The GPS data would be spatio-temporal data, lat/long = spatio, every x seconds = temporal. The FCD data apparently only supplies what can be easily captured within the car, such as acceleration and speed. The car doesn't know where it is, and unless it has a compass it probably doesn't even know where it's going. That's why you need the GPS.
You can calculate where a vehicle is and where it's going using only it's starting point, initial direction, and 3D acceleration data, but only if you have very accurate (meaning: expensive) equipment. Intercontinental ballistic missiles (ICBMs) have such systems, which is how they can be more or less accurately targeted. The equipment they use is probably far too expensive for traffic studies though.
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