Dangerous Drive © Vladimir Maravic, Vetta, Getty Images

Insurance is supposed to serve as a safety net, but that net can have some unexpected holes.

Even if you have full car insurance coverage, you could have less protection than you think because of exclusions and limits lurking in your policy's fine print. Car insurance policies can vary considerably when it comes to who's covered and when.

"Insurance is not a commodity," says Bill Wilson, associate vice president of education and research for the Independent Insurance Agents & Brokers of America. "Buying an auto policy is not the same as buying a 2-pound bag of sugar."

Instead, he says, it's more like buying a bicycle. You have to evaluate features, the company's reputation and reliability, and match the product with the way you plan to use it.

That's why it's important not only to compare car insurance quotes but also to compare policy details before purchasing.

"You should always ask for a full copy of your policy and read through the whole thing," says Penny Gusner, consumer analyst for Carinsurance.com. "It's better to know what coverage you have than to find out you don't have the coverage you need after something bad has happened."

Here are six examples of how coverage might be less than you expected:

No. 1: Lower liability coverage for some drivers

In some states, insurance companies can include "step-down provisions" for auto liability coverage; those lower the policy's liability limits to your state's minimum insurance requirement when you let a driver who's not on the policy use your car.

Say, for instance, you have liability limits of 100/300/50 ($100,000 for bodily injury liability for one person, $300,000 for all injuries and $50,000 for property damage in an accident). You loan your car to a neighbor, who gets in an accident. Under a step-down provision, your liability limits for that accident would automatically drop to your state's minimum required liability limits. Those vary by state, but are far lower than 100/300/50.

"I'd call your insurance agent to ask, especially if you have a relative coming to visit who will use your car," Gusner says.

No. 2: Confusing underinsured motorist coverage

Underinsured motorist coverage kicks in when the at-fault driver doesn't have enough liability coverage to pay all your medical bills after you are injured in an accident. (Uninsured motorist coverage pays for medical treatment when the at-fault driver has no insurance.)

That might sound simple, but the payout for underinsured motorist coverage is complicated. Why? The amount paid from the at-fault driver's liability coverage is deducted from the amount you can claim against your own underinsured motorist coverage.

Say, for instance, you face $50,000 in medical bills for injuries after a car accident; the at-fault driver has $25,000 in bodily injury liability coverage, and you have $25,000 in underinsured motorist coverage.

The payment from the other driver's policy -- $25,000 -- would be deducted from what you can claim on your $25,000 of UIM coverage. The result: Your underinsured motorist insurance would pay nothing, and you'd be $25,000 short of the $50,000 you need.

Your underinsured motorist coverage must be higher than the at-fault driver's liability limits to do any good.

In some states, you can "stack" your underinsured motorist coverage -- meaning combine the limits on two or more vehicles you've insured to get more protection. And in Connecticut, you can buy "conversion" coverage, so payments from your policy are not reduced by payments from the at-fault driver's insurance.

Gusner says you should make sure you know how your underinsured motorist coverage works and evaluate whether you need to increase the limits on your policy.

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