In every state it is illegal to exceed the posted (or reasonable) speed limits and offenders may be ticketed. Speed limits may be explicitly posted but may also be established by the state for specific settings. For example, a state may declare that the maximum highway speed is 55 mph, the maximum residential speed is 30 mph, and the maximum speed in a school zone is 25 mph. A police office can also pull a driver over if, given the situation and weather/road conditions, the driver was exceeding what would be considered a reasonable speed. For example, while the highway speed limit may be 55 mph, driving 55 mph during a blizzard or ice storm could be deemed unreasonable, dangerous, and, consequently, grounds for a traffic ticket.
Traffic tickets can be based upon absolute/fixed limits or upon prima facie limits, depending on the regulations of the state. The latter refers to states that allow drivers to give good reason for their speed, given the circumstances. In the case of the former, any speed above the set numeral limit is a violation, regardless of circumstances. In states that operate with prima facie, the violation is not exceeding the speed limit but rather exceeding what would be considered reasonable and safe. Because speeding laws can vary greatly, drivers should always operate with the utmost caution. This includes slowing down when near intersections, railroad crossings, curves, hills, schools, and in dangerous weather conditions.
There are several possible excuses for speeding, but they are not guarantees. If you are transporting a person with a medical emergency (injury, illness, pregnancy) to the hospital, the officer may not issue the traffic ticket and may even escort you to the hospital. A court emergency (with proper documentation) is another possible excuse. Finally, if the speedometer is broken, there is a chance the officer may let the driver off with a mechanical violation ticket.