I witnessed a hit and run car accident. Do I have to get involved?
You don't ordinarily need to get involved, legally speaking, if you witness any kind of car accident, even a hit and run. State laws don't impose any kind of legal duty on bystanders -- either to help out or to remain at the scene of an accident to tell law enforcement authorities what they might have seen as an eyewitness.
You might be thinking about so-called "Good Samaritan" laws, which are on the books in a handful of states. But those laws typically only apply to people who have already decided to try to help out and administer first aid or other assistance at an accident scene. Good Samaritan laws don't require someone to act at an accident scene; they protect people from incurring civil liability (being sued for personal injury for making an accident victim's condition worse, for example) when they do decide to take action.
So, you don't have a legal obligation to "get involved" if you witness a hit and run accident. It's up to you. Obviously, any information you can provide (description of the vehicle, description of the driver, how the accident happened) might be crucial in identifying the driver so that he or she can be held to face the appropriate legal consequences of a hit and run accident.
One other thing to note: If law enforcement, a victim, an insurance company, or an attorney gets wind of your identity and determines that you may have information that can aid a criminal investigation, you may receive a subpoena. This is a legal order that requires you to appear in court or at some other proceeding. It's not a request, but an order. If you ignore the subpoena, you'll face a "contempt of court" charge or some other legal sanction.