I was motoring along southbound on a four-lane Oregon road, barely overtaking another motorcyclist who I'd first seen over a quarter-mile ahead of me. By now the distance had shrunk by about two-thirds and, even over the wind noise I could tell from the considerable rumble that he was riding a big twin. I was still a few hundred feet behind when he gradually overtook a van towing a big house trailer. This rider had been favoring the right side of the lane, and when he got right up behind the trailer, he made a quick flick into the left lane, again on the right side, close to the van-trailer. It was going almost as fast as we were, and I could hear that he didn't accelerate when he drew alongside of it. He had probably been in passing position for almost a minute when the van-trailer signaled, then began to move over (a deer carcass lay in the road ahead). The rider apparently didn't see the signal and didn't react until the vehicle began crowding him. Then he pulled ahead next to the driver and gestured angrily. I saw the brake lights on the trailer, then the bike, illuminate, as the rider apparently tried to stay next to the vehicle. But the driver still had the obstacle to avoid and continued to move over while trying to brake behind the bike. The rider wanted to express his anger though and didn't get out of the way until he was finally on the shoulder, and both he and the other vehicle had slowed to less than 30 mph, and I'd had to brake to stay clear.
It was probably the worst pass I've ever seen. The rider was probably used to being heard by other drivers, and I don't know why he didn't see the van signal (he was ahead of the trailer and maybe the signal on the van itself wasn't working or visible). But he did almost everything he could to get into trouble. He and his bike were dull colors (black and blue, respectively and appropriately), his lights were not on, he stayed close to the vehicle he was passing, perhaps in a blind spot. Relying on loud pipes didn't work either, maybe because the van was loaded with several people and music was playing loudly.
Passing other vehicles is one of those things we do routinely and rarely think twice about, but there is always danger there. It seems that at least once a week I read about motorcyclists killed when other cars (or motorcycles) cross the centerline and hit them. And I can't help but believe that some of those "single vehicle" fatal accidents where a rider runs off the road and crashes for no apparent reason are caused by another driver. Cars cross centerlines unexpectedly because their drivers are drunk or distracted or don't see the oncoming bike.
You can't do anything about keeping other drivers from getting wasted or answering the phone, but you can make yourself visible. In particular make sure your headlight is working and that you use the high beam during daytime. That high beam not only makes you appear closer, but it can also separate you from the car behind you with its headlights on. It can also help a driver that you are overtaking identify you in his mirror before he changes lanes. During midday, bright, solid colors on your helmet and jacket are probably most effective in making you conspicuous.
Overtaking another car usually involves passing through the driver's blind spot, and making yourself conspicuous will help him see you before you hide there. However, you can also protect yourself by accelerating as you come alongside to minimize exposure time and by moving away from him, perhaps to a more distant lane, as you pass. And if you want to be heard, a good horn (which projects its warning in the right direction) is more friendly and effective than loud pipes.