While I am all for appropriate penalties for anyone who intentionally (and seriously) harrasses or places another person at risk or in fear for their safety, to be fair to law enforcement, the advent of camera phones and other digital imaging technology has lead to a proliferation of video vigilantes. Everybody seems to be out to catch someone else "in the act" and they all have photos or video to "prove" their case. Cops get tired of people waving pictures of delivery trucks blocking a driveway or proof that it is the neighbor's dog that pooped in their yard. You might be the fifth guy today who came in with video of someone who passed too close, or honked at you, or flipped you off. They are not very likely to pursue traffic violations based on citizen video unless there is significant criminal conduct and the video provides sufficient documentation. If you do have video/photos of a criminal act like assault, battery, vandalism, or theft that clearly shows the person comitting the crime, and law enforcement doesn't seem to want to take it seriously, you can take it to the city attorney, your city council or commission, or other governing body with appropriate authority. You also may be able to use the digital images as evidence if criminal charges are brought or in a civil court if you sue for injury or property damage.

I've only had one time I have been able to provide a useful photo to law enforcement (nothing to do with cycling) and that involved an accident between two other vehicles. One of the drivers, a man about my age and size, began physically threatening the other driver, a smaller older man. I snapped a photo of his pickup and license plate with my cell phone and dialed 911 while yelling to both drivers that an officer was on the way. The driver of the pickup swore at me and said something about that he wasn't going to take the blame for this (it was the other driver's fault anyway) and got in his damaged pickup and took off. I checked on the other driver and when an officer arrived a couple of minutes later he looked at the photo to get a vehicle description. I had given the plate number to the 911 dispatcher already, but it was clearly visible in the photo as well. I have no idea what eventually came of it all, but I got a call from the officer a couple hours later to take a statement over the phone.