I was a passanger in car whose driver was at fault. medical bills are over $100,000 but his insurance limit is $30,000. Who pays?

If the driver was a minor, you could sue his parents--they are then legally responsible for him. Or if the accident was owing to some failure of maintenance they were responsible for--e.g. they had bad brakes, knew about them, and never had them fixed--you could sue them. However, if the driver was an adult and the car was properly maintained, you probably can't sue the parents. To sue someone there must be liability--legal fault--and the mere fact that a person owns property, or that one person lives with another, does not establish liability. There must be something showing fault or liability.

You can of couse also sue the driver himself for the amount over the insurance (or technically, you sue him for the full amount, and the insurance pays up to its limit). If he has assets, you can then go after them (if you win), or try to garnish his wages.

Sometimes, unfortunately, there simply is no one with assets or insurance who you can collect from. This situation is why it's often a good idea to have underinsured driver coverage--it kicks in when you are injured by a driver whose coverage is inadequate to the damage/harm he causes.