Taken from the South Carolina DMV (Your laws may vary):
Remember, it is unlawful to harass, taunt or maliciously throw an object at or in the direction of any person riding a bicycle. Here are some of the critical points to remember:
1. When attempting to pass a bicyclist, you must treat the bicycle as any other vehicle you would pass.
3. The horn should be used to attract the bike riderís attention and not as a threat to get out of your way.
4. Drivers must be careful when driving close to cyclists and should maintain a safe operating distance between the motor vehicle and the cyclist.
5. Before passing a cyclist in a narrow traffic lane, wait until the traffic is clear in the opposite lane and then change lanes to pass the cyclist. Do not attempt to squeeze past the cyclist.
Edited those that did not specifically relate to passing.