NEW YORK -- With the arrival of the summer holiday season, many families plan trips that involve renting a car. Most don't give much thought to insurance until they're offered coverage at the rental car counter.

It can be a costly mistake if there's an accident and the driver doesn't have adequate coverage. At the same time, drivers shouldn't have to pay for insurance that duplicates coverage on their personal auto policies or is available via some credit cards.

"You have to do your homework and not wait until you get to the rental counter to start thinking about insurance," said Lisa Lee Freeman, deputy editor of Consumer Reports Money Adviser.

The two most important types of coverage for drivers are a collision damage waiver, sometimes called a loss damage waiver, and liability insurance. The collision damage waiver relieves the driver of financial responsibility if the rental car is damaged or stolen, while the liability insurance covers costs if the driver is in an accident and is sued.

Accepting the collision damage waiver and liability coverage from a car rental agency can add between $16 and $33 to the daily cost of the car, according to the Insurance Information Institute.

Jeanne M. Salvatore, a vice president with the New York-based institute, said families should check their personal insurance policies to see if they already have coverage.

"In the vast majority of cases, whatever coverage you have on your own personal auto policy is going to extend to your rental car -- with the big caveat that the car is being rented for pleasure, not business," Salvatore said.

So drivers who have comprehensive and collision coverage as well as liability coverage in their personal policies can decline coverage offered by rental car agencies, she said.

"If you're not sure, call your (insurance) agent and ask what coverage you have on a rental car," Salvatore said.

This isn't an option for families who don't own a car or who have dropped the comprehensive and collision coverage on an old car. In these cases, an alternative is to use a credit card such as Visa, MasterCard or American Express, that provides insurance protection as a benefit.

Visa, for example, offers a collision damage waiver program on its consumer credit cards.

"When you use that card to pay for the rental, the protection is automatically kicked in," said Seth Eisen, Visa's director of corporate relations. So drivers can decline the collision damage waiver offered by the car rental company.

"We estimate that cardholders who take advantage of the collision damage waiver will save roughly $10 a day on auto rental," Eisen said.

Still, insurance benefits can vary depending on the credit card issuer and whether the card is basic or premium, so consumer experts advise that drivers check their card issuers' Web sites or call the card companies to confirm coverage.

Consumers who take advantage of these credit card-related collision damage waivers may want to supplement it with the liability insurance offered by the rental agencies.

Rental car companies also offer other insurance options to customers, some of which may duplicate personal coverage.

These can include personal injury insurance, which provides a one-time payment if the driver or a passenger are maimed or killed, and personal property insurance, which covers the theft of goods from a car.

Freeman of Consumer Reports Money Adviser said most people don't need to buy either.

"If you have a health insurance policy, the odds are you're covered for personal injury," she said.

She added that theft of personal property often is covered in a homeowner's or renter's insurance policy and likely isn't necessary if families are security conscious.

"Common sense tells you not to leave anything valuable in your car," Freeman said. "And if you can't take things with you, you put them in the trunk out of sight."