Do heavy trucks crumble US highways?

There was no telling what kind of freight the 18-wheeler had aboard, but it was obviously extremely heavy. Puffs of black smoke rose from the twin exhaust pipes as the driver maneuvered in the heavy traffic, and the trailer squeaked loudly as it traveled over rough patches of road. In fact, that's mostly what the road was - rough patches.

The scene was northbound on I-91 just outside Hartford, Conn. But it could have been along almost any other major highway in the United States. These roads have taken unmerciful punishment over the years, and by unanimous agreement they need major repairs.

Who, or what, is responsible for that punishment? The question is central to the spreading strike by independent truckers - and to the increased federal fuel and road-use taxes they are protesting.

Nearly every transportation-interest group except the trucking industry contends that a disproportionate share of the damage is done by heavy trucks.

''Definitely,'' says Roger Mingo of the Federal Highway Administration (FHwA). ''By our figures, they cause about 90 times the pavement damage per mile that automobiles cause - and that's the lightest amount that anybody has found so far.''

Indeed, a 1976 California study often cited by those in the transportation industry found that all but 1 percent of the damage to the state's roads and bridges by vehicles was caused by heavy trucks and buses.

Charles Brady of the American Automobile Association's (AAA) highways division is quick to agree that heavy trucks inflict the greatest damage to roads. ''And that's not a subjective judgment; it's an objective judgment,'' he says. ''There have been beaucoup studies on this.''

Adds Eldon Yoder, a professor of civil engineering at Purdue University and widely hailed as the ''dean'' of pavement design in the US: ''The only argument that we come into is the magnitude of it. The fact that heavy loads cause more damage than light loads is undeniable.''

Others choose their words more carefully.

''There's no question in our minds that trucks do a certain amount of damage, '' offers Randy Russell, director of public relations at the American Road and Transportation Builders Association in Washington. ''The problem is how to quantify it.''

''I'm in a tough position to say categorically,'' maintains Woody Rankin, director of transportation safety for the Highway Users Federation. ''(But) you can design a highway just for passenger cars at lower cost than you can for trucks.''

There usually seems to be more wear in the ''truck'' lane of a highway than in other lanes, Mr. Rankin concedes. But he adds, ''What you don't know is how well the road was designed in the first place.''

That is the kind of qualifier that the trucking industry likes to hear. The American Trucking Associations (ATA) in Washington, an organization representing more than 16,000 companies in all 50 states, seeks to steer the discussion to weather and environmental factors. Officially, the organization does not support the strike by independent owner-operators.

One ATA source, who preferred not to be identified, conceded: ''Obviously, there probably is some impact of heavier-load vehicles'' on US highways.

But he and other ATA spokesmen complain that the Department of Transportation study on which the Surface Transportation Act of 1982 - referred to by most people as the nickle-a-gallon fuel-tax increase - was based ''completely disregarded tried-and-true factors'' contributing to highway deterioration.

By these, the ATA means snow, sand, the salts used to melt ice in winter, the quality of soil on which a road is built - and especially rain.''

Water is the chief enemy of pavements - water getting into chemicals within the pavement and combining with elements on the pavement,'' one of the ATA sources said.

Usually, water finds its way into and under pavements through cracks opened by the continuous passage of traffic over them. In states with harsh climates, this phenomenon and the winter-spring freeze-thaw cycle eventually result in potholes and heaving of the pavement.

The ATA also points to deterioration of routes that bar truck traffic as proof that highways wear out at similar rates regardless of what vehicles use them. Three of these are the Baltimore-Washington Parkway; the Garden State Parkway in New Jersey; and the Merritt Parkway in Connecticut.''

Using the design-life principle (roughly translated to mean the useful life of a road before it needs to be repaved or replaced) the Baltimore-Washington Parkway should have lasted more than 1,000 years,'' one of the ATA sources said. ''But it wore out in 20 years.''

There are stretches of road in New Jersey, he continued, that have lasted 40 or more years under truck traffic and are still bearing up well. He could not identify the road, however.

''No way,'' argues Debbie Lawler, spokeswoman for the New Jersey Department of Transportation. ''I would not believe it if he told me there was a stretch of road (in her state) that's 40 years old and still in good condition - except unless we did some major resurfacing on it.''

The lone exception, Ms. Lawler says, could be the New Jersey Turnpike, one of the world's busiest highways. But the turnpike authority, created in 1947, has ''tens of thousands of dollars per mile'' to work with from toll collections and can make repairs to the road quickly, she adds.

Likewise, a spokesman for the Connecticut Department of Transportation says there are sections of the Merritt Parkway that have not been repaved since the road was built in the 1930s and '40s and still are in good condition. On the other hand, I-91 has been repaved many times since it was opened in the 1960s, he notes.

Engineers, according to Mr. Brady of the AAA, talk in terms of ''design years ,'' meaning the accumulated passages of vehicles over the road, coupled with their combined weight and environmental factors. If a road with a design life of 20 years wears out in 10 calendar years, he continues, ''The engineer says, 'Well, it reached its limit.' ''

''It's very difficult to get this (concept) across to people,'' Professor Yoder says. ''It's like a new car. As soon as you begin to drive it, you start using it up.''

Mr. Mingo of the FHwA defends the controversial 1982 federal Highway Cost Allocation Study - the one criticized by the ATA - as ''the most comprehensive engineering study ever done.'' Its focus: to determine which vehicles are paying their fair share of highway maintenance costs.

The study is ''not perfect,'' Mingo concedes, ''but we've got the problem bracketed pretty well, we think.'' And it did take weather and environment into account.