Truck tractors and trailers hauling freight on the nation's highways are older than most company managers would prefer, a new study of equipment age shows.

The report, prepared for the Regular Common Carrier Association, a trucking trade group, also suggests older equipment could be contributing to truck safety concerns that have surfaced since the industry was deregulated seven years ago.Moreover, researchers hint that financial troubles tied to deregulation may be the cause of the aging fleets.

The median age of truck tractors as of Jan. 1, 1987, was four years, according to the survey that underlies the report. Two years earlier, median tractor age was slightly older, at 4.2 years.

Despite the decline in age, the report says prudent truckers prefer to replace tractors on a three-year schedule.

Given the intense pressures since deregulation and the resultant financial inability to adhere to that schedule, equipment continues to be older than sound managerial principles recommend, the report says.

Research for the study was conducted by Dabney Waring, director of cost research for the Motor Common Carrier Association, which represents the industry's rate bureaus.

Both the conference and the rate bureaus opposed partial trucking deregulation in 1980 and continue to resist further changes.

The report's conclusions are based on a three-year survey of 66 trucking companies.

But the analysis focuses on large truck lines with annual revenue above $250 million, including national giants like Roadway Express Inc., Yellow Freight System Inc. and Consolidated Freightways Inc. All 12 fleets with business volume above $250 million participated in the study.

Although the median age of tractors fell slightly during the study period, the survey shows that 45.6 percent of all tractors operated by major carriers were more than 5 years old at the end of 1986.

Straight trucks, which comprise a small portion of a national carrier's fleet, aged from an average of 6.4 years in 1984 to 7 years in 1986.

Most trailers, conversely, were newer in 1986 than two years earlier. A 1983 law that altered truck size and weight rules and allowed twin-trailers on most highways spurred a trailer-buying binge in the mid-1980s.

The median age of trailers for major trucking companies was 2.9 years in 1986, down from 5.1 years in 1984.

The report's authors say the survey measured only the duration of time since the tractors and trailers were purchased. No attempt was made to calculate the equipment's useful life, which would have required an analysis of miles driven, engine replacement timetables and technological improvements after


Russell Capelle, director of research for the conference, said future equipment studies might include more precise operating data.

Beyond equipment age, the report also shows major, less-than-truckload carriers continued to convert their fleets from 45-foot trailers to 28-foot models.

Just over 50 percent of all trailers operated by major fleets in 1984 were 28-feet long. Another 38 percent were 45-feet long.

But by 1986, shorter trailers comprised nearly 70 percent of the total fleet and 45-foot models just 21 percent.