What is the minimum speed law on most US highways?

Most US highways do not have a posted minimum speed but state laws generally require driving at a reasonable speed for conditions and prohibit impeding traffic without good cause. Some Interstates post a minimum speed (typically 40-45 mph) for vehicles that cannot maintain prevailing traffic speeds. A driver going significantly below the prevailing speed (often defined as 20+ mph below the limit) can be cited for impeding traffic. Slow drivers, especially in the left lane, create dangerous speed differentials with other traffic. If you cannot maintain reasonable speed (vehicle problems, towing, learner driver), use the right lane and consider using a route below highway grade.