Generally speaking, I would think you have to answer these questions (if you are the plaintiff). Interrogatories are part of the discovery process, and the defendant is usually entitled to get this type of information because it may lead to other things that are relevant to the defense of their case.
To be certain, you should discuss this with your Philadelphia accident lawyer. If you do not provide this information, I assume the defense attorney will file a motion and asked the court to compel you to answer the questions. I assume the court will grant this motion. Again, you should discuss this with your own accident attorney.