I spend a lot of time on my bicycle in traffic, and since I don't (always) go as fast as a car I necessarily get passed by lots of cars. From my perspective, on the other side of that white line or squeezed over to the margin, I can tell that some drivers are nervous, anxious, impatient or even angry at my forcing them to steer carefully past me. Believe me, I'm anxious at that moment as well, though in a seasoned, been-there, done-that, here-we-go kinda way..
For those drivers who are nervous passing a bicycle in traffic: Thank you! That shows an appropriate appreciation for the sheer physics happening between your metal vehicle and my body during that exchange. If you follow these simple guidelines, we'll both get through it safely.
How to pass a cyclist? First of all: look ahead. Look waaay ahead. What's in your lane and what's in mine? If there's anything there that's not going as fast as you or me, don't pass. If you try to pass me while I'm passing another cyclist or going around a pedestrian, we're going to come together in a not-so-happy way. And don't underestimate my speed. We cyclists can often cruise on flat terrain at 25 mph or more. Wait until it's safe for everyone and everything involved.
Second, if it's clear, then go on by, but please give me as much space as you possibly can. Even if you're centered in "your" lane, if you pass me at 35 mph. with only three feet between us, that is a scary experience for me. Plus, one of us could blow a tire, or swerve slightly to avoid an obstacle in our paths, and close that three-foot margin in a deadly instant. Animals can run out, and there are always potholes and impediments on the road. So as much as you possibly can, give me a wide berth, even if it means going into the next lane to do so. Be sure to flash your lights at oncoming traffic if you go into their lane.
Also, pass peacefully. Don't suddenly hit the gas. This can be loud and scary to the cyclist and the fumes from "gunning it" are noxious.
Cyclists: All of the above, plus: Before passing, shout out "on your left" and observe the pass-ee before passing. Did they hear you and move over, or are they blissfully rocking to their iPod