If I witnessed a hit and run last year but didn't see the plate number of the car that left scene, am I legally obligated as a witness to sign an affidavit?

You have no legal obligation to sign any statement.  If you are properly served with a subpoena, then you may be compelled to attend a deposition or trial and give your testimony.

There are some pros and cons, however, to giving a statement.  The pros are that you let the parties know what your anticipated testimony might be.  This may help the parties settle the case without having to bother you with appearing at a deposition or trial.

The cons are that the statement can be used against you in a deposition or trial.  Rest assured that the attorney for the party who is hurt by your testimony will use you prior written statement to challenge your credibility.  Therefore, if you give a statement, make absolutley sure everything in the statement is true and accurate.

One thing you can do is give an informal telephone statement to the lawyers so long as they agree (preferrably in writing) that they will not record the conversation.   This way, the lawyers know what your testimony is going to be, but at the same time they do not have some recording or written statement that they can manipulate against you.

If you are summoned to appear in court or at a deposition, then you do have to be part of the process.  However, you are never required to sign a statement that you do not agree with.  In fact, signing a false document can cause you more problems, than the inconvenience your being bothered with right now, like perjury since this is an affidavit.  If you want to sign it, then manually make the corrections, make a copy, and the return the original.  I would also suggest sending a copy of the affidavit to the other party so they know what your statement is as well.  If this attorney is shady, you don't want him re-editing your affidavit and sending it forward in an incorrect state.  Keep the record strait from the beginning to the end.