"Stacking" refers to an increased amount of auto insurance coverage against uninsured and underinsured motorists, based on the number of cars you insure -- and this type of coverage typically comes in handy only if your luck is particularly bad. According to a report in USA Today, about one in seven drivers lacks insurance, so the decision as to whether to carry UM/UIM coverage -- and whether to choose stacked coverage -- comes down to how likely you think it is that the one-in-seven driver will find you.
UM and UIM Coverage
UM/UIM insurance covers you if a driver without insurance or with little insurance hits your vehicle. Your insurer pays what the other guy's insurance would have paid – or should have paid – if he had carried sufficient insurance. In some states, this coverage extends to property damage, but in others, it covers only bodily injury.
You can only stack your insurance if you have more than one car, and even if you do, your state's laws may limit your options. Stacking increases your UM/UIM coverage limits based on the number of vehicles you're insuring. For example, if you have two vehicles, your coverage doubles. If you have three, your UM/UIM coverage triples. If you're injured and your medical bills exceed one car's limits, you can tap into the other's to make sure you can afford the treatment you need. The cars don't necessarily have to be on the same policy, and in some states, stacking provisions only apply across policies if each of your vehicles is covered by separately. Some states don't allow stacking at all.
If you don't stack your coverage, you can't "borrow" from another of your vehicles' insurance when the UM/UIM coverage on your first car is depleted. Each of your vehicles' coverage stands alone. If you're injured, your insurance extends to the UM/UIM limits you took out on the vehicle you were driving at the time. The limits do apply per accident, however. For example, if you have $20,000 in UM/UIM insurance, you're covered up to $20,000 for one accident and another $20,000 if you're unfortunate enough to tangle with a second uninsured driver at a later time.
Adding UM/UIM coverage to your policy will make your premiums go up. You can waive this coverage in most states, but if you encounter an uninsured or underinsured driver, your medical bills are yours to deal with. Likewise, if you stack your coverage, you'll pay even more because you're effectively doubling or tripling the amount your insurer is obligated to pay if you have an accident. Like all aspects of auto insurance, how much more stacking will cost you depends on your insurer's individual rates. In some cases, the price difference between stacked and unstacked coverage might not be all that prohibitive.
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