You would want to verify that any vehicle you drive is insured. The owner of that vehicle is required to insure it. Your SR22 policy would be secondary to the car owner's insurance which would need to be on the vehicle and be primary. You could still get in trouble for driving that car if there is no insurance on it and thus cited for driving without insurance. In many states the car owner could also be cited for allowing you to drive their uninsured car on the roadway.
If you borrow a car from a friend and get into an accident in that car, your friend's insurance would kick in first. But if, for example, the accident was your fault and the damage to the other driver's property exceeded the Liability limits on your friend's policy, your SR22 insurance policy would cover the excess (up to policy limits). Without your friend (the car owner) having insurance on the vehicle there is no primary insurance and thus the insurance related to your SR-22 certificate of financial responsibility would not usually be used.
All this being said if you were not at fault in the accident than in most states even though the car you were driving was uninsured your friend could make a claim against the at-fault party for the damages to his car.
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