"It's safer to move close to the speed of the flow of traffic."

Alternately, if you're passing more cars than are passing you, you're clearly going faster than the speed of traffic, and based on the totally unscientific evidence of driving multiple offshoots of 95 in the DC area, this seems to be how a good chunk of people drive.

As an experiment, I've tried to stay closer to the speed limit (not more that 5/10 over), and I've noticed that driving is a totally different experience when you don't concentrate on 'passing that asshole who's driving too slow' in the lane you want to be in. It's almost... pleasant.