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Speed limits are in effect until otherwise posted or noted. So once you enter a 55 MPH zone it stays that way until you see a sign raising it back up to 65 MPH. Unfortunately not every road is properly sign posted, so it is best to assume you are still in the lower speed limit zone until you actually see a sign telling you otherwise. Often in an inner city area, all roads will abide by the lower speed limit noted when entering the city limits, but not all will have speed limit signs.
Some speed zones are named, such as a school zone or safety zone. These reduced speed areas will often only have an "end of school zone" type sign and not a speed limit sign. Here it is presumed that you can return to the previous speed limit you were under when you pass the"End of..." sign.
Unfortunately for visitors to an area, speed limits are not always posted on every road, so one needs to pay attention to the signs you do see and their relationship to civilization, two lane roads in housing areas, neighborhoods often have the lowest speeds 25-35, two lane roads in business areas a bit higher 35-45, four lane roads in the city 50/55, outside the city 65/70. And to compound the issue, each city sets it's speed limits a bit differently :(