With the passage of Memorial Day, the summer driving season is in full swing. And for many vacationers, that will include driving in a rented car.

Rental cars are often convenient, and in today's competitive market careful consumers can turn up reasonable prices in many parts of the country.

But for the unwary, the cost of the car may be only a fraction of the daily tab. The rest? Insurance, which could boost the rate by as much as half, according to some studies.

Insurance is a necessary expense of driving, and it is risking a great deal to go uncovered. But the insurance sold at car rental counters is expensive, and it may duplicate coverage you already have.

Just how expensive was highlighted last week by the U.S. Public Interest Research Group. The consumer advocacy group released a report on "collision damage waivers," which are sold by car rental companies and which purport to release renters from liability for damage to the car if there is an accident.

Surveying major rental car companies operating in the Washington area, the group found that the collision damage waiver can increase the cost of renting a car by as much as $12.95 a day -- an increase of 48 percent, in the case of a couple of companies.

The group said company representatives responding to in-person and telephone requests for price information failed to mention the collision damage waiver without prompting, but at the counter some 20 percent of the agents "engaged in hard-sell tactics" to persuade the consumer to purchase the waiver.

USPIRG's investigators, who posed as consumers, also found it difficult to extract information concerning driver activities or circumstances in which the waiver would be void. These include driving on unpaved roads, allowing an unauthorized driver to drive the car, driving while impaired or not sufficiently alert, or using the car to push or tow another vehicle.

Lucinda Sikes, a lawyer with the group and co-author of the report, called collision damage waivers "a classic consumer rip-off ... sold with deceptive statements under coercive circumstances at exorbitant prices."

A representative of the American Car Rental Association, however, expressed surprise at the tone of the report and Sikes's remarks.

The collision damage waiver "is an option ... designed for certain groups of people to be there if they want it," said Jan M. Armstrong. There is no coercion, she said, adding that she understands that only about 20 percent of renters now buy the waiver.

Collision damage waivers have long been controversial. Companies have been accused off and on in the past of using deceptive tactics to sell them. The National Association of Insurance Commissioners has recommended that states ban the waivers.

The car rental industry has argued that such a ban, which a few states have already imposed, would raise the cost of renting cars because companies would absorb the cost of accidents without any offsetting revenue.

They also say that smaller companies, which are more likely to offer discount rates, would be hurt more than larger firms by such a ban.

Sikes said rates in Illinois, where waivers are banned, have risen only 8 percent. She added that her group favors a federal ban.

The waivers are still available in most states, however, and the consumer's best approach is to understand them and then decide whether or not to buy.

To begin with, the waiver is just that -- a waiver. It is not insurance. Under a standard car rental contract, the company reserves the right to hold the consumer responsible for any damage to the car while it is rented. If the consumer buys the waiver, the company waives this right under most, but not all, circumstances.

If the renter is an average motorist, he or she may already be protected from this sort of liability by his or her own auto insurance policy. However, drivers of older cars, who do not carry collision or comprehensive coverage, may not be.

So the first step before renting is to call your insurance agent or company and ask what, if any, coverage you have for rental cars.

Second, if you charge your rental car on a credit card, it may provide insurance. American Express and some issuers of Visa and MasterCard offer this coverage. The next step is to call the issuer of the card you plan to use and ask what protection you may have through them.

Business travelers should check with their company travel office -- most business travelers are covered by their employers.

After this exercise, most people will find they don't need the collision damage waiver.

On the other hand, foreign travelers visiting the United States and those Americans -- primarily big-city dwellers -- who don't own a car may be well advised to spend the extra money. "It's just a case of preparing for the trip," said the car rental group's Armstrong.

The same is true for some other types of insurance sold at rental car counters. These coverages -- some of which are true insurance -- may be useful, but they, too, often duplicate existing protection.

One is the loss damage waiver, which covers vandalism and theft. Many companies include this in their basic cost, but some are splitting it out and charging extra.

Another is personal accident insurance, which covers death or injury. Your auto or medical insurance may duplicate it.

Then there is personal effects coverage. That may duplicate your auto or homeowner coverage, but deductibles on homeowner policies are often high, leaving little meaningful coverage for vacation luggage. If you're traveling light, you may prefer to take your chances on an uninsured loss.

Finally, there is supplemental liability coverage, which provides extra protection from claims by others. This has been criticized as expensive in the form sold by auto rental companies and again may be duplicative.

The best defense against buying unnecessary coverage as well as against leaving yourself unduly exposed to risk is a thorough review of your existing personal and business coverage before setting out for the rental counter.