Starting this week, just such a smarter navigation box is hitting the market. Called the Dash Express, this $400 product looks a lot like units from better-known firms such as Garmin (GRMN) and Magellan. Like them, it uses GPS satellite signals to locate your car on an easily seen map, and to route you to destinations and places of interest, using both visual and spoken instructions.

But, unlike any other in-car navigation device I’ve seen, each Dash Express, from a Silicon Valley start-up called Dash Navigation, becomes part of a network, connected to the company via the Internet. Each device not only receives and displays information, but transmits it as well, acting as a “probe,” as Dash calls it, to measure local traffic speeds. This information is compiled by the company and then broadcast back to all other Dash units in your area, almost instantly painting streets on your map with color codes to indicate traffic speeds.

I’ve been testing a Dash Express in and around my home base of Washington, D.C., and, while it isn’t perfect, I like it a lot. If the company sells enough units to create a solid network, Dash could radically improve in-car navigation.

That “if” is the big catch with Dash — in order to get its special benefits, enough units must be sold in your city to feed the network with sufficient traffic data. According to the company, for most cities just “several hundred” units would be enough to provide more than half of the significant traffic data it requires for major roads during normal commuting hours.

Meanwhile, Dash, like some of its competitors, makes use of limited traffic data provided by a commercial vendor. This information, which mainly covers major highways, is presented as a dotted line on the Dash maps, to indicate that it may be stale. By contrast, fresh input from Dash’s own network is presented as a solid line.

For the next 30 days, the Dash Express will be available exclusively at Amazon.com (AMZN) for $399.99. You get three free months of Dash’s service, including traffic and other features, such as wireless map updates. After that, the service costs between $9.99 and $12.99 a month. That fee includes the cost of the Internet connection used by the Dash Express, which is achieved using both cellphone and Wi-Fi networks.

To test the Dash, I had to create a tiny two-car network. My colleague Katie Boehret and I each drove the same route in Dash-equipped cars, about 15 minutes apart. The route included everything from the smallest residential streets to large local commuter arteries to the jammed Washington Beltway.

Katie went first, and by the time I retraced her route, my Dash unit’s screen was ablaze with solid-color streets indicating the traffic speeds she had encountered: green for free-flowing traffic, yellow for moderate congestion, red for stop-and-go conditions. Even two-lane local roads, the kind where traffic data are almost never available, were colored in.

Once Dash begins selling, the company won’t rely much on the information provided by a single driver like we did. It will average and weight the information it receives, to eliminate odd results from especially fast or slow drivers, and to emphasize the newest data. Each Dash reading will time out after no more than 25 minutes, turning solid lines into dotted ones as a warning that the information may be old.

I did run into a couple of glitches during the test. For one small road Katie had traveled, I received no Dash data. And on the return trip, Dash tried to route me right into a Beltway traffic jam, even though its screen showed that area in red. The company is working a future feature, called My Route, that would allow savvy drivers to order the device to use the local routes they prefer, to avoid such jams.

Dash Express has a host of other nice features, explained online at dash.net. Instead of giving you one route to your destination, it offers three choices, one of which supposedly incorporates current traffic conditions. It allows you to type in a destination on a personalized MyDash Web page and have that address sent wirelessly to your Dash unit, ready to be selected. You could even have a colleague or friend send you a destination while you are driving, so you don’t have to pull over to type it in or, worse, try typing while driving.

And the Dash also connects to the Internet to perform searches for local businesses, and then routes you there. Plus, you can create your own lists of favorite places and points of interest, or share those created by others and send these to your Dash from the MyDash Web site.

Dash Express finally brings the power of the Internet, and of community information, to auto navigation. If it becomes popular, it could be a big deal.

• Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

The slender Motorola Q, despite software that is markedly inferior to that of the Treo 700p, is wooing some users because it is much slimmer and now can be had for just $99, versus $299 for the Treo. The Nokia E62 is about the size of the Q and also costs just $99 these days. The tiny BlackBerry Pearl is just $199. And Samsung has introduced the skinny BlackJack for $199, too.

So, this month, Palm is striking back with a lighter, thinner, cheaper model of its own, the Treo 680, which is being offered by Cingular Wireless at $199.

Meanwhile, T-Mobile has introduced a new slim, light competitor called the Dash. It has built-in Wi-Fi wireless networking to supplement the slower cellphone data network. And it costs just $149.

I’ve been testing the new Treo and the Dash. Both are OK, but neither is as good as it could be. The new Treo still has great software, but it makes some compromises and still fails to match the new competitors in slimness, lightness or price. The Dash has very nice hardware, but is hampered by lousy software.

The Treo 680 is shorter than the Treo 700, because it doesn’t have the 700’s protruding antenna. It’s about 11% thinner and 14% lighter. But it’s still much larger than the new class of Q-type competitors. The Dash is slightly wider than the new Treo and about the same length. But it’s much thinner and lighter. The Dash feels great in your hand because it has rubberized paint.

This new Treo works just like the 700p. It uses the same Palm operating-system software, which is much easier and faster than the Windows Mobile software used by the Dash and the Q. As I have noted in the past, simple operations like deleting an email or displaying your calendar are usually one-click processes on the Palm OS devices, while they often take two or more clicks, or involve opening menus, on the Windows devices.

Also, the Treo 680, like the costlier 700p, comes with better functionality for handling Microsoft Office documents than the Dash does, even though the latter uses Microsoft software. The 680 has the same large, high-resolution screen as the 700p. By contrast, the screen on the Dash, while bright and vivid, is lower resolution.

But the new 680 is less capable than the 700 series Treos. First, it runs on a much slower cellphone network, called EDGE. This EDGE technology isn’t broadband speed, and is only about one-seventh as fast as the networks from Verizon Wireless and Sprint that the Treo 700 uses. Cingular does have a new network with speeds comparable to Verizon’s and Sprint’s, but the Treo 680 can’t take advantage of it.

Second, the Treo 680 is a big step backward in terms of its camera. The camera’s resolution is only about a third of a megapixel, while the camera on the Treo 700 — and the Dash — is 1.3 megapixels. Finally, the stylus on the Treo 680 is cheap and just plain awful. It actually bends when you use it.

The Dash is also stuck on the slow EDGE network technology because that’s the best data network T-Mobile currently offers. It makes up for it with built-in Wi-Fi, which is much faster than EDGE, and potentially much faster than the Verizon and Sprint cellphone data networks.

In my tests, I was able to use the Dash for email and Web browsing via Wi-Fi in my office, my home and a couple of coffee shops. The Wi-Fi setup and connection process was fairly easy, and T-Mobile has added software to the Dash that guides you through setting up access at Wi-Fi hot spots it operates in airports, Starbucks shops and other locations.

In fact, T-Mobile offers a data plan for $30 a month that includes both cellphone Internet service and access to its Wi-Fi hot spots. This is on top of the cost of a voice plan.

There are some downsides. The Dash doesn’t automatically switch on Wi-Fi. You have to do it manually. And, in my tests, it was much slower using Wi-Fi than a laptop was. For instance, in my home, on my very fast Wi-Fi connection, the Dash got just under one megabit per second, while a Mac laptop inches away got over 14 mbps. On a slower network in my office, the Dash got around half a megabit per second, while a computer inches away got 1.3 mbps.

I also found the keyboard on the Dash to be more cramped and harder to use than the one on the Treo. And the touch strip it uses to control volume didn’t work well.

If you have always wanted a Treo, but couldn’t handle the price tag, the 680 may be for you. Just be prepared for its slower speed and inferior camera. If you yearn for Wi-Fi in a slender smart phone, and can put up with a clumsy software interface, go with the Dash.

• Email me at mossberg@wsj.com.