I'm with chrome and Cdub. I do believe there is a law, and you'll see the signs posted on many of the major freeways that states, "slower traffic keep right". It is not your job to regulate the speed of the fast lane. If traffic behind you is approaching at a higher rate of speed than you are going, simply merge to the clear lane to the right as soon as it is safe and surrender the passing lane to the higher-speed traffic. It's a very simple concept of fewer and fewer people seem to understand these days. That said, he was totally in the wrong for cutting into you and the blame for the accident rest completely on him, but riding in the fast lane at the speed limit when there is a clear lane to the right is a total dick move.