Let's say a vehicle enters an inactive school zone going exactly the speed limit of 30mph. Then the school zone (20mph) turns on while the vehicle is still inside.

What is the current speed limit for that driver?

If it is 20mph, is there any allowance for drivers to adjust their speed or are they instantly breaking the law?

My interpretation has been that the speed limit remains 30mph in this case for the vehicles that entered an inactive school zone.


  • 1
    Do you mean if you enter a school zone at 6:59:55 and 5 seconds later, while you are still in the zone, the limit changes? Is there an electronic sign that changes, or is there a fixed sign like "School zone, Speed limit 20 MPH 7 am- 4pm weekdays"?
    – Damila
    Oct 16, 2019 at 14:10
  • Yes to your first question. The sign says "School, speed limit 20 when flashing" with a flashing light at the beginning and end of the zone. I can add a picture in original question if it helps. I'm in this situation regularly in my morning commute. The specific school zone is quite long and police presence there is common.
    – 5AM
    Oct 16, 2019 at 14:30
  • 1
    Interesting, so "when flashing" but no posted time? Sorry, I don't have an answer for you but I agree it seems unfair to start the flashing after you enter (and then charge you). If it was a posted time, I would think the driver is responsible for knowing the time.
    – Damila
    Oct 16, 2019 at 14:34
  • 3
    Seems like gradually and safely reducing to the new speed would be the wisest course of action.
    – Justin
    Oct 16, 2019 at 15:40
  • 2
    I've had similar thoughts as I drive through a school zone that has the same signage that you posted. I haven't tried this defense yet, but I always visualize the sign (and the line in the road indicating the beginning of the zone) as a magic wand. Every time a car passes the line, the wand taps the car and says 'I set your max legal speed to X' where X is either the reduced speed (if flashing) or the normal speed. In that sense, the restriction would be placed on the vehicle (cannot go faster than X) rather than the road itself (cannot be traveled on at speeds over X). Great question!
    – Brian R
    Oct 17, 2019 at 16:51

2 Answers 2


While this question may or may not have a theoretical answer, the practical answer is that you should slow down to 20 mph if there is any chance that the school zone speed limit will be in force at any time that you pass through it.

If you do, you will definitely not be cited.

If it is at all a close call (say within five minutes of being active), normal differences in time keeping between unsynchronized watches and a lack of any way to prove precisely when you vehicle was where and traveling at what speed, mean that the question of whether the school zone was in effect at the time you were cited will be a question of fact to be resolved based upon the credibility of the witnesses. The witnesses will probably be only the citing officer and you. As a practical matter, you are going to lose the credibility contest on this question of fact 95%+ of the time, and that credibility determination will not be an issue that could be raised on an appeal.

Therefore, you should slow down before entering the school zone if it is remotely close to being time for it to be effective to leave a sufficient margin of error against traffic law enforcement officer inaccuracy, which includes any time that the school zone takes effect while you are in it.

Even if you could perfectly prove that you were half way through the zone when the speed limit fell from 30 mph to 20 mph, the argument that you needed to break suddenly to comply doesn't cut it. Violating a speed limit is a strict liability offense and you have a duty as a driver to anticipate what you need to do to comply with the law at every stage of your journey. Even if your speed was legal when you entered the zone, you knew or should have known that it was on the verge of being triggered and should have slowed down in advance.

  • 2
    "If you do, you will definitely not be cited." Also, you know, you're risking hitting someone's kid and are a jackass. Nov 12, 2020 at 12:29

At least in NC, the law is unclear on the issue. However, it is my (and thanks to Brian R's comment, not everyone's) experience that such signs, flashing or with posted times, are posted every few feet within the zone.

Thus, even if the first sign you pass was not flashing, if subsequent ones begin flashing, you should reduce speed.

It may be a good idea to invest in a cheap dashcam which you have on in your morning commute. This way, in the event you are pulled over after passing a school zone sign which is not flashing, you have evidence to show the judge.

Also check the laws on the state where you live to see if there are any applicable statutes.

  • 3
    "However, it is my experience that such signs, flashing or with posted times, are posted every few feet within the zone." In Texas, I have never seen this. There is often one sign at the beginning of the zone and one sign at the end (sometimes just a line on the road at the end). Rarely there will be some kind of overhead sign 0.5-1 miles in advance of the beginning of the zone to warn of the upcoming change (if the reduction in speed is drastic).
    – Brian R
    Oct 17, 2019 at 16:53
  • 1
    Or, just don't cut it so close and slow down before you get there if it is close to changing. Usually, unlike in the image shown, there is some sign in or near the zone that announced when it is effective, and the effective time of school zones is often a matter of statute as well.
    – ohwilleke
    Nov 11, 2020 at 22:33

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .