Under what condition is it unlawful to drive less than 55 m.p.h. in the left lane of an interstate highway?

Driving less than 55 m.p.h. in the left lane of an interstate highway can be considered unlawful primarily when traffic congestion prevents safe driving at that speed. The left lane is typically reserved for passing and is intended for vehicles that are moving faster than the general flow of traffic. Under normal circumstances, if traffic is light, a slower vehicle may be less of a concern, but when traffic congestion makes it unsafe to maintain the speed limit, it becomes critical for drivers to adhere to the flow of traffic in order to avoid accidents and ensure safety for all road users.

This regulation helps maintain a smoother traffic flow and reduces the likelihood of rear-end collisions that could occur when faster moving vehicles are impeded by slower vehicles in the left lane. While there may be exceptions in adverse weather or when vehicles are alone on the road, those situations do not specifically apply to the legality of driving under the set speed limit in the left lane during congested conditions where safety is a priority.