[Aloisius]

In California at least, the rules for entering a roundabout are to slow down when approaching, yield and prepare to stop to traffic in the roundabout and continue when there's a big enough gap in traffic to merge safely.

While in a roundabout, there's no passing permitted so you're essentially moving at the slowest vehicle's speed (which can be you). This rule can cause a problem if you're in the inner lane or a multilane roundabout and want to exit when a car is to your right when people don't choose their lanes properly when entering.

Highway merges have somewhat different rules in that you must be at or near the speed of traffic when merging instead of slowing down and you're not permitted to stop unless absolutely necessary.