The short answer is yes, there only a few times that it would not be the back car's fault. The insurance company my just be avoiding paying the claim. This has the sound of a bad faith denial and they have been taking some heat for that sort of issue with from the Washington State Insurance Commisioner lately. Don't be bullied.
If you have been injured due to someone else's negligence, you may consider filing a personal injury lawsuit. If your injury is not serious you may want to pursue other options.
Prepare For Success
Gather all pertinent documents and any evidence at your disposal. Much of the record keeping should be done from the moment of the accident, before you even meet with your personal injury lawyer. Write down everything you remember about what happened, before, during and right after the accident. Videotapes, photographs and witness names and testimony will help a great deal. Then contact a personal injury lawyer. Your personal injury lawyer will take your case through civil court proceedings and will try and contact the other party to reach a settlement. Filing an official lawsuit may not be necessary.
Act Timely and Be Realistic
After suffering a injury you only have a certain amount of time to file a lawsuit. The statute of limitations varies by state. Illinois' is two years. If you miss the statutory deadline for filing a case, your case is thrown out of court. You may read about people being awarded millions of dollars after suffering minor injuries but serious "emotional distress" but these cases are rare. Be realistic about the damages on which you may collect reimbursement.
Work With Your Attorney
Your personal injury attorney will listen to your account of what happened, review your case carefully, and determine whether or not you have grounds to sue. If you decide to go ahead with a lawsuit, your personal injury attorney should handle all the legal proceedings and keep you informed every step of the way. Of course, you can file a lawsuit by yourself, but having a personal injury attorney is highly advisable, because trying to navigate the legal system without any expertise is very difficult. Personal injury lawyers draw almost entirely from court decisions and theories published in the legal community. Your attorney will explain the laws that apply and that will govern your case. Make sure you ask questions and understand the answers your lawyer advises you about.
It is usual that a rear end collision is the fault of the 'rear' driver; but it is NOT necessarily automatic. There is a presumption that the car in the rear is responsible; but the presumption is rebuttable. For example, what if you were driving down I-5 at 55 mph, and suddenly the driver in the lane next to you quickly swerved two feet in front of you - so that you struck them in the back bumper? That is a situation where the car that was struck might be found at fault.
It sounds like the other party's insurance company is denying your claim because their insured is claiming your son did something like that. As a practical matter, an insurance company will be forced to "believe" its insured unless the facts overwhelmingly establish that its insured is wrong.
So what can your son do? Did the police come to the scene? Is there a police report? The police report might help prove responsibility. It might also list other witnesses. This is why it's always a good idea to call the police to an accident. People's stories often change once they leave an accident scene.
Hopefully your son was insured himself. Assuming he was, he should report the accident to his own insurance company. Assuming he had collision insurance coverage, they will take care of the damage to his car (but for the deductible); and if he had PIP coverage, they will take care of his related medical bills (if he was hurt).
If he was hurt, it's time for him to call a personal injury attorney for help. If he wasn't hurt - so that all we're talking about is the damage to his car - he should let his own insurance company deal with it. And if he didn't have collision insurance, he can always sue the other driver in small claims court. (If he did have collision insurance, he can still sue the other driver in small claims court for his deductible.)
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