# Speed limit based on 85th percentile: is it really the safest? **URL:** \ **Category:** Factual Questions\ **Created:** [August 23, 2015, 1:03pm UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600 "2015-08-23T13:03:01Z")\ **Posts on this page:** 6\ **Page:** 3
**Author:** ![Kimballkid](https://avatars.discourse-cdn.com/v4/letter/k/b5a626/32.png) [@Kimballkid](https://boards.straightdope.com/u/Kimballkid)\ **Post date:** [August 27, 2015, 6:32pm UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/41 "2015-08-27T18:32:28Z")
> [@bob\_2](#): > > I have often felt that in this country, the speed limits are subject to the whims and politics of the local authority. I think this has more to do with it than anything. Here in South Dakota, the state legislature passed a bill that raises the gas tax by 6 cents per gallon, raises the vehicle excise tax by 1 percent, raises license plate fees by 20 percent and allows townships and counties to impose additional property taxes for roads and bridges. But to appease the masses and quell revolution, they raised the speed limit on the (two) interstates from 75 to 80 mph. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [August 27, 2015, 6:35pm UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/42 "2015-08-27T18:35:26Z")
> [@bump](#): > > Well, no. They’re a symptom of a bigger problem, which is the fact that there is a huge speed differential between the two lanes. If one lane’s going 90, and one’s going 60, there’s no middle ground for anyone who wants to go any other speed, UNLESS we want to weave in and out of traffic, or go one of the two other speeds. The phrase “weave in and out of traffic” is obviously meant to be scary, but yes if you want to go faster than the slowest lane on a multi-lane highway you do occasionally need to change lanes. Changing lanes is not a dangerous maneuver if you use your mirrors and turn signals. The whole idea of a multi-lane highway is that it can accommodate people doing a wide range of speeds because you can always pass, but that doesn’t work when someone decides they’re going plenty fast and get to stay in the left lane regardless of what traffic behind them is doing. Like I mentioned earlier, if you ever drive on a European-style highway this all becomes pretty obvious, and speed differential is not the problem. To take an extreme example, on the unlimited sections of the autobahn the right lane is often near bumper-to-bumper with trucks going 80-100 KPH but the left lane is full of people doing 200 KPH. It works because when you pass you spend as little time as possible in the left lane. If you want to go 120 KPH, yes, you will be constantly going from the right to left lane and back but when people actually do it it’s quite safe and works very well. ---
**Author:** ![Orwell](https://avatars.discourse-cdn.com/v4/letter/o/dfb087/32.png) [@Orwell](https://boards.straightdope.com/u/Orwell)\ **Post date:** [August 27, 2015, 6:39pm UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/43 "2015-08-27T18:39:19Z")
> [@bump](#): > > But on say… I-45, between Houston and Dallas, or certain stretches of I-35, it’s sort of a 90 or 60 option because the traffic is heavy enough to where you’re literally going back and forth every few miles, and I more or less refuse to go either of the other speeds. So the 90 mph contingent loses out. And there you have it… the confessions of a left lane hog. Going back and forth is what you should be doing, not tying up traffic. “Going back and forth every few miles” doesn’t sound like a burden to me. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [August 28, 2015, 12:57am UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/44 "2015-08-28T00:57:33Z")
> [@Ludovic](#): > > I-95 in the Carolinas definitely needs 3. The laws must be different there because there are a _lot_ more trucks in the Carolinas that park in the left hand lane going 1 mph (or less!) more than the traffic in the right hand lane, even compared to other 2 lane regions of I-95. I once spent about two hours trying (vainly) to get past a left lane hog on I-26 once, who (doing c. 2 below the limit) simply refused to move over despite multiple opportunities to do so. The right lane was congested just enough that none of us could easily pass him to the right-most of the time there were like 12 cars backed up behind the essobee, with the semi’s and the grandmas to the right. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [August 28, 2015, 1:09am UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/45 "2015-08-28T01:09:37Z")
> [@Orwell](#): > > Oh, and BTW, I am not one of those drivers who is going 90 mph, or who is aggressively tailgating. I usually drive 5 to 9 mph over the limit. I am glad to move over to the right to let a faster driver through. It doesn’t upset me in the least. I don’t either, unless they are parked 2 feet from my bumper when I cannot move over-like they somehow expect me to magically merge into the vehicle to my immediate right like I am phantom or something. Like the wacko airhead the other day (3 lanes), I had moved into the left lane to pass some slowpokes, was about to merge back to the middle one (where I prefer to cruise given all of the traffic in the right one constantly entering and exiting the highway)-when I saw the truck in the far right lane with all of his splendid left turn signals going off like the 4th of July. So I waited to pass him completely to give him room to merge to his left, and all the while she is shaking her fist at me in her rear-view mirror. Stupid bitch never noticed that his signals were on. Finally got the margin I needed to safely move over-except that she had simultaneously changed right along with me. Meanwhile the moron in the truck had never actually merged having simply forgotten to turn the fucking thing off. She zipped in front of him then right on back to the left again, where she immediately glommed onto the tail of another victim. Bah. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [August 28, 2015, 3:37pm UTC](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600/46 "2015-08-28T15:37:08Z")
> [@Orwell](#): > > And there you have it… the confessions of a left lane hog. Going back and forth is what you should be doing, not tying up traffic. “Going back and forth every few miles” doesn’t sound like a burden to me. Every few miles at 75 mph is something like every couple of minutes… for a very large portion of a 4-5 hour trip. It’s a colossal pain in the ass. And really, why is it my obligation to enable people to exceed the speed limit by 15+ mph, by the expedient of screwing myself around and potentially getting stuck behind the slowpokes? It’s not like traffic is getting “tied up” if I’m going 80 in the left lane, just because the people behind me want to go 90; the real problem is the slowpokes going 65 or slower in the right lane. [Previous page](https://boards.straightdope.com/t/speed-limit-based-on-85th-percentile-is-it-really-the-safest/728600.md?page=2)