better to be expended on the busier roadways, and in the towns protecting and responding to complaints. People much prefer local visibility, seeing them around town, in the coffee shop (no donut jokes here) and driving through the neighborhoods.

Leave a Comment

If you would like to make a comment, please fill out the form below.