Thousands of vacationing American motorists have been hitting the road in search of sunnier climes or snow-capped ski areas. Unfortunately, this year, they're finding the road is hitting back.

The nation's interstate highway system -- the backbone of our country's hughe skeleton of interlacing roadways -- is crumbling.

Almost a third of the system -- about 11,000 miles -- is in dire need of resurfacing or rebuilding and the rest is going fast, according to The Road Information Progran (TRIP), a Washington-based nonprofit research and information agency.

What it means to the American motorist is more bone-jarring potholes, more inconvenience, more aggravation and more expense.

The breakup of the interstate system comes as no surprise to Donald S. Knight, TRIP's executive director, who points out that the life expectancy of pavement averages 16 years and many interstate stretches have long since passed that mark.

"The once gleaming ribbons of concrete are coming apart at the seams and it poses a whole new set of concerns for American travelers," said Knight. "The cost of maintaining the system is going to be huge."

Most of the interstate system was built after World War II by the federal government for national purposes. "Fortunately for us," he said, "the emergency has never come and the system has been used by cars and trucks rather than troops, and for vacations rather than evacuations."

Although the federal government provided the lion's share of the funding to build the highways, it puts up litttle or nothing to repair and maintain them, Knight said. "The maintenance of the interstate highways has been largely left up to the individual states. Since highway pavings is an energy -- and labor -- intensive business, inflation has sent road maintenance costs soaring 220 percent since 1967. Many states now find themselves with extensive mileage of gift-horse highway that simply can't be maintained with existing revenues."

The TRIP executive pointed out that even though the interstate account for only 1 percent of the nation's highways, they carry nearly a fifth of its traffic. "We're going to have to come up with some sort of national consensus on whether or not we're willing to pay the price for the benefits offered today by th interstate system," he said.

Ironically, it is the American motorist's efforts to conserve gasoline that is helping to put them, their cars and the interstate in the hole. Gasoline consumption is down, and along with it are the tax revenues that states use for highway maintenance. On the federal level, the Highway Trust Fund has been driven into the red for the first time in history. The fund was created by Congress in 1956 with the primary purpose of building the interstate network. It derives its monies from federal taxes paid on gasoline, tires and other automobile equipment.

"With conservation, the 55-mph speed limit, lighter cars and smaller engines all cutting into fuel consumption, the tax revenues are down," Knight said. "However, because of bad road conditions, the motorists isn't saving that much."

He said TRIP studies show the average American driver is now spending $110 a year for fuel wasted by driving over rough and broken pavement. The fuel waste results from a loss of traction caused by vibration and an uneven power flow through a vehicle's drive train.

Adding insult to financial injury is an additional $45 a year, TRIP says, that the average driver shells out for vehicle maintenance costs caused by bad roads. Those costs show up as excessive tire wear and damage to brake, steering and suspension systems.

"The amount of money wasted by driving over bad roads is becoming increasingly significant as the cost of fuel continues to climb," Knight said. i"And rough winter weather, of course, brings many bad roads into full bloom.

"This winter, more than any past winter, drivers unfortunately won't be able to escape the expense and anxieties of bad road driving by using interstate highways."