For four years, the truckers had seen it coming -- a distant but onrushing speck in the road.

They threw up detours, jerry-rigged roadblocks, threatened head-on collisions. But it came just the same. The Department of Transportation last week released its first major highway cost allocation study in two decades, and it found, as expected, that heavy trucks are paying only 65 percent of their fair share of federal road-use taxes.

The truckers, who fear the study could add another $2 billion a year to a tax bite they already consider too big, have begun bracing for a battle in Congress.

"The study doesn't scare us one damn bit," says Edward V. Kiley, the American Trucking Association's senior vice president. "Their methodology is preposterous . . . . It's ludicrous."

But bravado aside, the industry is already finding itself on the defensive. In its long gestation period, the report proved to be one of those rare documents that rearranged the terms of a public policy debate by looking at long-settled issues in fresh ways.

"Basically the fight has been over defining a new conventional wisdom over how you should compute road use costs . . . and the truckers have seen most of the important questions go against them," says Lee Lane, director of an in-house think tank of the Association of American Railroads, which has been cheering on all efforts to raise the tax load on its trucking competitors.

As the cost-allocation study lands on Capitol Hill, it is dovetailing with two other big-ticket, long-festering highway issues. The truckers are pressing for more permissive federal regulations on weights and sizes, and Transportation Secretary Drew Lewis is pushing to boost all federal highway taxes by the equivalent of 5 cents a gallon of gasoline.

Though the White House has put Lewis' tax proposals on hold, most observers believe that the nation's deteriorating roadways and its dwindling Highway Trust Fund will demand that all three issues be taken up in a major package of highway bills, probably next year. Already, the secretary has been openly offering a trade-off to the industry: bigger trucks for more bucks.

In the battles that lie ahead, the politicians will take center stage. In those just completed, the economists and engineers ruled the day.

As soon as Congress commissioned the study in 1978, all the key questions in the big truck fight became methodological.

The first federal highway cost-allocation study, conducted from 1956 through 1965 as the interstate system was being built, was based on something known as the incremental method. It determined how thick highway pavement had to be to handle automobile traffic, then calculated how much thicker it needed to be to accommodate heavy trucks. The trucks were assigned special taxes for the incremental costs.

This time around, the DOT has chosen what economists call a consumption-based approach, which involves assigning costs on the basis of damage actually done to the roads by different classes of vehicles.

DOT says the change came in response to the changing nature of the interstate system itself. "We're not building a system anymore, we are maintaining one," says Charles Swinburn, deputy assistant secretary for policy at DOT. The system is now more than 95 percent complete, and many of the sections that were built in the 1950s and 1960s are badly in need of repair. As a result, an ever-increasing percentage of the $7 billion annual trust fund revenues now go into repaving.

Kiley has a different explanation for the change. He attributes it to the fact that "economic theoreticians" have moved in on a domain formely ruled by highway engineers. "They've got their theories for everything; they even wanted to assign a social cost to the fact that trucks make noise," he says. "The problem is, we don't pay taxes in theoretical dollars."

Whatever the rationale behind the change, the truckers knew that once it was made, problems lay ahead. They were well aware, for example, that the most widely accepted test of roadway damage, conducted in 1962 by the American Association of State Highway and Transportation Officials, found that damage increases exponentially with weight. A fully loaded tractor-trailer semi combination does the same pavement damage as 9,600 cars, AASHTO found.

The industry has always taken issue with those findings, and when it saw that "damage" would be the basis of the new cost-allocation study, it commissioned two studies of its own. They both concluded that the chief culprit in highway damage was not weight but weather and age.

The railroaders immediately came up with their own study, which, predictably, rebutted the truck findings. Lane likes nothing better than to recall the reception the truckers got when they presented their weather studies to AASHTO last year. "As soon as the presentation was over, one of the highway engineers from the back of the room said, 'You mean to tell me that it rains more in my right lane than my left?' "

But Kiley is no slouch, and he knows how to fight anecdote for anecdote. "They don't allow trucks on the Baltimore-Washington Parkway," he says. "Explain to me why that pavement is coming apart."

Kiley insists that the trouble with the federal report is that "with all of their fancy new theories, nobody has found a way to put the weather into a computer."

He also says that the methodological change forces the truckers pay twice--once to build the roads to higher specifications, and then again to repair them. DOT says there is no double counting; the repairs are for future use, and therefore constitute a whole new category of cost.

In the course of the four-year debate over the study, the railroads and the truckers have seen their longstanding rivalry reach new heights. The stakes are considerable. The government has estimated that up to 70 percent of rail revenue is earned from cargo subject to truck competition, and in the last 30 years, the truck share of the market has been growing steadily at the expense of the railroads.

So the inside elbowing has been intense. "This is a big-ticket item for us," says Lane. At one point, when he discovered that DOT had hired the same Texas consultant who had done work for the truckers, "I came out of my chair four feet," he says. Fortunately, from the railroad vantage point, the government put a different spin than the truckers had on the consultant's data.

The two economic interests have been fighting not only at the federal level but from state to state--where roughly two-thirds of all road-use taxes are levied. Whenever the issue comes up, the railroaders dutifully present the legislature a report showing how much can be raised by assessing higher truck taxes.

They have also tried to thwart the efforts of the truckers to raise the weight limits in the three so-called "barrier states"--Missouri, Illinois and Arkansas. They are the only ones that have a 73,280-pound weight limit rather than the prevailing 80,000-pound limit.

Last year the Missouri legislature raised the state limit to 80,000 pounds, but the voters, responding in part to an ad camapign financed by railroad interests, overturned the decision in a referendum.

Nor are the railroaders the only interest opposing the trucks. Environmental, automobile and taxpayer groups are banding together as the highway issue heats up on the Hill.

By itself, the trucking industry is considered the most powerful lobbying force in the road transportation field, but the combined forces of its foes is formidable. Kiley is trying to remain philophical, but the anger shows. "People see potholes," he says, "and they need scapegoats."