My friend Bill and I look up a steep, heavily wooded footpath in Rock Creek Park. We're finishing an hour of fast mountain biking on a crisp Saturday. We figured we'd have one more lung-burner before heading home. Not today.

We are riding slowly, almost at walking speed along a flat, wide path that runs along the creek about a half-mile south of the U.S. Park Police's Rock Creek substation. Bill and I know mountain biking on these trails is illegal and we therefore don't do it often, although the dirt, root-bridled paths are ideal for high-speed rides.

"That's the same guy," Bill says. I glance up ahead and see a shirtless runner slowing, about 100 feet in front of us.

I think little about this. We had passed the guy earlier in our ride on another trail and he apparently had said something to Bill about not biking on the trails. Such minor exchanges between hikers, joggers, bikers and dog-walkers are common, and Bill said this guy had been neither incendiary nor especially adamant.

I pedal onward. Bill drops back. But as I approach, the runner is standing still in the middle of the trail. I begin moving to my right -- conscientious lawbreaker that I am -- to indicate that, hey, we may carp at each other, but we're all out here to have fun.

In the 1990s, I frequently rode my mountain bike illegally on Rock Creek Park's hiking trails, especially after learning while researching an article in 1994 that biking does no more trail damage than hiking -- at least on dry trails -- and far less damage than horseback riding. Older and at least slightly wiser, I now do most of my biking on the legal trails of the Schaefer Farms trail system in Germantown, Patapsco Valley State Park in Ellicott City and Gambrill State Park, west of Frederick. But every now and then, when I crave a ride and am pressed for time, I duck into Rock Creek for a quick hit.

Park Police don't have an estimate of how many people pedal the trails. Based on my observation over 15 years (since mountain biking began flourishing) I'd call it no more than a few dozen. Whether biking, running or walking, I rarely see another rider on the trails, but I do see numerous mountain bike tire tracks that I know are not my own.

I also know from experience that many Rock Creek Park walkers and joggers view trail bikers with disdain, and thus I am always polite -- slowing down when approaching people, ensuring they see and hear me before I pass, offering a casual greeting. That is my strategy today.

But the jogger steps to his left, toward me. I move further right. He shifts further left. We are now three feet from each other and it now appears he wants to do more than talk. Aside from being a generally peaceful guy, I have another issue: I am still clipped into my bike pedals (damn high-tech shoes!) and have slowed to a crawl.

It is suddenly clear from this guy's body language that he has decided to attack me. But deep down I still don't believe it will happen. As I move yet further to my right -- off the trail and into the brush -- he starts coming at me.

"What's the problem, Ashcroft?" I ask, but he is already lunging, leading with his shoulder. I turn my shoulder in to meet him and just like that, we are fighting. I roll with my bike and somehow click out of my pedals. Looking up, I see the jogger in full flight above me, headed for my torso with a pointy knee and fists flailing.

I also notice something odd. This grown man has not a single hair on his body, like some alien attack mannequin. Nor does he appear to have much in the way of muscles, an observation confirmed when he punches me in the face. I barely feel it. But I do feel his knee as it connects with my ribs.

Still on my back, I repel him with two kicks and spring to my feet ready for a real brawl, the type I haven't engaged in since high school almost 25 years ago. My heart rate is up, pupils dilated. The fight-or-flight verdict is in. I start to charge.

But I am stopped in my tracks by a stream of burning spray -- do I taste cayenne? -- that floods my eyes and nostrils. I pause just as Bill sprints up to help out; he too is blasted with the pepper spray. Even as my eyes sear with the fiery pain, I realize that this confrontation was not unplanned.

Now standing about eight feet away, our adversary methodically shifts the spray from Bill's face to mine, alternating the blasts for about 20 seconds before sprinting off.

We are bewildered more than anything. Bill and I yell unprintables at the fleeing coward and walk over to the creek to rinse the pepper spray from our faces. I don't feel injured (though after the adrenaline recedes, I realize I have a bruised rib and a jammed thumb from rolling off the bike). We pedal slowly to the cop station to file an assault report.

Can't We Get Along?

This is my first physical encounter in 30 years of use of dozens of trail systems in the area: Rock Creek Park, the C&O Canal towpath, the Billy Goat trail along the Potomac River, the Cabin John trail, the multi-use trails mentioned above. My activities span the gamut, from mountain biking, jogging and dog walking to taking photographs and strolling to relieve a hangover.

I have had words with other trail users -- spawned by my illegal mountain biking or by someone's failure to control an aggressive dog -- but I rarely feared that any of those verbal snipes would escalate to a real fight. And I never imagined that, out there along the woodsy paths with the chirping birds and docile deer, were trail users so hostile or imbalanced that they spent their Saturdays armed with pepper spray, looking for trouble.

The police who take our report are flabbergasted, too, saying such outbursts without a robbery (or more sinister) motive are unheard of. U.S. Park Police in Rock Creek issue "no more than three of four" citations a year for illegal mountain biking, said Sgt. Scott Fear, the force's public information officer. "We rarely get calls about it; it is not a big issue." Park Police give verbal warnings to most trail riders because, Fear says, "most people don't know they're not supposed to be doing it."

The National Park Service bars mountain bikes from unpaved trails in Rock Creek Park "for the protection of park resources and to reduce conflict among visitors," Park Service spokesman Gerry Gaumer explained.

Fear noted that police in Rock Creek Park get many more calls about unleashed dogs, and they do reprimand people for violating the leash law. "That is a big problem -- much bigger than mountain biking," he said. Still, police issue "only a handful of tickets" per year for such breaches.

Many trails in the Washington area are multi-use, including the C&O Canal towpath: all non-motorized users are welcome and dogs must be leashed. The park does mandate that cyclists dismount when in the presence of groups of pedestrians, but the policy is rarely observed and almost never enforced. The paved Mount Vernon trail in Virginia, also non-motorized, has a stay-to-the-right policy and a 15-mph speed limit. In-line skaters, with their wide arm swings, pose the biggest problem there, said Audrey Calhoun, superintendent of the George Washington Memorial Parkway, which oversees the trail.

But Calhoun, like most local trail managers, said serious conflicts on the paths are extremely rare. Most users of local trails understand that the land is public. By my observation, they do what they can to peacefully share the space.

Still, trail use has risen dramatically over the past 20 years, and that means more potential for conflict. Authorities do not keep statistics on the number of fights that stem directly from trail-use disputes, but local park managers acknowledge the potential.

"I spent a Saturday on the C&O Canal towpath pulling my granddaughter out of the way of bikers," said Naomi Manders, volunteer coordinator for natural surface trails for the Maryland-National Capital Park and Planning Commission. "Weekends are just too busy on that trail," she added.

Bill Justice, chief of interpretation for the C&O National Historical Park, noted a "tremendous increase in use" over past 10 years. Visitation to that park, which runs from Georgetown to Cumberland, Md., peaked at more than 4 million visitors in 2000 and has dropped only slightly since then. "Any time you have that volume, there is a potential for conflict -- between fishermen and birders, joggers and bikers, all kinds of groups." But Justice said he could not recall one incident of a physical fight erupting on a trail.

Share the Road

Scott Scudamore, president of the Mid-Atlantic Off-Road Enthusiasts (MORE), a 350-member mountain bike club for the Washington area, said shared-use trails are a necessity. "There is only so much real estate," he said. "If we gave every [activity] a separate trail, there simply wouldn't be enough to go around."

At the same time, Scudamore does not advocate what I did -- i.e., pedaling on trails that are closed to mountain biking. "We really try to discourage people from poaching. When [mountain bikers] poach, we lose credibility." He's right and I know it, but I meekly remind myself that I only do this on occasion and only in Rock Creek Park.

MORE works with land managers at 24 trail sites covering more than 300 miles of single-track trails. Only one of those trail systems -- Fountainhead Regional Park in Fairfax Station -- is bike-only; the rest are multi-use, although some of those reserve a few individual trails for pedestrians only. The group estimates that roughly 10,000 mountain bikers are active in the Washington area.

The best shared-use trails, Scudamore said, "have good sightlines and no blind corners or really steep fall lines," two features that facilitate surprise encounters between bikers, hikers, joggers and equestrians -- a recipe for conflict.

Scudamore cited the Schaefer Farms trail system as an example of a well-designed network.

"There are hills but not a lot of screaming downhills, and there are great sightlines," he observed. Schaefer Farms is popular with local bikers because it serves a variety of skill levels, with many log piles that more-advanced bikers can ride over, tight single-track trails and nice stretches of fairly flat dirt routes. The trails wend through scenic forest and open fields and across streams.

Patapsco State Park, south of Baltimore, is also fairly well designed, he said, although it has longer hills than Schaefer Farms and thus encourages more downhill speed.

Even in poorly designed multi-use parks, most conflict is avoidable, Scudamore asserted. "You see very few problems when everybody acts like they were taught to in kindergarten -- share the space."

Pete Webber, communications director for the International Mountain Bicycling Association (IMBA), said physical confrontations on trails almost never happen.

"The perception of conflict is far greater than the reality. A study in New Zealand found that people who don't use the trails -- like a lot of government regulators -- believe it is happening, but when you ask trail users they say, 'No, I don't really ever have conflicts on the trails.' "

To minimize potential for clashes, Webber advises focusing on the moment at which two trail users pass each other.

"That is the key interaction. People are not having problems when they are in there alone, and they're not having problems at the trailhead." At the moment of truth, Webber said, cyclists should "say 'Hi,' don't startle people, move to the side and pass at a reasonable rate of speed."

By his formula, I did everything right, though he declined to say whether calling an attacking jogger by the name of a government official violated protocol.

To Each, Etc.

From a trail management standpoint, IMBA advocates diverse trail networks where, for example, an expert mountain biker would not be on the same path as a parent walking a child in a stroller.

"You get seven miles into the woods on a technical, single-track trail and anybody back there will be an experienced trail user. So even if you have a hiker and a mountain biker on that trail, they both should know how to treat each other," he said. Standard trail etiquette: Bikers yield to hikers; hikers yield to equestrians.

Manders said trail users need to be especially considerate around horses, which might not recognize a cyclist or even a backpacker as a human being and thus might get spooked.

"When a horse gets unsettled it starts pumping hormones. Once that happens, you're in trouble because the animal could bolt and throw the rider," Manders said. She recalled an encounter on the C&O Canal towpath where her horse started to freak out upon seeing two people wearing big backpacks. "Luckily they were smart and quickly took off their packs. Then the horse let a big sigh -- like, 'Oh, it's just people' -- and we went along. But for a moment it was pretty scary."

Manders cites loose dogs in local parks as the biggest conflict issue facing trail users. "Even a nice big friendly Labrador can be a problem if it runs up to someone who is afraid of dogs," she said. "We have had a number of issues with horses and dogs where we were sure the dogs were going to attack. Dog owners need to show some etiquette."

That's probably true for all of us trail users. For me, it means sticking to bike-approved trails, which I have done since my memorable encounter with the jogger. And now I realize this will lead to better political footing for mountain bike advocates who say we deserve dedicated bike trails.

Either that or I'll start carrying a Tex-Mex picnic lunch -- you know, something that calls for a lot of cayenne pepper.

John Briley is author of the Health section's Moving Crew column and anchor of the Crew's bi-weekly chats on washingtonpost.com.

These maps show some of the more popular multi-use trails in the area. Most are paved and designed to accommodate pedestrians and bikers of all speeds -- assuming mutual respect.