Good question, I checked my policy, it covers any damage to personal property inside the vehicle in the case of an "accident". It does not however cover "theft" of property within the vehicle, which would be covered by my "property insurance". I guess you could argue if bikes are technically "inside" the car, when mounted to the trunk! Probably best to check with your own insurance company.

A quick google reveals that people were usually able to claim it either under the auto or property insurance (including when they backed up their car and accidentally smashed their own bikes against a wall or pole).

In any case, I would be more worried about my car in an accident than some easily replaceable bikes!