[aidenn0]

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.