Nowadays it's rare to see a dusty traveler standing roadside with his thumb sticking out — a gesture that's evolved from a signal of soul searching and wanderlust to a cause for suspicion in security-minded America.

But security concerns are only partially to blame for why hitchhiking lost favor in the American imagination, challenging true believers to keep the spirit alive decades after Jack Kerouac's "On the Road" inspired a generation to thumb their way somewhere, anywhere, on a journey to self-discovery.

Americans' growing wariness with people they don't know well is one reason you don't see many hitchhikers (or maybe just no longer notice them) on the road, says Erve Chambers, chairman of anthropology at the University of Maryland, College Park.

"We're just an anonymous kind of society," he said.

Mark Holmberg, a 35-year hitchhiking veteran who began in his teens, agrees there is a general lack of empathy for those standing on the side of the road. As a columnist for the Richmond Times-Dispatch, he occasionally heads to the highway with thumb extended and writes a recurring series of columns about his experiences.

In hitchhiking, he's found, empathy must go both ways.

"You tend to get picked up by people beat up by life and (who) had to live through hard stuff," Holmberg said. And though he's had to fight his way out of threatening situations, the camaraderie has been worth the trip.

"You're a listener a lot of times. And once you're talking, the miles just disappear."

In some ways that empathy is characteristic of a generational divide, says William Falk, chairman of sociology at the University of Maryland.

"People with a recollection of the Depression realized you could be affluent and be touched by hard times, that you yourself were one bad break away from . . . losing everything you worked for," he said.

At the same time, the affluence Falk refers to is largely responsible for a decline in hitchhiking. Cars' increased affordability over the past couple of generations has helped make hitchhiking something to be met with suspicion.

"Now if you see somebody hitchhiking, the level of trust has so changed, you think, 'What is this guy's scenario?'" Falk said.

Or as Sgt. Rob Moroney of the Maryland State Police put it: "In today's society, most normal, contributing people have automobiles, can support themselves and not have to hitchhike."

Meaning, "Someone involved in criminal activity may use that as a ruse."

The proliferation of automobiles in the United States — to the point where there are now more cars than drivers — also begat the rapid expansion of the interstate highway system, where in almost all cases it is illegal to hitchhike, except for the on-ramps.

Yet hitchhiking lives on in the Washington, D.C., area, albeit in a form little resembling its freewheeling origins.

After all, a guy's gotta commute.

Enter "slug lines", a practice exclusive to metropolitan Washington, where city workers living in Northern Virginia anonymously carpool with other workers along designated pickup spots throughout the region so that drivers can use high-occupancy lanes that make rush-hour traffic tolerable.

Slugging, at least two decades old, is remarkably well-organized — maps and pickup points can be found on the Internet — is supported by city officials and, by anecdotal estimates, moves 10,000 people. According to local lore, they are called slugs after bus drivers' term for fake coins, when drivers pulled into stops expecting to make a pickup but instead found everyone waiting for the next carpool.

So why no hullabaloo about the dangers of randomly hopping into a different person's car each day of the work week? Falk has one theory:

"Here there's some sort of screening mechanism, a sort of fraternity . . . where you are presumed to be OK because you're a city worker," he said. "I'd guess that everybody doing this looks like a semi-well dressed person, they're probably getting almost entirely 8-to-5 workers and that's why this ride system is possible."

Meanwhile, Holmberg plans to continue the traditional hitchhiking route, so to speak, for the same reason he's been doing it for nearly four decades.

"Get to know your fellow countrymen, step out of your comfort zone, experience something totally new and different," he said.

But even he realizes that in today's world, it's a tougher and tougher sell.

"I don't think everybody's built to do it," Holmberg said. "We've changed so much, and things are so comfortable: We all have food to eat and a place to sleep. And people don't like asking for help."