For example, say the speed limit on a certain road is 40 MPH, and say a certain someone was doing 45 on this road. Technically, if a cop clocked someone doing 45 when the speed limit is 40, he could pull that someone over and issue a ticket. What I’m wondering is if there is a law that precludes punishment for trivial, petty “crimes” or infractions such as this. Another example would be jaywalking or loitering.

I ask this because I find it hard to believe that a cop could punish you for going 5 MPH too fast. Is there perhaps a minimum of how much you break the law before you can be punished?


3 Answers 3



The laws specify what you can and what you cannot do. If the intent of the authority was that you were allowed to drive at 45 mph, you would have a speed limit of 45 mph, not a speed limit of 40 mph. If you go at 41 mph, you are breaking a law and can be punished.

That said, law enforcement officers usually have some leeway on how to enforce the law, and they could very well let it pass with just a warning (or even ignore it if they have more pressing issues); the circunstances of it are specific to every situation and officer.

The only point that could be made would be if the difference was so small that it could be argued that it can invalidate the evidence on the basis of margin of errors. If the radar catches you driving at 41 mph but the error margin of the radar is 5%, you could argue that you were driving at 39 mph and that the reading is due to the error in the radar1.

That would enable you to challenge the evidence (but here the point is not that you are allowed to drive at 41 mph but that there is no proof that you were driving at 41 mph). From what I know, most police forces will be aware of that and avoid issuing fines unless you are well above that margin of error2.

1In fact, in Spain word of the street is that radars are set to account to possible margin of error of the radar, plus possible margin of error of the vehicle speedometer -even if it is the vehicle owner's responsibility to ensure that it works correctly- and some leeway.

2Some people post on the internet the "magic formula" of how many % of speed you can go over the posted speed limit based on those calculations. Of course those magic formulas rely in the radar and the speedometer being 100% accurate and the driver never getting distracted a few seconds and passing it.

So, even assuming that those magic formulas are correct, if either the radar or the speedometer are not accurate or the driver gets distracted for a few seconds, you are at risk of getting a ticket.

  • +1 for "the point is not that you are allowed to drive at 41 mph but that there is no evidence that you were driving at 41 mph",. though "proof" would be even better. Commented Oct 12, 2018 at 12:32
  • Of course, what cops usually do is clock you at 45mph in a 40mph zone, then write you a ticket for 41mph with a warning that if you try to contest, they'll tell the judge your "real" speed.
    – pboss3010
    Commented Oct 12, 2018 at 13:17
  • 1
    @pboss3010 Wondering if 'telling the judge your real speed' isn't perjury of some sort. I think an officer is certifying that the speed he has on the ticket is the 'actual speed' he witnessed the driving going. To change his mind later and tell the judge 'hey, I was lying the first time - he was really going xx mph faster' seems to me to be shady.
    – mark b
    Commented Oct 12, 2018 at 15:37
  • I haven't been in a court where someone tries that argument, but the one I got actually listed in the notes the "actual observed speed". I can imagine the officer telling the judge that it's his discretionary correction for angle/uncertainty.
    – pboss3010
    Commented Oct 12, 2018 at 15:56
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    @markb What actually happens is that the measured speed was 45mph, but the measured speed may not be exact. A measured speed of 45mph then might be proof that you drove at 41mph or faster. If the maximum error is 4mph, then it is also evidence that you didn't exceed 49mph.
    – gnasher729
    Commented Oct 13, 2018 at 21:33

The closest thing to what you're asking is something like Maryland's speed camera law. Speed cameras (free-standing automatic ones, not the "radar guns" carried by police) are somewhat controversial so in the law that authorizes them, Maryland requires a 12mph buffer before the camera activates. For example, in a 35mph zone, you have to traveling at 47mph for it to snap a picture. This doesn't mean an officer standing next to the camera can't go after you for driving at 36 mph though.


You can be punished if there is proof that you went just 1mph too fast. If you lived in the UK for a while, you probably know in which areas this will actually happen, and where not.

Since there must be proof and speed cameras are not 100% precise, it is often required that the camera measures for example 5mph above the limit, which would prove that you drove 1mph above the limit. If you drove 41mph, you could be measured at 45 or at 37, so you might get fined or get away with it. At 48mph you could be measured at 44mph (4 miles error), which would only be proof that you did 40mph, and get away with it.


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