Campsites are filling up across the U.S.

Although it's still early in the season, there are already signs that more Americans will be vacationing in campgrounds this summer.

Bookings are up at many parks — in some cases by as much as 30 percent. And some campgrounds were filled on weekends even before Memorial Day, the traditional kickoff of the camping season.

Many in the camping community believe that the bad economy is causing sudden interest in the great outdoors, with people who are worried about finances forgoing a stay at a resort or a trip overseas in favor of a sojourn in the woods that can cost just $10 a night in campground fees. Gas prices that are significantly lower than last summer's (albeit rising) may also be affecting decisions to hit the road.

Suzi Dow, who with her husband, Fred Dow, writes guides to U.S. Forest Service campgrounds, said that back in April, traffic to their site, www.forestcamping.com, reached 300,000 hits a day, a rate they don't normally see until summer.

Although certain parks have always been magnets, camping has been declining for a generation, according to the National Park Service. In 1980, there were 3.93 million overnight tent stays at Park Service properties, including the country's 58 national parks, which contain 861 campgrounds ranging from primitive (pit toilets) to modern (hot showers); last year, 2.95 million stays were registered.

General visitation is down, too, prompting the Secretary of the Interior, Ken Salazar, whose department includes the Park Service, to announce that 147 of its sites would waive park entrance fees this weekend and two other weekends this summer (July 18 and 19, and Aug. 15 and 16).

"I'm optimistic," said David Barna, the chief of public affairs for the National Park Service, whose budget this year increased to $2.5 billion from $2.4 billion last year. This does not count $741 million from the American Recovery and Reinvestment Act of 2009, some of which will go toward campground upgrades like restroom remodeling and the resurfacing of access roads later this year. "Typically, when the economy is not in good shape that bodes well for the park service," Barna said.

Indeed, bookings at the 141 national park campgrounds in the National Recreation Reservation Service were up 9 percent from January through March 26 over the same period last year. The reservations are handled by ReserveAmerica, a private company, through the site www.recreation.gov and at 877-444-6777 (a fee of $10 is added).

Although many experienced campers booked six months ago, the good news for latecomers is that there is still plenty of room at campgrounds that take reservations, particularly during midweek, when demand is lower.

Cancellations do occur, and there are first-come-first-served campgrounds. (Arrive in the morning, when the previous night's campers are checking out.)

In addition to national park campgrounds, there are 5,800 campgrounds in national forests plus thousands of state park and privately run campgrounds across the country.