Driving below the posted speed limit is legal throughout the United States, provided the driver is not creating an unsafe obstruction to the normal and reasonable flow of traffic. Posted speed limits represent maximum lawful speeds, not mandatory minimums.

Drivers traveling 55 mph in the right lane are generally operating within safe and lawful norms, especially since:

• many states require trucks and trailers to travel at or below 55 mph
• right‑lane travel is the appropriate place for slower speeds
• maintaining a predictable, steady pace is a key safety factor

However, driving significantly slower than surrounding traffic — especially in the left lane — may violate laws against obstructing traffic and can create hazardous conditions.

Tim Castleman, founder of the Drive 55 Conservation Project, promotes reduced speeds as a way to:

• conserve fuel
• reduce emissions
• decrease dependence on foreign oil

While the movement has passionate supporters, Castleman acknowledges that public resistance and safety concerns — particularly regarding speed differentials — remain major challenges.

Driving slower can improve fuel economy, but the optimal speed varies by vehicle. For most passenger cars, peak fuel efficiency occurs between 45–55 mph.

Examples:

• A 2001 Chevrolet Malibu achieved 35 mpg at 55 mph, dropping to 25 mpg at 70+ mph.
• Every 5 mph over 60 mph can feel like paying an extra $0.30 per gallon.
• Hybrids like the Toyota Prius may reach 50 mpg at 55 mph, dropping into the 30s at 65 mph.

Driving too slowly (e.g., 35 mph on a highway) can reduce efficiency due to low‑gear operation and extended engine runtime.

While slower speeds can save fuel, large speed differentials increase crash risk.

AAA and traffic safety experts warn that:

• drivers going 55 mph in a 70–80 mph flow may experience tailgating
• aggressive passing maneuvers become more likely
• unpredictable lane behavior increases collision risk