It depends where you ride. I find it happens more often in high-traffic-volume situations (probably because the traffic makes the drivers more frustrated, just in general, before they even encounter you on your bike). It happens much less often, in my experience, on quieter back roads or rural roads.

Unambiguous road rage incidents I've experienced (as opposed to incidents where it could have just been total incompetence/impatience on the part of the driver):

1. Climbing a very steep hill one day on a road without a shoulder, an idiot driving a lawn care company truck with a trailer comes up behind me, then guns it around me and yells "get a motorcycle if you want to ride on the road, or get the f*** off". He then pulled into a nearby home's driveway, and glared at me like he was ready to fight me if I stopped to argue with him.

2. I've had fast food bags and drinks thrown at me a couple of times by passing drivers or passengers, which I would also interpret as unambiguous road rage.

3. Worst incident of all: I was about to cross a very busy and wide 8 lane (not counting turn lanes) road from a side street with a light. The side street green cycles are very short, and so I have to book it across in order to barely make it before the opposing green lights up. One day I approached the light on the side street right as it was turning to yellow, and knew that I wouldn't have anywhere near enough time to make it through. So I stopped to wait for the next cycle, which evidently really pissed off the driver behind me (she wanted to gun it through the yellow/red light, and I prevented her from doing so because I have to take the lane there (no bike lanes, right-turn-only lane to my right, left-turn-only lane to my left). She actually opened her door and started screaming and cursing at me, and I ignored her for a few seconds. Then she started revving her engine, and I finally turned around and gave her the one-finger-salute and told her to STFU (probably bad judgment on my part, but she was totally losing it and really pissing me off). She actually got out of her car and told me she wanted to fight me (mind you, this is a short, fat, middle-aged woman who is trying to pick a fight with a 6'1" male in his late twenties). I just started shaking my head and she finally got back into her car as the light cycle changed.

Incident #3 was the most unbelievable and surreal experience I think I've ever had while riding.