I witnessed a silver Mustang full throttle from an intersection today and was wishing there was something i could do. (Or, substitute some other clear traffic infraction or dangerous driving — whatever premise is needed to make this question sensible.)
Theoretically, with a dashcam record as evidence, would it be illegal for me to give chase and attempt to get the driver to stop, place him under civil detainment, and call a local or state enforcement officer?
While local law varies, most jurisdictions do not authorize citizens' arrests for traffic violations. Generally, citizens arrests are authorized only for misdemeanors and felonies outside the traffic code.
Indeed, there are many traffic offenses for which law enforcement is authorized only to stop a violator and issue a ticket, but not even law enforcement is authorized to actually arrest someone. And there is no such thing as a citizen's citation or ticket.
Citizens arrests also usually require that the citizen witnessed the crime in progress, but that requirement would be met in this case.
If the case had been a hit and run, however, a citizens' arrest probably would have been legally authorized, although the wiser course of action would still be to call the police and to follow the offender at a safe distance.
The circumstances under which one may make a "citizen's arrest" vary depending on the jurisdiction. In most the citizen must have actually witnessed a violation of law. In some the violation must be of a specified severity, and a traffic offense would not qualify.
In any case such a pursuit might well involve driving dangerously, and that would be a violation of law.
Also, once the alleged violator stops, it is plausible that a fight breaks out, and that is potentially dangerous, and might involve violations of law.
One possible course of action is to call the police on a hands-free phone, and follow the offender, reporting one's location, and thus the offender's. The police could use this to stop and perhaps arrest the offender. However, the police will probably not go along with this unless they regard the offense as somewhat serious.
Anecdote: One time ~ 35 years ago I was stopped at a red light. Car in the lane next to me zooms through the intersection. A cop was in the intersecting road, and he turned and followed and pulled the driver over, and I assume gave him a nice big ticket. I thought that was fantastic - immediate justice for breaking the law. But there is no way I would have considered going after the driver myself, despite the clear violation of the law.
As others have pointed out, there are a lot of potential problems with pursuing a traffic law violator yourself, including both legal issues and risk of physical harm to yourself. Don't do that! However, calling the police - 911 or similar emergency number - does make sense if you see:
For almost anything else, grin and bear it. Police aren't going to arrest someone because you said they ran a light. Hopefully they will do something else stupid, but not injure or kill anyone, and get caught. Most people who run lights (or drive recklessly, etc.) don't do that just once.
OP isn't specifying a country, so I'll speak for Germany. There is no part of the StVO (Straßenverkehrsordnung, road traffic regulations) which gives rise to a civilian becoming a "traffic enforcer".
Hence, you would be best served by not breaking any law yourself when witnessing an offender. The safest bet would be to report the event to the police and record and provide evidence, hoping that they will take care of it. If for whatever reason you decide that it's a good idea to follow the other car, you'd still need to move within the speed limit, and you cannot simply decide to hinder them, push their car to the side or anything like that.
This kind of responsibility (i.e. to apply force to a fellow citizen) is (at least in Germany) clearly regulated by laws related to the Verkehrspolizei (traffic police) together with regular police law.
As an example for where the law makes this clear: if you are a Schulweglotse (a person with a yellow coat and a flag, helping school children to cross roads safely, often at busy traffic lights), the relevant law clearly states that although you have to be trained by the police in your job, you can not regulate traffic in any way, shape or form. For example, when you get this training, you are clearly told that if "your" traffic light has an outage, you are not allowed to regulate the traffic flow (much less do anything about any offenders).
So, TLDR: since there is no law in Germany making you a "civilian traffic enforcer", and since the opposite is explicitly stated in the few edge cases where other laws could be interpreted that way, the answer to your question is "yes" - if you do something illegal while forcing a potential wrongdoer, then you are liable just like anyone else if push comes to shove, and are putting yourself at the mercy of the courts.
Note that all of this says nothing about ethics or morals. Obviously, you can construct extreme cases where you can or maybe even should, as a good human being, decide to do whatsoever you wish, while accepting the consequences (i.e., whatever the law may or may not throw at you at the discretion of the court).
PS: Note that a dashcam is not allowed in some countries. I.e., in Germany, not only is dashcam evidence not admitted in court, but it may even be held against you due to the cam conflicting with EU privacy regulations.
PPS: Just report that Mustang if you recall the license number. If the driver pulls several such stunts, and other citizens report them as well, then over time the police might just pay them a visit; or if they get involved in an accident later, having such reports may count against them indirectly, in court.
