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Ok, now the long version.
I was riding about 30bmph aka 25mph in a 25mph zone street. This car behind me suddenly accelerates, crosses the double-yellow line, and then dangerously cuts me off with very little room. I gave the driver a long press of the horn.
You can probably guess what happened next: the car got stuck at the next light. I pulled up right behind the car, gave another long press of the horn, and pulled over to get a closer look at the driver with the intent of telling the person what a crummy thing that was. As it turns out, the car was going in the same direction as I am and turned left.
So now she speeds up again, in another 25mph zone, and gets stuck at the next major intersection where you can't turn right on a red. About a minute later I pull up, whip out my phone, and take a picture of her license plate. I also happen to have a photographic memory of cars

When I got home, I went online and googled the hotline number for bicyclists (I know... read on) to report dangerous drivers. The info below is specific to Colorado, but I'm guessing that most states would have something similar:
- Get the vehicle's license plate number. Stop and write it down. The more descriptive info you have the better.
- Anyone, not just bicyclist, can and should report dangerously aggressive drivers.
- From your cell phone, dial *277, or from a landline dial 303-239-4501
http://bicyclecolo.org/page.cfm?pageId=731Bicycle
According to the State Patrol website, they enter this information into an aggressive driver database and send a warning letter after receiving three complaints. If the State Patrol receives additional complaints they send a trooper to make personal contact with the registered owner of the vehicle to take appropriate enforcement action.