What does a posted speed limit of 55 mph indicate?

A posted speed limit of 55 mph indicates the maximum speed allowed under ideal conditions. This means that, under normal circumstances such as clear weather and good road conditions, drivers should not exceed this speed. It's important to note that while this is the maximum speed, drivers must always adjust their speed according to the actual driving conditions. For example, if the weather is poor, such as during rain or fog, or if road conditions are hazardous, drivers should reduce their speeds to maintain safety.

The other options do not accurately reflect the meaning of the posted speed limit. For instance, a speed limit is not a minimum requirement but rather a maximum guideline. Additionally, speed limits apply at all times, not just during the day, and driving conditions, such as weather, should always be considered when determining a safe speed to drive.