Speed limit based on 85th percentile: is it really the safest?

I’ve read arguments by a certain organization that the safest way for a locality to set speed limits on a road is by determining the speed that 85 percent of drivers are traveling at or below.

Is there any actual data to prove this is the safest method? Why the 85th percentile? Why not the 90th, or the 80th? Does the amount of speed limit enforcement make a significant difference? For example, what if 85 percent of drivers want to go up to 70, but they have a very strictly enforced 55 mile per hour limit? What about other road users, such as bicyclists and pedestrians?

I’d like them to talk to the people who built the roads first and see what speed they built them for. If the road’s built for 55 mph and people want to drive 75, too freakin’ bad. If you consistently have to drive that fast because you’re late for work, leave home sooner. If you just like to drive that fast, use a freeway that’s built for it. Why put the lives of everybody on the road at risk just because some people get their knickers in a twist because there are numbers on their speedometer they never get to use?

AFAIK, speed limits are set by consulting civil engineering manuals that set SLs by traffic volume, time of day, locales and other factors.

Of course, it’s more complicated than just that.

I have often felt that in this country, the speed limits are subject to the whims and politics of the local authority.

A case in point is Stratford Upon Avon. There are a couple of roads on the approach to the town that have a 40mph limit. Strangers frequently can’t believe it as the roads are built up and have street lights, so look like 30 limit roads.

One road however, runs past some very large houses set well back from the road but has a 30 limit and a camera to catch the foolhardy. I am sure that the fact that a couple of councillors live there has nothing to do with it.

Safest would probably be a limit of 10 mph, savagely enforced. Other considerations (e.g. the value of people’s time) are what call for higher speed limits.

The 85th percentile rule is partially true, but taken out of context and way too simplistic overall. Reall all about it here: Methods and Practices for Setting Speed Limits: An Informational Report

Part of the 85th percentile argument is that strictly-enforced low speed limits actually make it harder to catch people who are actually driving at dangerous speeds. If you figure the cops can only pull over a small percentage of the total pool of speeders, a low speed limit that makes that pool much larger makes it way easier for someone who’s really speeding to get away with it.

How do we know that the people driving over the low limits are not driving dangerously? They could be low for some reason that is not immediately apparent to drivers. That’s why I was hoping for some hard data to support or rebut the argument.

Did you read my link? There’s over 100 pages of data.

Then again, a lower speed limit gives them a much more target-rich environment, so if they’re on a quota, they don’t have to wait so long for somebody to come by at 10-15 mph over the limit. If they scale the penalties to increase based on percentage over the speed limit, on the rare occasion they nail somebody for going 75 in a 55 zone, they’ll get a lot more money than nailing somebody for going 75 in a 65 zone.

Which all kind of ignores the question of whether you’re defining “driving at dangerous speeds” as “driving way faster than everybody else” or “driving way faster than the road is designed to handle”.

Well, the fundamental basis of it is research which has consistently found that the frequency of collisions is lowest at or slightly above the average speed of traffic, and highest significantly below: Solomon curve - Wikipedia

You can quibble all you like about the engineering minutiae of whether a road is “designed for” 55 MPH or 70 MPH, but all that is pretty much dwarfed by the safety implications of speed relative to the flow of traffic.

I think the fundamental misunderstanding you may be having is that the speed limit alone has a major effect on average speeds. Absent really intensive traffic enforcement it generally doesn’t, although granted that’s easier to do these days with traffic cameras. If there really is some hazard not apparent to the average reasonable driver, traffic engineering manuals recommend taking steps to make that hazard more apparent (warning signs, etc) or traffic calming (speed bumps, etc) not simply lowering the speed limit on its own.

Are you saying that a freeway designed to be safe at 55 is perfectly safe to drive on at 70, so long as everybody wants to drive 70? I believe I saw this scientific principle in The Music Man… it’s called “The Think System.”

In almost all cases, there’s a fairly large margin of safety built in.

Have you forgotten the transition from the de-facto national speed limit of 55 mph? There was not a single stretch of highway that was rebuilt. Most state highways went from 55 to 65, and freeways went from 65 to 75 (or 80).

And despite the higher speed limits, the danger from speeding still primarily comes from the interaction between the speeder and other traffic, not because the speeder is exceeding the safety limit of the road design.

Even when a curve has an advisory speed posted, you can almost always go 10 above that speed, and still be perfectly safe.

The thing with “designed speed” at least in the context of the US Interstates is that they were designed to be safe for at least 70 MPH… in cars from the 1950’s. As far as the design considerations of the road go, neither the speed limit nor the average speed people actually drive are usually anywhere near the actual safe limits of the roadway.

The whole idea of the “set limits at the 85th percentile” idea in highway engineering manuals is that in most circumstances drivers are pretty good at determining a safe speed to travel on a given road is. Even if somehow getting those average speeds down might result in slightly fewer crashes for a given section of road, trying to do so solely by implementing a low speed limit only has the effect creating more of a speed difference between vehicles, which creates more crashes and is thus counterproductive.

Of course one giant caveat to all this is that it only talks about frequency of crashes, not intensity. It is possible that you can have a situation where an artificially low limit leads to more crashes but fewer serious ones. That’s particularly worth considering in city streets where pedestrians and cyclists are involved. On the highway though, I don’t think there’s much of an argument against the 85th percentile though (outside of maybe the old energy saving one back in the 55 MPH days.)

Safe speed for a new 1959 Chevy Impala and a brand new BMW Coup on the same exact road with the same drives is still way in the BMW ( or similar ) car.

NASCAR does fine at 190 and they are racing and bumping on purpose, F-1 is insane but both places are no place for a stock 59 Chevy.

Back to which road to drive questions. What & where is which one dangerous, all with same driver? Add the spectrum of drivers and I’ll fly a small plane thankyouverymuch… :smiley:

Not really.

I write just as many tickets with the speed limit on my highways at 65 as I did when it was 55. And I never write for less than 15 over. (Meaning 70 in a 55 and 80 in a 65). There is always someone speeding at 15 or more per. Always.

As of now there are no 70 limits where I work, but the guys I know that are on the job in areas that have the 70 limit told me that they can write 10 tickets an hour not stopping until 85. There are is always someone going 15 or more over the limit. Always. Doesn’t matter if the limit is 5 or 95, city street, rural highway, or interstate freeway. In a relatively short period of time someone always comes zipping by at least 15 over the posted limit.

If the limit was 100 I’d get someone going 115 with a few minutes of taking post and turning on my laser. I guarantee it!

Let’s see how much I can remember from tech school :smiley:

The 85th percentile speed could be considered how fast the drivers ‘want’ to go, based on how comfortable they are in their surroundings and why they are travelling. Existing roads use traffic surveys to work out the 85th percentile. I think new roads probably get it out of a roading manual.

The road designer takes the 85 percentile speed and designs or redesigns the road to be safe for a large truck travelling at that speed. This is the design speed. Or they might design the road so the traffic goes at a certain 85% speed.

The speed limit is theoretically the same as the design speed but can be affected by political considerations, such as backseat traffic engineering by residents, budget, or politicians wanting an easier pull out of their drive, etc. Artificially lowering speed limits can be dangerous because it increases the speed differential between slow and fast traffic.

Pedestrians getting run over or the like is a sign that the design speed is too high. Since no one wants to tear up useful roads to redesign them, traffic calming measures like speed bumps get installed to artificially lower the 85th percentile speed as well as the design speed. (Which is why they feel artificial and jarring when you’re driving.)

This certainly was the case during the national speed limit era in the U.S… “55 Saves Lives” was possibly one of the biggest lies of the 20th century. When gasoline surged in price in 1973/'74 people drove less and hence fewer highway fatalities.
Today most states have limits of 70 or higher and the rate of highway fatalities goes down most years, continuing to disprove the 55 nonsense. But it was good for bringing in big bucks to states and municipalities and insurance companies.

To be fair, the reduction of car crash fatalities has mostly to do with improved safety features, such as shoulder harnesses and airbags.

One argument I often heard for the 55MPH limit was to “smooth out” the flow of traffic. Highway engineers theorized that some drivers will only go 50-55MPH no matter what, while others will speed up to the limit and beyond. Reducing the speed limit, supposedly, would force everyone to drive the same speed, which would reduce accidents overall. In my experience, that never really seemed to be the case.

Yes, the manual I linked to in post #6.

Two important details to note. First, state laws override everything. If a state raises its highway limit from 65 to 70, then it applies to every mile that the 65 did whether that is 85th percentile or not. In my city all streets have a 30 mph limit. That supersedes individual street calculations (except for a few rare slower exceptions).

Second, the 85th percentile is a computation that tries to account for the varied individual tastes. The OP seems to think that the 85th percentile is an absolute, not a guideline with best expectations. The actual best value for any individual section of any individual road may be different from the 85th percentile value. But you can’t change a speed limit every 500 feet and you can’t have speed limits of 48.83838323 mph. Speed limits are a balance of multiple factors which need to be given a number to a precision of no more than 5 mph.