By Jim Carlton, Wall Street Journal

KETCHUM, Idaho — As mountain biking surges in popularity, federal land managers are building dozens of miles of special backcountry trails to accommodate adrenaline-junkie riders and cut down on crashes with hikers.

Dozens of one-way, so-called flow trails allowing bikers to ride downhill at breakneck speeds have been built across the U.S. in recent years, with more in the works. Many of these trails are built on Western public lands managed by the U.S. Forest Service and Bureau of Land Management. They are specially designed with jumps and berms to allow riders to flow unimpeded down the mountain without stopping, making the downhill-riding experience akin to skiing or sledding.

Lake Tahoe is joining the country in expanding mountain bike opportunities. Photo/LTN

“It’s like a luge,” said 30-year-old Jeremy Newberg of Hailey, Idaho, after riding a flow trail called “Punchline” in the state’s Sun Valley this month. “It’s just awesome.”

As well as thrilling riders, the trails provide a legitimate alternative to guerrilla trails carved out illegally in some areas. And supporters say the flow trails help solve what backcountry visitors say is a growing problem: bicyclists careening into fellow trail lovers on foot and horseback.

“Most trails, you don’t know if someone is coming around the corner, but this you can trust going down,” says Tiff Koehn, a 37-year-old, Spandex-clad corporate pilot, after hurtling down another Sun Valley flow trail called “Forbidden Fruit.” It takes just three to five minutes to barrel down that 1.3-mile course.

More than two dozen locations nationwide now boast multiple flow trails, according to the International Mountain Bicycling Association.

The expansion in the number of these trails comes as mountain biking has grown from being the obsession of a handful of enthusiasts 30 years ago to a sport with 7.1 million participants, according to the Outdoor Foundation, a nonprofit group in Washington, D.C. Bikers are now the No. 2 users of backcountry trails after the 32.6 million hikers in the U.S., according a 2010 report by the group. Amid this growth, hikers have been struck by cyclists and seriously injured, and horses have thrown riders after being spooked by bikes.

“If [cyclists] are coming downhill, we have to be alert,” says Bob Anderson, a veteran horseman from South Lake Tahoe.

The conflicts have gotten ugly at times. In 2002, someone booby trapped a trail in Santa Barbara County with a wire designed to knock a rider off a bike. No serious injuries were reported, though some riders were shaken up by falls. No arrests were made, according to news reports at the time.

So far, the flow trails are drawing support from a wide spectrum of backcountry users. “The problem is when hikers, bikers and dogs intermix,” says Nick Hilbers, a 55-year-old hiker from Beaverton, Ore., who recently alerted local officials to an illegally made mountain-bike trail in a nearby city park.

These illicit trails have sprung up in several areas in recent years, helping spur the growth in sanctioned runs on federal lands. In the Lake Tahoe Basin, for instance, as many as 150 miles of unauthorized trails ringed the surrounding mountains a decade ago, says Garrett Villanueva, a Forest Service assistant engineer for the basin.

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