When auto insurance companies insure your car, they're also covering you as a driver. Your driving history, along with the condition and age of the car, affects how much you pay for coverage. If you share driving duties with your partner or your teenage child, your auto insurer will take that into account.
The primary driver is whoever uses the car the most. If you buy your son his own car, he's the car's primary driver, even if you also pay for his coverage. The choice of the primary driver has a big effect on your premiums. If you have a clean driving record and your partner has a string of traffic accidents on her record, rates will be lower if you're the primary. If she's the one who does most of the driving, though, you should list your partner as primary all the same.
Anyone who uses your car on a regular or semi-regular basis is a secondary driver. You can have more than one secondary driver (for example, if your spouse and your daughter both drive your car). Every secondary driver you list on your policy adds to the cost, particularly if it's someone young -- teen drivers are a high-risk group -- or with a bad traffic history. Putting a secondary driver on your policy still works out cheaper than if the driver takes out a separate policy on her own.
If your family has two cars, your insurer expects you to share: You can't list yourself as primary on both policies, even if you're the driver who uses both cars the most. Instead, you have to be primary on one vehicle; on the other, you must be the secondary and pick another household member as primary. If you do most of the driving in both cars, take the primary slot on whichever vehicle gets you the best premiums. Usually your insurance is cheapest when the best driver is the primary driver on the more expensive car.
If someone borrows your car in an emergency, that isn't usually an issue: Either your insurance or the borrower's insurance should cover any damages. It's only drivers who use your car regularly that your insurer worries about. If you want to add a secondary driver -- for instance, you and your partner have started sharing a car -- talk to your insurance agent. Different companies may differ in the exact definition of "secondary driver," so make sure you conform to the rules.
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