I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear that this happened.
Putting a turn signal on doesn't give a person the right of way. If I've pulled off the road to the right and used my left turn signal, I have to wait until the way is clear before proceeding, whether I'm merging back into traffic, turning into a driveway on the left, or making a U-turn. The cars currently on the road, driving forward, have the right of way over a signalling vehicle pulled over on the side of the road. If the insurance company will not pay for the damages, your son can sue the mail carrier (and the postal service) for the cost of repairing the damages to his vehicle.
There's a concept in the law called "negligence per se," which says that, if you break a law and then an accident occurs, you're AUTOMATICALLY responsible. That means your son can bring the evidence of the ticket to civil court and use it to show that the carrier caused the accident. Another option is to send a demand letter to the insurance company, via certified mail, giving them one last chance to pay or have their insured get sued. Your son can write it himself (unless he's under 18), or he can hire a local attorney to do it. But if he goes to court, he should be able to win, based on what you've said.
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