Hardly anyone challenges the assumption that light-truck sales in the United States will grow indefinitely. Big Three executives boldly predict that light trucks will account for at least half of total light vehicle sales by the turn of the century.

Since 1990, light trucks have grown from 33% of the total light vehicle market to about 43% currently, with General Motors Corp., Ford Motor Co. and Chrysler Corp. controlling some 85% of that market. Not only have light trucks generated substantial sales gains for the Big Three, they've also contributed to substantially higher profits.

Consider that the Big Three make about $6,000 on an average compact sport/utility vehicle (SUV), nearly double the $2,500 to $3,000 they make on the average compact or midsize car. Ford officials haven't disputed one Wall Street analyst's prediction that its new Expedition could pump $12,000 per unit into the bottom line.

Given their success, it is no wonder the Big Three are banking on the truck trend to continue. Their optimism is reflected in an increasing number of new light-truck models and variations of current ones to be launched over the next several years, many aimed directly at conventional car buyers.

Detroit also is confident that current light-truck buyers will continue to repurchase those types of vehicles as they age.

Will this wonderful wave roll on forever, like a surfer's vision of the endless summer? Probably not. In fact, it is not likely that light trucks will reach half of the total light-vehicle market by the turn of the century, or even by the end of the next decade. In the short term, limited availability of lower-priced light trucks will make it difficult for penetration to go much beyond 45%.

Let's run through the math. To reach 50% penetration, an additional 1 million light trucks would have to be sold annually. That assumes a steady 15-million-unit light-vehicle market. That additional million units will be difficult to come by considering the lack of high-volume, lower-priced light trucks that will be launched during the next several years. The one remaining opportunity to substantially increase light truck sales is with new lower-priced models, primarily compact SUVs and vans, that are priced below $20,000.

This is due to the fact that demand for light trucks priced above $20,000, especially over $25,000, is close to the saturation point. It is increasingly unlikely that manufacturers will be able to pull many more buyers away from the passenger-car market. Some compact SUVs and vans priced over $25,000 already are beginning to meet with sales resistance, with manufacturers having to resort to incentive programs to keep sales moving. For example, Chevrolet is offering a $1,250 rebate on Blazer, while Isuzu is knocking $2,000 to $2,250 off the sticker price of Trooper.

Because about 55% of the passenger-car market represents models priced under $20,000, automakers will need to attract buyers of those models to reach their lofty 50% bogie. While manufacturers are investing in new lower-priced light trucks, these are not large-volume programs. In fact, when you add up all of the new lower-priced light trucks scheduled to be launched through the balance of the decade, a maximum of about 400,000 units in annual sales will be achieved.

Meanwhile, other factors will pull buyers out of the light-truck market at that time. Factors such as demographic and attitudinal changes will result in many buyers staying in or moving back into passenger cars.

From a demographic standpoint, much of the baby boom generation will be moving into their 50s by the turn of the century. These are the people largely responsible for the rapid growth in light-truck sales. As they age and move into the empty-nest stage of their lives, their vehicle preferences likely will change. Although most light trucks are evolving into more "car-like" and comfortable vehicles, they likely never will match the comfort level found in passenger cars.

One could argue that the loss of these maturing baby boomers in the light truck market could be offset by Generation X, but their size is not large enough to fully make up the decline in older buyers. Moreover, it still is not a foregone conclusion that Generation X will take to light trucks the way baby boomers have.

Probably the most volatile and controversial factor in the long-term direction of the light-truck market is simply the rationale for buying them. Much of the momentum behind the light-truck market, primarily with SUVs and full-size pickups, reflects their status appeal. These vehicles, which make up 60% of all light-truck sales, have become very trendy in most markets across the country.

For many buyers, the image appeal of these vehicles outweighs their more utilitarian aspects in the decision process. Unfortunately, conventional consumer research tends to understate the importance of status or image. Most people are reluctant to say they bought a vehicle because it was the trendy thing to do.

For these buyers, the SUV is an extension of their wardrobe. It is interesting to note that the SUV boom follows the fashion trend toward more outdoor or "active" apparel that began in the late 1980s. Call it "Northern Exposure" chic.

But today the only place you see Maurice, Holling, Dr. Joel, Maggie and the other charming characters of Cecily, Alaska, is on syndicated reruns.

Automotive trends, like popular television series, don't last forever.

The light-truck segment always will remain a strong and profitable part of the market, but it is unreasonable to expect its growth to continue at the current pace into the next decade. Manufacturers that are banking on that growth not only will lose sales volume, they'll miss out on the next wave.