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When do speed limits change? According to here it occurs at "the exact point of the sign", however this is not very precise. Is it when the front of the car passes the point or the rear of the car passes? For example if there was a change from 30 to 25 and you were driving 26 mph when your front passes the sign, but 25 when your rear passes the sign, could you get a ticket?
I understand that this is never a practical concern for most cars. But say you have a huge big rig or something.
In the U.S., this almost certainly depends on jurisdiction. My understanding is that many places have laws that provide only basic default speed limits and require drivers drive at prudent and safe speeds. Violating posted limits constitutes prima facie evidence that your speed is not reasonable or safe, but the offense isn't that you were doing X in a Y. Given that, the exact placement of the sign might not even be meaningful. The real question is where the conditions which warrant a reduction in speed begin, and whether your speed was prudent and safe at the time the alleged violation occurs.
Under this theory, if the speed limit were changing for no good reason and you could prove it in court, you might beat the ticket. I wouldn't bet a large amount of money on such a venture being successful.
Perhaps an interesting legal question is at what point the posted limit creates the conditions for recorded speed becoming prima facie evidence of imprudent or unsafe driving. Again, my gut says that the zone begins as soon as:
a reasonable man driving at a prudent and safe speed in the same situation would become aware of the posted limit; and
the conditions which justify the reduction in speed (a curve, intersection, construction, etc.) occur
A speed limit sign is actually the entrence to a speed zone. Let's say for the sake of the argument there where a camera enforced speed zone. It would capture photos of any vehicle that enters the zone at any speed greater than the posted speed limit for that zone.
Edit: Entry is defined as the moment the zone plane is breached.
Edit I don't have a text source to reference. I did have a Judge tell my wife that because she came to a stop beyond a stop sign she broke the plane imposed by the stop sign.
