Many people and vehicles use APRS to broadcast their location. Their radios act as beacons, and they can be seen online here: http://aprs.fi. (Give it a minute to load – you may have a lot of data to show in your area.) This map provides up-to-date, accurate location information showing people, vehicles, boats, etc., as they continuously transmit packets GPS of data to other APRS-compatible radios. Take a look and see for yourself.
You can see a recent snapshot of APRS activity across the U.S. here, from late 2010: http://www.aprs.org/maps/USA-Turkey-10.png. As you can see, most of the APRS traffic takes place in urban areas or near highways, but plenty of people in rural areas use APRS too.
