# Speed Limit Signs: When do you have to slow down? Do you have to be at the reduced speed at the sign or slow down at the sign? **URL:** \ **Category:** Factual Questions\ **Created:** [November 8, 2021, 3:50pm UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985 "2021-11-08T15:50:29Z")\ **Posts on this page:** 7\ **Page:** 3
**Author:** ![Ignatz](https://avatars.discourse-cdn.com/v4/letter/i/3be4f8/32.png) [@Ignatz](https://boards.straightdope.com/u/Ignatz)\ **Post date:** [December 16, 2021, 1:22am UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/41 "2021-12-16T01:22:14Z")
Cars, Trucks, Night, for example. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [December 16, 2021, 9:47am UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/42 "2021-12-16T09:47:33Z")
In the UK there is a general tolerance of 10% + 2 for speed limits. This is **not** law though and is not applied everywhere. Given the inaccuracy of speedometers (most read 1 or 2 mph under at 30), you are _fairly_ safe doing 35 in a 30 limit, but it’s pretty risky to rely on it. ---
**Author:** ![its\_the\_daddy](https://avatars.discourse-cdn.com/v4/letter/i/bb73d2/32.png) [@its\_the\_daddy](https://boards.straightdope.com/u/its_the_daddy)\ **Post date:** [December 16, 2021, 10:34am UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/43 "2021-12-16T10:34:03Z")
I am part of a Community Speed Watch group in the UK. We go out under quite tight conditions - we are trained and insured, and our locations are pre-defined - for an hour or two a week. We report speeders to the police, who issue warning letters (but no prosecutions, on our evidence). In a 30mph area, we are told not to report speeds under 35 (ie 10%+2) seen on our (uncalibrated) instrument, This instrument is approved for our use, by the police. ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 16, 2021, 11:11am UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/44 "2021-12-16T11:11:35Z")
> [@AHunter3](#): > > The little town of Ludowici GA became internationally famous for its speed trap. Sign as you’re approaching town repeated the highway speed limit of 50 or 60 or whatever it was. A few yard beyond it, with no prior warning, a sign said 25 mph, and a police officer _lurked_ there and would ticket out of town (and especially out of state) drivers. Yeah, that’s one of the roads leading to Ft. Stewart, and there are/were speedtraps on all of them. Local law enforcement just _loves_ to skim money off young soldiers in a hurry to make it home for Sunday evening formation after a long weekend. ---
**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:** [December 16, 2021, 10:21pm UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/45 "2021-12-16T22:21:00Z")
Here’s one I encountered the other day: My commute home takes me through a school zone (20 MPH when sign is lit), on a road that’s otherwise 35 MPH. Usually, I get caught by it. But this day, I saw that it wasn’t lit. “Hm, I guess they must not have school today”, and so I kept on at 35 MPH… Except that it turns out that I was just a bit earlier that day than usual, and just as I was passing the sign, it started flashing. Strictly speaking legally, if I couldn’t brake hard enough to get under 20 by the moment I passed the sign, was I speeding? What if I was slightly past the sign, and hence couldn’t see it, at the moment it turned on? ---
**Author:** ![echoreply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/echoreply/32/3641_2.png) [@echoreply](https://boards.straightdope.com/u/echoreply)\ **Post date:** [December 16, 2021, 10:57pm UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/46 "2021-12-16T22:57:03Z")
> [@Chronos](#): > > But this day, I saw that it wasn’t lit. Getting caught by changing school zones is a thing that bothers me, too. I drive through a school zone that is nearly half a mile long. If I enter at just the wrong minute, it can be off when I enter, but then activate while I’m still in the zone, and well out of site of the flashing lights. Fortunately (I guess for drivers, not for kids) the cops around here don’t do much to enforce school zones. ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [December 17, 2021, 12:29am UTC](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985/47 "2021-12-17T00:29:47Z")
One of my annoyances with speed limit signs involves school zones. They don’t tend to have a “School Zone End” sign, leaving you wondering how ^@!##$ long you gotta drive 20 mph. [Previous page](https://boards.straightdope.com/t/speed-limit-signs-when-do-you-have-to-slow-down-do-you-have-to-be-at-the-reduced-speed-at-the-sign-or-slow-down-at-the-sign/953985.md?page=2)