Traffic's speed (United States)

You must obey the speed limit. If no limit is posted, drive no more than 55 mph
(88 km/h). Often, it is common sense to keep your actual speed below the posted limit.
For example, the legal limit on a slippery or fogged-in expressway might be 55 mph
(88 km/h), or even 65 mph (100 km/h), but the safe speed to drive would be much
lower. Even if you were to drive at 50 mph (80 km/h) on that hazardous highway, a
police officer could ticket you for a speed “not reasonable” for the conditions.

To keep a smooth traffic flow, some highways also have minimum speed limits.
If you drive slower than the minimum speed you can halt the traffic flow and create a dangerous condition. Even if there is no minimum speed limit, those driving too slow can be as dangerous as those who drive too fast.

Know that some cities have speed limits less than 55 mph (88 km/h) that is not always posted. For example, the speed limit is 25 mph (48 km/h) in New York City unless another limit is posted.