The fun with the car-navigation system started in downtown Baltimore. I had been loaned a "car-nav"-equipped Acura TL by Navigation Technologies, the Rosemont, Ill., map-database developer behind many of these systems, and I decided to challenge the trip computer by not taking either harbor tunnel.

Right after I pitched it this curve ball -- as I cruised past Camden Yards, actually -- the screen froze on a map of the next turn. A flurry of button-pressing got me no farther than its setup menu, and after I tried to reboot it (i.e., turned the car on and off) at a stoplight, the system refused to start up at all.

Downtown Baltimore, inconveniently enough, is one place where I could use some computer-assisted, satellite-linked navigation, but the screen stayed dark for the rest of the drive to Philadelphia. An Acura spokesman said this was probably caused by a random error in the 340-megabyte hard drive the system uses to store its data -- my initial suspicions to the contrary, it wasn't my fault. (It's pretty sad how we've been taught to blame ourselves when our gadgets don't work.)

The next day, the car still thought it was in Charm City until I wheeled out of the driveway, allowing its miniature antenna to pick up the Global Positioning System (GPS) satellite signals that tell where "here" is. Then it functioned quite nicely, getting me to and from the country town of Mendenhall, then in and out of downtown Philly.

When it works, this $2,000 system is pretty slick. (Other car manufacturers, such as BMW, Mercedes and Lexus, offer comparable systems, as do aftermarket vendors such as Garmin.) You enter an address through the touch-sensitive screen or with a tiny joystick; you can also select a destination off a map, from a list of previous destinations or by picking a business in NavTech's database of restaurants, shops, hotels and other establishments. (Sadly, this otherwise impressive directory lacked the two bakeries I was looking for in South Philly, so my day ended without the hoped-for cannoli.) The system takes a minute or two to compute directions -- a processor upgrade wouldn't hurt here -- which it then coaches you through, turn by turn, in a synthesized sort of geisha-girl voice.

When you go "off route," it refrains from scolding while it recalculates directions, which can be pretty savvy: After I rejected its U.S. 50-to-New York Avenue routing on my return trip and continued south on the Anacostia Freeway, it knew enough to send me through the obscure Howard Road U-turn needed to cross over to the Southeast-Southwest Freeway.

After a while, I found it comforting to have this R2-D2 in the dashboard to steer me through erratic signage and confusing intersections. It also helped to settle some nagging routing questions -- for instance, whether the quickest way to get to one friend's Palisades abode is via Foxhall Road or MacArthur Boulevard.

Too many times, though, poor little R2 must have been reading the map upside-down. On Interstate 76 in downtown Philly, the system ignored an exit marked "Thirtieth Street Station," instead recommending an off-ramp a mile before the train station. Back here, it sent me from the George Washington Parkway to I-395 -- then across four lanes of traffic to the left-hand exit for U.S. 1, only about 2,000 feet down the highway. That was dangerous enough at 9 p.m. on a weeknight; in rush hour, it would have been flat-out impossible. And on my way to return the car in downtown D.C., the computer directed me into the service lane of K Street, then advised a gratuitously illegal left turn onto 15th Street.

I don't doubt that such a system can be genuinely useful; in Japan, for instance, car-nav systems are everywhere (it may help that Tokyo employs one of the world's most inexplicable and unhelpful addressing systems). I also don't doubt that the information this kind of gear delivers to drivers will only get better -- earlier this year, NavTech demonstrated how future car-nav systems could incorporate real-time traffic data. That will be commuting nirvana, at least until everybody gets one of these systems and drivers are back to where they started.

But the developers are going to have to overcome the same problems that the programmers of handwriting-recognition software, Web-screening filters and voice-input systems all deal with: Getting a computer to think -- and learn -- like a human being. And that's a problem that makes computing trip directions look simple.