It depends on where you live and what kind of unit you’re looking at. Most GPS devices that show traffic get their data from a handful of providers like Clear Channel and Navteq, which collect their information from numerous sources. News of a coming pileup reaches your dashboard as text data over FM airwaves and updates every five to 15 minutes. This system works great for major freeways in big cities, but neither the network nor the data services cover rural areas and side roads, and the FM signal can’t deliver much beyond basic information.

Two new systems, Garmin’s nüLink and LiveTraffic from TomTom, instead use the cellular network to beam alerts to the nav units. That means you can receive updates more frequently and in a wider area, and you can find out details such as whether the slowdown is due to construction or an accident. But the service will cost you a few bucks a month.

If GPS makers and cellphone carriers can reach an agreement, we could eventually have a system like the one being rolled out in Europe that gets information from every idle cellphone on the road, providing truly ubiquitous traffic coverage.