I will admit that as a motorist I will sometimes come up behind cyclists and pace them for a little bit. However I usually try and give them at least a car's length room in case they need to take the lane. If after about 10 seconds it looks like they're content with holding their line then I'll pass them at no more than 5MPH in excess. I will also confess to doing a little bit of following if the bike (or the rider) happens to err... catch my eye in a special way. In either case, a little wave always seems to work at letting them know my IFF signature.