With enough cars participating you wouldn't need continuous data. Maybe a minute a day. Send with no identification, just the GPS coordinates. The unit should be smart enough to only send the minute of data when the car is in transit and has had a few minutes of travel on either side of the minute window.

The unit could also have regions defined as high density areas and those that aren't. The ones that aren't could be sent days later because that data is more for verifying roads than for immediate traffic information.