Depends a lot on where you are located.
Your family would likely report you missing, in time your car will be found and they will look for you or your body.
If your body is not found, your family will have the hassle of trying to have you declared dead. This can be a very long process (in some places, I'm talking years.) Your estate cannot be settled without a death certificate, so your phone will continue to ring as bill collectors try and track you down.
IMO, it's a really mean thing to do to your family and friends. If you're planning to jump in a place where you will not likely be found, please leave a note in your car making your intentions clear. At least then they won't always wonder...