It will depend on the requirements of your particular car insurance company, but if your son's college is 100 miles or less from your house then yes, typically you will need to include him on your car insurance policy.
Car insurance companies' guidelines differ so you will need to discuss if your son is required to be on your car insurance policy while he is away at college with a car insurance agent from your specific provider.
Most automobile insurance companies require that you include all household members on your policy, including any children away from home or at college that make the occasional trip home. Even if your son lives on his college campus likely his permanent address is still your residence thus still making him a household member in the eyes of your car insurance company.
In general, an auto insurance company is allowed to consider resident operators of an insured vehicle, including children that are away at school. This is because the insurer is using classifications that reflect a possible exposure for liability, in event that bodily injury or property damage occurs due to your child's operation of the vehicle.
If your son previously drove your vehicles when he lived at home before leaving for college, then most insurance companies would believe the probability of him driving your car when he comes home to visit is high and thus he should be covered by your insurance.
You say however that your child does not drive your vehicle at all when he is home, if that is the case you should see if your automobile insurance company will allow you to exclude him from your policy. Excluding him from the policy means he would not be extended any insurance coverage if he did drive your car and was in an accident, even if he had to take your car out due to an emergency situation.
If your son is attending a college at least 100 miles away from home, you may be able to get a discounted rate for him or take him off your policy completely, without needing to sign an exclusion for him. If this will be allowed depends upon your car insurance company's guidelines.
Because car insurance companies' guidelines vary on if a child must be on your policy or not and if an exclusion will be needed to not place him on as a driver, it is important that you discuss your situation with your company's insurance agent who can advise you properly on your specific situation and what is required.
If your automobile insurance company does require you to place your son on the policy and your rates go up then let CarInsurance.com help you find low cost car insurance. Our online car insurance quote takes a few minutes and then we provide rates directly from multiple insurance companies in 60 seconds or less.
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