I think it is highly overrated that one needs to keep up with fastest traffic for safety. I'm in AZ, just drove over highway 17 w/ 75 MPH limit. I did 65 and was in packs of numerous other slower vehicles at same speed. Faster vehicles simply changed lanes and went around slower vehicles. There were several smaller passes and traffic slowed to 40 MPH at times even in left lane. Watching for slower traffic is fundamental in driving, much like watching for traffic signal or stop sign. It is much safer to stay within the vehicles comfort zone than try to keep up with the fastest traffic.