Varies from state to state since traffic laws are per-state in the US, but in most states the traffic merging is supposed to yield to the traffic staying in the

Varies from state to state since traffic laws are per-state in the US, but in most states the traffic merging is supposed to yield to the traffic staying in their lane. You also are supposed to maintain a "safe distance" (where the law might be literally "safe distance" or might give a specific units) which at freeway speeds would mean leaving enough room for zippers but I don't know anyone who has ever been cited for following too closely, except after an accident caused by following too closely.