Not all people can judge relative speed to flowing traffic. Whether incoming car up to highway speed, cars on left lane should have moved to right lane to make transition even smoother. The car on the left lane should adjust his vehicle to match the traffic on right lane and merge to the right. This is easier to do because sometimes merging lane sometimes is not long enough and some car has problem to build up the speed especially heavy vehicle.

This is the correct method to ensure smoother flow of traffic.