In general, one driver reporting the plates of another is not likely to result in a citation being issued. Even if a citation were to be issued, I'd assume the initial caller would be required to testify in court, thus turning it into you word against theirs--an easily winning case. But what if multiple people call someone's plates in? I've read that officers are more likely to issue a citation in this case, but it seems there's an obvious exploitation. For example, a group of friends could conspire against a common enemy by all reporting their plates. Or let's say someone in a car with multiple people in it believes they have been cut off. Everyone in the car could then report the accused driver, getting him a ticket by default. Where is the line drawn and how possible are these scenarios to result in convictions? If taken to court, couldn't the accused driver simply say they weren't even on the road at the time? What's the point in witness based citations (without any proof but a testimony) even being issued?
The citation could be written but it should be easy to beat. In the circumstance that you describe the accused could just say the he was not driving his car at the time. If the prosecution is a sham with all your friends saying you all saw this thing, then the defendant can bring all of his friends to say that he was at some other place. The benefit to having a cop on the scene is that the cop will get ID and positive identification.
The other thing the cop has is expert judgment. If a cop sees unsafe lane movement and writes a ticket, his testimony in court carries more weight than the "it was not unsafe!" testimony of the accused. Keep in mind, getting all of your witnesses to court on the same day, after continuances and other delays, and getting them all to tell the same story to overcome the presumption of innocence is no small feat.
All that to say, it's easier with police as witness.
There are two mechanisms which could get you where you want to go. Private prosecution and citizen's arrest.
Private prosecutions are a part of history but not strictly unavailable at the state level. You could write up some charging documents to see if you can get your target indicted.
Citizen's arrest is interesting because the rules are fairly unclear unless governed by a specific statute. There are two necessary parts of citizens arrest and the first part gets the attention.
The first part is the detention (arrest). The crimes for which a citizen can legally detain a suspect are likely defined by state statute. The second part is giving testimony about the witnessed crime. Let's assume that you see a felony happen and watch the perp walk into a gas station bathroom. You are prepared to arrest, do part number one, and then call the cops, to do part number two. But while you wait for the guy to come out of the bathroom a cop walks up. You explain the felony and the cop goes into the bathroom and arrests the guy. It's purely witness testimony that led to the arrest and the indictment (physical evidence - let's say there is none) and even the trial.
You successfully get the guy indicted and when you show up to testify you tell the story as you witnessed it. Now the accused gets to take the stand.
When he takes the stand he says, "it was the other guy." When asked, "what other guy?" He says, "I was in the bathroom and a guy ran in and went into the stall. Then a cop came in and arrested me."
Reasonable doubt right there. How much more if it's a bunch of friends ratting on some lone driver? Police need evidence. Prosecutors need evidence. Judges need evidence.
Yes. This happened today to a friend of mine in Plymouth Meeting, PA, which is what brought me to this page searching for information. The officer issued a citation based solely on the account given by the witness - there literally is no evidence, other than her account, that a traffic violation was even committed.