I don't profess to be an expert, in anything really, so take this with a grain of salt. When you are in heavy traffic and are stopped on the highway look closely at the pavement. The ruts or grooves are much rougher than the surrounding less used pavement. It's just like sand paper, the rougher it is the more resistance will be associated with it. I don't know what that equates to when you're talking about gas mileage but I do believe that it helps to not drive on the rougher parts of the lane (as long as you can do it safely without bothering other drivers). And every little thing you can do adds up to MPG.
Check out what Consumer Reports has to say on rolling resistance.