I've never had anyone come closer than a couple of feet from me, which is what I'd consider too close. At least, not that I've noticed.

Lately I've been having a hell of a time getting people to pass me even if there's plenty of room. This evening I was on a hilly part of my commute and a car followed me for about 15 minutes at 10 to 15 MPH. They would NOT pass me even when I motioned. There were several times when there was plenty of room to do so. The couple of times that I thought about stopping to force them to pass me, they had dropped back several hundred feet. Weird.