Is every speed limit too low?


Traffic engineers believe that the 85th percentile speed is the ideal speed limit because it leads to the least variability between driving speeds and therefore safer roads. When the speed limit is correctly set at the 85th percentile speed, the minority of drivers that do conscientiously follow speed limits are no longer driving much slower than the speed of traffic. The choice of the 85th percentile speed is a data-driven conclusion — as noted Lt. Megge and speed limit resources like the Michigan State Police’s guide — that has been established by the consistent findings of years of traffic studies.

Yet most speed limits are set below the 85th percentile speed.

Last Sunday, I decided to see what kind of gas mileage I could get on my motorcycle. I drove 90 minutes into downtown Vancouver, keeping to the speed limit the entire time (except for the 30 seconds I didn’t and this happened). I got passed by everybody. Semis, minvans, old men in Buicks. Even thought the speed limit was 100kph (about 60mph), literally no one was doing the speed limit.

  • That “85th percentile” figure led to the increase in interstate highway speed limits outside of cities in Maine a about two years ago or so, from 65 to 70 M.P.H..

    One stretch of i-295 between Portland and Brunswick saw an increase in accidents (including fatalities) and has been dropped down to 65 again for that portion of highway.

    So, your mileage may vary, or that percentile study for Maine didn’t treat each length of highway separately. (I’d imagine the big stretch of near nothing between Bangor and Houlton on I-95 could be set to speed limit: ludicrous speed.)

  • satcomer

    I once received a ticket for doing 36 MPH in a 35 MPH. The darn rookie cop gave me a ticket (along with a line of 8 cars). He gave me a $50.00 fine or loose a day of work (that would cost much more). Sometimes this carn counties mounties back off once in while!

    The funny thing he didn’t like my Military ID at all either that really pissed me off! I even emailed the cops name to County headquarters. Then low and behold 35 other people received the same bogus tickets and I then read in paper how that cop had a stern talking too!

    • i have a hard time believe they were issue tickets for 1 MPH over. neither his equipment nor yours is that accurate.

      • satcomer

        Believe it! It was in Northern Virginia!

        • Mo

          Did this happen near the end of a month?

        • I can believe this a would happen in Northern Virginia. that place is a notorious speed trap — the whole of it. I used to live there — and have to admit that many of the drivers deserve the tickets, though. Whew, what a mess.

          Here is Oklahoma they drive worse, but there never seems to be a cop around when I’m being passed while still doing 5 or so MPH over the limit, and being cut off three ways to sunday.

          • drx1

            Only Georgia and some tourist traps are worse – in terms of traffic cops.

        • drx1

          NOVA cops can be uptight jerkwads and once you get out of the “city” area … it can be worse – in the Old Dominion …

  • SockRolid

    “Yet most speed limits are set below the 85th percentile speed.”

    Two words: Ticket Revenue.

  • I just moved from a 65mph state to a 70mph state. I can’t get used to everyone passing me. Not knowing the roads very well yet doesn’t help.

  • Paul Chernoff

    On the highways around DC I find variations in speed. On the beltway (55MPH) and I95 (65MPH) I can at or a little above the speed limit in the right lane safely (except when some idiot is driving 5-10 miles BELOW the speed limit). But on 270 if I drive the speed limit in the right line I will probably be plowed into and everyone is whizzing by me.

    • GlennC777

      I remember portions of the beltway twenty-plus years ago that were at a constant eighty-five or so when traffic allowed. I’ve done thousand-mile drives across the west with hours and hours at ninety-five plus. I remember driving in Montana during the years when there was no speed limit at all, passing and getting passed at very high speeds. I have driven in Germany on the autobahn at absolute top speed, which was approximately one-oh-five-mph in an (even-then) old Mazda 323, which had surprisingly excellent brakes.

      I say this for no reason at all except that I sometimes enjoy driving fast and your post created this succession of memories.

  • I think highway speed limits are too low around here, but at the same time a lot of the city roads have limits that are too fast. And drivers can’t be counted on to obey those city limits. They can’t be counted to slow down for school zones, either.

    There’s a huge difference in impact between hitting someone at 25km/h vs. 60km/h.

    Edit: And like we’ve discussed, the Coquihalla speeds are insane. Nobody should come down a twisty mountain at those speeds.

  • GlennC777

    This is a good, meaty article. Worth reading. Thanks for posting it.

  • neil_hiblythe

    This is how it plays out in the UK, national limit is 70, everyone drives at 80. If it was set it 80 they’ll all be doing 90, no-one wants to be seen to behind anyone as it adversely effect their social status, they are all the best driver and their car will protect them.

  • David Stewart

    Studies have shown people tend to drive as fast as they feel is safe on the road regardless of the posted speed limit. In building safer roads and cars we’ve pushed drivers to drive faster even as we maintain historical speed limits.

    • drx1

      most of those roads – or interstate highways in the US were designed for 75MPH and are fairly safe at higher speeds – depending.

      There is a newish toll road between Austin, TX and San Antonio – about 80 miles or so – and the posted speed for most of it is 85MPH and so long as people drive safely, 95MPH is not bad either – of course brakes, tires and awareness are always key – more so the faster you go.

      Now I35 between Dallas and Austin is crazy – I feel like I’m in an X Wing (yet not as cool) flying down a trench, bumper to bumper at 80MPH + and no shoulder at all – just the jersey barrier wall and “BIG EFF OFF” trucks all round – also going crazy fast and no margin for error. That’s a bit scarey and there are accidents all the time.

  • George

    Long rant coming: In Montreal, all of the major roads that used to be at 60 and 70 km/h have all been lowered to 50, and all the secondary roads have been re-signed at 40 or 30 km/h.

    Large stretches of highway are now 70 when before they were 90 or 100, and some boulevards have even been lowered below 50 to 30 with no real justification.

    Legend for Americans:

    30 km/h -> 20 mph 40 km/h -> 25 mph 50 km/h -> 30 mph 60 km/h -> 35 mph 70 km/h -> 45 mph

    I don’t mind limits of 40 or 50 km/h on residential streets, and on some of these streets even 50 is too fast, but for a freeway service road with no lights, sidewalks or pedestrians to be posted at the same 50 km/h is beyond ridiculous. The only reason for that to exist is to be abused as a speed trap, and these places often are abused in that way.

    However, the poorly-set speed limits is the least of it – Montreal has been waging a war against drivers for a while now, and they are winning. Traffic has been made severely worse through a series of artificial obstructions and construction:

    • Many green lights on major roads have been reduced to 30 seconds or less, sometimes with reds much longer than that.
    • Most lights have been desynchronized – get a green, the next light turns red. Combined with the short lights above, it’s a lot of stress trying to make the next light before it turns red.
    • Many lights now have four-way all-reds for 30 seconds for pedestrians, wasting time, including in many places where it doesn’t make sense. Many pedestrians get confused and still cross on green anyways.
    • Most other lights now present a straight green arrow for the first 10 seconds of that 30 second green, so cars can’t turn, whether there is a pedestrian or not.
    • Many roads have been reduced in lanes, from 3 lanes to 2 lanes or from 2 lanes to 1 lane, and with extra stop signs added, including on major boulevards. On busier roads this results in huge bottlenecks.

    I have nothing against transit and when I was working and going to school downtown, I took it all the time. However, people should be encouraged to use transit by making transit better, not by making driving a god-awful experience. Even if we had fully-automated cars, it would still take forever to get anywhere because of the artificially-imposed delays.

    This seems to be uniquely Montreal thing, because I’ve travelled in many other cities and countries, and nowhere are the delays as artificial as they are here. As an example, in Barbados, the road network is much smaller, and just about every road is one lane per side with no parking even in the built up areas. It’s hard to ever go faster than 50 km/h just because that is the natural limit. However, getting around is MUCH faster, and the main reason is that there are NO four-way stops (not a SINGLE one), and cops focus on real crime and traffic safety, not on enforcing speed traps. It’s hard to even exceed the speed limit anyways.

    When you’re not stopping at a stop sign or desynchronized light every 500 feet, it’s amazing how much distance you can travel even at 50. It’s also amazing how much more polite drivers are when they don’t have to deal with all of this crap.

    In driving through Europe, I’ve found that much of it is like this as well, in spite of their anti-car reputation.

    If only the low speed limits were the only problem! 🙂 Until this mentality changes, and I don’t see it changing anytime soon, driving is only going to become more frustrating and feel more like a crime with every passing day.

  • SDR97

    I did a similar experiment on a 30 mile drive from San Diego to the suburbs once. I did the exact speed limit the whole way and counted cars I passed and cars that passed me. I was passed by 300 cars. I passed only 30 — and many of those were semis exiting the freeway. I felt unsafe the whole time.

  • Jakelson

    A lesson from my father when I was learning to drive, which I have never forgotten, know the speed limit, but it is safer to drive the speed of traffic.