Huh? Like where? Like on the freeway? Because nobody has a "right of way" when it comes to merging on the freeway. Once the car enters the freeway onramp, it is technically also in a lane of traffic, and their only responsibility is to accellerate to the speed of the traffic in the other lanes, but It is both parties' mutual responsibility to adjust accordingly to allow them to merge if the lane runs out. As it's not like the car merging would have anywhere else to go, and can't just stop on a freeway if the traffic is all moving along. They have to keep moving, so it is both something that drivers on the main thoroughfare must be conscience of, and also of the mergers.
However, if you're talking about people coming from some side street or driveway or whatever, into a highway or road, then of course the main road drivers have the right of way, unless there is a stop light or stop sign or something, and if it was stop sign, usually the person there first goes, or if both cars came to the intersection at the same time, it's the person to the right who gets the right of way.