When it comes to Mapquest, don't get Pittsburgh map maker Bob Firth started.

He knows he is going against the tide of a wildly popular innovation that has guided map-averse motorists from Altoona to Albany, from Boston to Billings, from Meadville to Morgantown.

But Mr. Firth, a competitor, doesn't trust Mapquest, even if tens of millions of others do. To bolster his case, he points to the Internet mapping service's route from his old office on 100 Ross St. to his Regent Square home.

Take this route, he says, at your own peril.

"What they ask you to do is the most illegal thing you could ask someone to do and live," says Mr. Firth, president of Informing Design Inc., which makes color-coded maps and designs road signs. "Well, you wouldn't live. You would probably die. Or have a really, really bad accident."

A Mapquest printout directs drivers down Court Place from Ross Street. Then it says to merge onto I-376 East, which would require motorists from Court Place to turn left at Grant Street across traffic flying down the incoming ramp of the Boulevard of the Allies, in addition to cars coming the opposite direction on the boulevard. That's despite two signs that say "No Left Turn" and two signs that say "No Turns."

"Picture this," Mr. Firth says. "You just got in town. You couldn't make a left. Ha. Ha. Ha. You are like a goner," panicked in the middle of one of the busiest intersections in Downtown.

Google Local and Yahoo! Local and Maps, he says, also direct you to make the same "dangerous, illegal and horrible turn."

Mr. Firth, a Blackberry-carrying, iPod-toting cartographer, says he is not some wild-eyed Luddite. He just thinks the existing online mapping services are often plain wrong or send out-of-towners the long way on routes that locals never use. He also believes they make people focus on tiny increments of information and ignore important contextual clues and road signs that would orient motorists to their surroundings.

"It is the illusion of mastery because you have all these instructions coming out," says Mr. Firth, who is negotiating with the city of Montreal to do the first prototype of his own online mapping service.

But a Mapquest executive said they receive very few complaints of errors. And Americans have embraced its mapping service, which turns 10 this year. As the dominant online mapping service, its site had more than 41 million unique visitors during December, or 68 percent of the visitors to map sites, according to comScore Media Matrix, which rates search engines.

The company is so entrenched in popular culture that it has become a verb. "I'll Mapquest it," people will say, cutting off offers of directions from human beings.

"People really, really like and use these online mapping tools" -- more so than other Internet services, says Greg Sterling, an analyst of the Kelsey Group in Princeton, N.J.

In fact, 87 percent of Internet users have done online mapping, leading all other online activities including news, e-commerce and gaming, the Pew Internet & American Life Project reported in August 2004.

Mapquest, which was bought by AOL in 2000, contracts with companies that provide the raw mapping data. To get users from Point A to Point B, it applies a complex mathematical formula that favors right turns over left ones and main roads over back ones. Users can type in beginning and end street addresses or more general categories such as hotels near a particular airport or simply "Mount Rushmore."

"When it comes to finding places, no one does it more accurately or relevantly," says James Greiner, vice president and general manager of Mapquest.

For many people, a printout with numbered instructions of turns and tiny map fragments is preferable to unfurling a large map, reading small print and then jamming the rumpled map back into the glove compartment. "You can pull over and read it," says Randy Kress, 50, of the North Side. He recently followed Mapquest directions that took him right to the door of a friend's new condominium in Washington, D.C.

And let's face it: A computer printout is often more succinct than human directions, filled with confusing travel tips such as "make a left where the Kroger's store was."

Denise Catalfamo, 49, of Dormont prefers Mapquest to people's guidance. It got her to many tourist destinations during a recent trip to San Francisco. And it doesn't get her lost on the circuitous roads of Pittsburgh. "Instead of maps, I use Mapquest. If I am going somewhere, a party, a new restaurant, I don't bother to ask for directions."

"It has gotten more accurate," Mr. Kress says.

Others disagree.

"Mapquest turns a 20-minute drive into an hour-and-a-half drive," says Heather Chirdon, 31, of Shaler. "Living in Pittsburgh doesn't help. It's not like we have the best, well-marked maps." Or road signs. Venturing into unfamiliar neighborhoods as an auditor for Retail Grocery Inventory Specialists, she now uses Google Local instead of Mapquest.

Eric Shephard, a social studies teacher at West Allegheny High School, says Mapquest is generally useful, although it has sent him the wrong direction down a one-way street.

Mr. Greiner of Mapquest says less than one-tenth of 1 percent of its users e-mail the company with feedback. Of that group, less than one-tenth has anything negative or constructive criticism about the directions. Any errors are forwarded to the vendors, who develop their raw data by riding around streets with laptops with global positioning system software.

So what?, says Mr. Firth. How many lost motorists are going to bother to complain to an impersonal online mapping service?, he asks.

Mr. Firth says people who rely on online maps often ignore important contextual clues such as road signs or rivers because they are so narrowly focused on what the printout tells them to do. "Relying on these driving directions is like kids relying on a calculator and never mastering the multiplication tables."

Another criticism is that once you make a wrong turn, it is often hard to get back on track.

An executive at Yahoo, which offers drag-and-drop maps and multipoint driving directions, says you may be able to get back on course if you opted for a printout of a map. "It is still good to have a paper map in your car," says Jeremy Kreit-ler, senior product manager of Yahoo Local and Maps. Yahoo is starting to put the text on highway signs on its directions, he says.

Mr. Kreitler says the proof that online mapping services work is that so many millions of people use them.

"Compared to asking friends and stopping at gas stations, it actually works pretty good. I have the worst sense of direction," Mr. Kreitler says. "If it weren't for stuff like this, I would get lost all the time."