Final answer:
If no speed limit is posted on an open highway, the legal speed to drive typically defaults to a 'reasonable and prudent' speed, often around 55 to 65 mph, depending on the state or country's laws. This is subject to enforcement discretion and road conditions.
Step-by-step explanation:
If there is no posted speed limit, the default speed limit on an open highway varies by jurisdiction, but in many cases, it is implicitly set at a 'reasonable and prudent' speed, which could be around 55 to 65 miles per hour (mph). For instance, in the state of Texas, when no specific speed limit is posted, the maximum speed is set at 70 mph during the day on highways outside of an urban district with a population of less than 25,000.
However, drivers should always consider road, traffic, and weather conditions, and adjust their speed accordingly to ensure safety. If you exceed what is considered reasonable, you may be subject to a traffic ticket. Enforcement can vary, as there may be some leeway built into speed regulation enforcement due to speedometer inaccuracies, but pushing the limits further increases the risk of getting a ticket.