# Speedometer Checks on Highways **URL:** \ **Category:** Factual Questions\ **Created:** [September 23, 2010, 12:50am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719 "2010-09-23T00:50:51Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![GregH](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@GregH](https://boards.straightdope.com/u/GregH)\ **Post date:** [September 23, 2010, 12:50am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/1 "2010-09-23T00:50:51Z")
So, how often are these used? Seems like some government has gone to a lot of time and expense to plant these markers on the edge of the highway. Has anyone here used these? Are they geared towards a specific group of people? ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [September 23, 2010, 1:27am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/2 "2010-09-23T01:27:04Z")
say what? you talking about those mobile radar trailers? ---
**Author:** ![pyromyte](https://avatars.discourse-cdn.com/v4/letter/p/d26b3c/32.png) [@pyromyte](https://boards.straightdope.com/u/pyromyte)\ **Post date:** [September 23, 2010, 1:38am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/3 "2010-09-23T01:38:45Z")
Are you talking about highway mile markers? If so, here is the purpose: > [@](#): > > Milestones are constructed to provide reference points along the road. This can be used to reassure travellers that the proper path is being followed, and to indicate either distance travelled or the remaining distance to a destination. Such references are also used to by maintenance engineers and emergency services to direct them to specific points where their presence is required. This term is sometimes used to denote a location on a road even if no physical sign is present. This is useful for accident reporting and other record keeping (e.g., “an accident occurred at the 13 mile mark” even if the road is only marked with a stone once every 10 miles). [wiki](http://en.wikipedia.org/wiki/Milestone) I have timed myself between mile markers as a check of my speed, but it turns out they [aren’t really all that accurate](http://mankatofreepress.com/local/x525176674/Ask-Us-Mile-markers-that-aren-t). If you’re talking about paint markings used by [aerial police](http://dailyrecordnews.com/news/article_302db7ac-bc32-11df-babc-001cc4c002e0.html) to catch speeders, then I guess they’re geared toward the use by police. Or maybe you’re talking about these [“optical speed bars”](http://www.sciencedaily.com/videos/2006/1003-slowing_down_speeders.htm) intended to trick drivers into slowing down ---
**Author:** ![GregH](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@GregH](https://boards.straightdope.com/u/GregH)\ **Post date:** [September 23, 2010, 1:40am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/4 "2010-09-23T01:40:12Z")
Sorry- my question is far too vague. There is a sign at the side of the highway that says “speedometer check starts at Mile 0”, or something similar. Then, a series of markers at 1-mile intervals follows. So, who uses these? How often are speedometers faulty these days? ---
**Author:** ![campp](https://avatars.discourse-cdn.com/v4/letter/c/c5a1d2/32.png) [@campp](https://boards.straightdope.com/u/campp)\ **Post date:** [September 23, 2010, 2:02am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/5 "2010-09-23T02:02:35Z")
> [@GregH](#): > > So, who uses these? How often are speedometers faulty these days? Why couldn’t someone’s speedo be suspect. 😕 Also, different sized tires/wheels than the car was designed for will affect speedometer accuracy. ---
**Author:** ![pyromyte](https://avatars.discourse-cdn.com/v4/letter/p/d26b3c/32.png) [@pyromyte](https://boards.straightdope.com/u/pyromyte)\ **Post date:** [September 23, 2010, 3:23am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/6 "2010-09-23T03:23:16Z")
Apparently MnDOT at least… > [@](#): > > There are markers precisely a mile apart that MnDOT uses to calibrate its equipment. > > One is on Highway 22, with a mile’s southerly boundary marked with a “begin calibration” sign just south of North Victory Drive (County Road 3) on the east side of the road (right if traveling north). There’s an “end calibration” sign a mile north. –[link](http://mankatofreepress.com/local/x525176674/Ask-Us-Mile-markers-that-aren-t) Like campp said there a lot of reasons a speedo could be off. I know that police interceptors have calibrated speedometers, but I am not sure what the calibration procedure is. Presumably they use the marks you’re talking about. [pic of police speedo.](http://www.flickr.com/photos/16831499@N00/671372744/) ---
**Author:** ![gravelrat](https://avatars.discourse-cdn.com/v4/letter/g/96bed5/32.png) [@gravelrat](https://boards.straightdope.com/u/gravelrat)\ **Post date:** [April 29, 2022, 4:48pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/7 "2022-04-29T16:48:57Z")
Lol no to all above… They are to defeat the argument citizens bring to court."how am I supposed to know my speedometer is wrong?? " When arguing about a speeding ticket. ---
**Author:** ![MikeF](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@MikeF](https://boards.straightdope.com/u/MikeF)\ **Post date:** [April 29, 2022, 7:13pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/8 "2022-04-29T19:13:25Z")
I’ve seen signs that say “Start measured Mile” and “End Measured Mile” but never knew why they were there. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [April 29, 2022, 7:17pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/9 "2022-04-29T19:17:00Z")
> [@pyromyte](#): > > I have timed myself between mile markers as a check of my speed, but it turns out they aren’t really all that accurate You can drive one direction, then turn around and drive the other direction and take the average. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [April 29, 2022, 7:25pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/10 "2022-04-29T19:25:06Z")
> [@gravelrat](#): > > They are to defeat the argument citizens bring to court."how am I supposed to know my speedometer is wrong?? " “Your honor, I was checking my speedometer at a range of speeds to verify its accuracy, and the officer pulled me over during the 90mph phase of the calibration procedure.” ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 29, 2022, 9:11pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/11 "2022-04-29T21:11:26Z")
> [@Riemann](#): > > You can drive one direction, then turn around and drive the other direction and take the average. How would that help? If the mile markers are closer than a mile, then your measured speeds will be too high in both directions. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [April 29, 2022, 9:27pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/12 "2022-04-29T21:27:01Z")
> [@Chronos](#): > > your measured speeds will be too high in both directions It’s a fair cop. Does the FQ “serious answers first” rule reset after a hiatus of 12 years? ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [April 29, 2022, 9:37pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/13 "2022-04-29T21:37:58Z")
Ha ha. I could see this as an XKCD. ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [April 29, 2022, 9:59pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/14 "2022-04-29T21:59:49Z")
> [@Riemann](#): > > You can drive one direction, then turn around and drive the other direction and take the average. Surely, you would only do this if you are chasing the [World Land Speed Record](https://www.fia.com/fia-world-land-speed-records#:~:text=The%20current%20holder%20of%20the,sound%20barrier%20at%20Mach%201.016)? You would need a decent stopwatch too as you would have to do each mile in less than 4.72 seconds. ---
**Author:** ![mmmiiikkkeee](https://avatars.discourse-cdn.com/v4/letter/m/edb3f5/32.png) [@mmmiiikkkeee](https://boards.straightdope.com/u/mmmiiikkkeee)\ **Post date:** [April 29, 2022, 11:09pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/15 "2022-04-29T23:09:29Z")
Do you guys seriously have speedometer check zones as opposed to odometer check zones? I’ve never seen the former but many of the later. I can’t see how a driver can make use of distance indicators to check their speed unless they have a stopwatch and calculator and either jot down notes and hit buttons while they drive or pull over and figure the math out. Checking your odometer is easy though; just look at the numbers when you pass a sign and check if they are where they’re supposed to be at the next sign. I actually discovered my truck was under-counting by about 10% one time by using these signs. ---
**Author:** ![Bootb](https://avatars.discourse-cdn.com/v4/letter/b/e19b73/32.png) [@Bootb](https://boards.straightdope.com/u/Bootb)\ **Post date:** [April 29, 2022, 11:52pm UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/16 "2022-04-29T23:52:24Z")
> [@mmmiiikkkeee](#): > > Do you guys seriously have speedometer check zones as opposed to odometer check zones? I’ve never seen the former but many of the later. > > I can’t see how a driver can make use of distance indicators to check their speed unless they have a stopwatch and calculator and either jot down notes and hit buttons while they drive or pull over and figure the math out. It’s really quite simple–all you have to do is drive at 60mph for one measured mile. If you cover that distance in 1 minute, you know that your speedometer is accurate. All you need is an ordinary watch. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 30, 2022, 2:09am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/17 "2022-04-30T02:09:14Z")
You might want someone in the passenger seat handling the stopwatch, so you can focus on your driving. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [April 30, 2022, 2:10am UTC](https://boards.straightdope.com/t/speedometer-checks-on-highways/554719/18 "2022-04-30T02:10:54Z")
> [@Chronos](#): > > You might want someone in the passenger seat handling the stopwatch, so you can focus on your driving. - You’re hired.