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.