They started out decades ago as handy vehicles for farmers and builders to haul things like lumber and fertilizer.

But now, with as many as six seats, roll bars, seat belts and cushy bucket seats, so-called side-by-sides have become the luxury sedan of off-roading. They are so popular with families and baby boomers that they are on pace to soon outsell their smaller cousins — all-terrain vehicles.

Side-by-sides are invigorating the power sporting equipment industry that has otherwise been clobbered by years of economic malaise and safety concerns. Side-by-side sales are growing roughly 15 percent this year, industry analysts estimate.

But with success has come scrutiny — and a probable showdown between safety officials and recreational enthusiasts. While side-by-sides appear to be far less prone to accidents than ATVs, federal officials are pushing toward mandatory regulations to make them safer.

Side-by-sides have been linked to an estimated 160 deaths and nearly twice as many injuries since 2003; ATVs, by comparison, are associated with 800 deaths and 130,000 injuries a year. An estimated 530,000 side-by-sides and 10.2 million ATVs were in operation by the late 2000s.

Unlike ATVs, which have handlebars and an open saddle, side-by-sides — the seats are beside one another — have a steering wheel and a roll bar. They reach speeds upward of 60 mph, and in some states, no driver’s license is required.

“As a utility vehicle where people are using them on the farms, they are perfectly appropriate,” said Donald Mays, senior director of product safety at Consumers Union, publisher of Consumer Reports magazine. But without rigorous stability tests, he said, “The concern that I have is these things can flip over, particularly when they are used on uneven terrain.”

Industry officials maintain that the vast majority of the accidents are caused by human error. And they worry that the government will require them to make side-by-sides so wide and so low to the ground to decrease the chance of rollovers that they will no longer be able to go on trails.

“Their mandatory standard is unrealistic from an off-road vehicle standpoint,” said Steve Nessl, marketing manager for the ATV and side-by-side group at the Yamaha Motor Corp.

Robust demand

The regulatory debate comes as manufacturers rush out new models to meet the robust demand. Polaris Industries, which markets a fleet of side-by-sides including a two-seat, 88-horsepower model, posted record profits in its most recent quarterly earnings. Its shares are up 53 percent since the first of the year.

“I went from selling roughly two to four side-by-side vehicles in a year to now I sell four a month,” said John Resciniti, the owner of the Motorcycle Mall in Belleville, N.J., which began selling Polaris side-by-sides a year ago. “It’s insane.”

Dave Crocker, senior partner at Power Products Marketing, which tracks the industry, said sales of side-by-sides, which cost about $7,000 to $17,000, fell with other recreational vehicles in 2009 but rebounded in 2010 even as the rest of the industry’s products — like ATVs, snowmobiles and dirt bikes — continue to lag.

This year, he said he expected roughly 240,000 side-by-sides to be sold in the United States; a decade ago, about 100,000 side-by-sides sold each year. Recreational users account for roughly a quarter of sales, he said, with the remainder bought by hunters, farmers, fire departments and for other utility uses.

By comparison, 330,000 ATVs were sold in 2010, and sales have declined from 10 to 30 percent in recent years, according to the Recreational Off-Highway Vehicle Association. Some analysts, including Crocker, believe that side-by-side sales have already surpassed ATV sales.

The Rhino effect

Some version of the side-by-side has been around since the 1980s, but the early models were relatively slow-moving utility vehicles. In the late 1990s, however, more people began using the vehicles for recreation, a trend that was accelerated when Polaris pushed past the 25 mph speed limit in 1998 with its Ranger model.

But the real propellant was the introduction, in 2003, of the Yamaha Rhino, the first side-by-side designed for recreation. In its peak year, 2007, Yamaha sold 44,000 Rhinos, Crocker said. Other manufacturers, including Polaris, soon followed with their own recreational models.

“It enabled people to buy these things and go on these incredible trails, and your son or your wife or your girlfriend can sit next to you,” he said.

Safety issues

But the Rhino became the subject of hundreds of lawsuits and a 2009 recall to address safety concerns, primarily rollovers that have led to injuries and some deaths. Among the victims was Ellie Sand, 10, of Ohio, who was killed when a Rhino she was riding in flipped in 2007.

“They are very narrow, they are top-heavy, and unfortunately they give the visual perception that they are safer because you see a steering wheel, you see a roll bar, you see a seat belt,” said her father, John Sand. “The concept is a nice concept. I just think they need to do things to make them safer.”

Yamaha officials maintain that there is no defect.

“We see nothing wrong with the Rhino and have defended it aggressively,” Nessl, of Yamaha, said.

He noted that his company had won all 10 lawsuits that had gone to trial; Yamaha officials declined to say how many other lawsuits filed against them had been settled.

Even so, Inez Tenenbaum, the chairwoman of the Consumer Product Safety Commission, said her agency had rejected drafts of voluntary standards proposed by the industry as too weak.

“We have to make sure there is stability and standards that prevent rollovers,” she said.