On interstate highways with four or more lanes, what is the minimum speed, and what is the minimum when the posted limit is 70 mph?

• A. 50 mph minimum; 40 mph when posted limit is 70.
• B. 40 mph minimum; 50 mph when posted limit is 70.
• C. 60 mph minimum; 75 mph when posted limit is 70.
• D. 40 mph minimum; 70 mph when posted limit is 70.

Answer: (B)

On multi-lane interstates, a minimum speed is set to keep traffic moving safely and to prevent very slow vehicles from impeding faster traffic in the left lanes. The standard minimum in these four-or-more-lane areas is 40 mph, which provides a baseline pace so that vehicles aren’t traveling well below flow. When the posted limit is 70 mph, the minimum increases to 50 mph to match the higher speeds typical on that roadway and reduce hazards from significantly slower vehicles in fast-moving traffic.

This helps explain why the other options don’t fit: the general minimum isn’t 50 mph, and the minimum isn’t raised to 70 mph just because the limit is 70 mph. The 60/75 pair isn’t the standard rule either. In normal conditions, aim to keep at least 40 mph, but at a 70 mph limit you should strive to stay around or above 50 mph to maintain safe flow.