You drive through the same part of town every day. It starts out in a 25-mph zone right near your house, but it changes to a standard 55-mph zone about two miles along in your daily commute. Since you know the area and you know the sign is coming, you usually start accelerating before you actually get to it. This way, you can get up to 55 mph when you reach the sign. Is this legal?
While it is very common, this is technically illegal. Speed limit signs do not mean you should already be going at that speed when you hit the sign. There is no gray area or buffer zone in front of the sign. As long as you have not passed it yet, you are still in the old speed limit zone. It does not change until the exact spot where they put up that sign.
So, if you are going 45 mph and you are still 100 yards from the sign, the police could pull you over. They do not care that you can see the sign and that you are still 10 mph under that limit. If the sign is still in front of your car, you are actually going 20 mph over the limit. It has not changed yet.
As simple as the idea of a speed limit sounds in theory, you can see how easy it is to make an easy mistake and get a ticket. If this happens to you, make sure you know about all of the legal steps you can take.