Check out this graph:
The three traces (speed versus distance) originate from GPS tracks taken between the same point A and the same point B, on different occasions, using three different transportation modes: driving, cycling and public transit (bus+train).
It is pretty obvious which is which. You can even recognize and identify traffic light stops, sections of freeway, bus stops, bus versus train rides (and also – ahem – speeding).
Which makes one think that it should be possible to algorithmically deduce the trip type and nature from just the GPS data, with pretty good accuracy. There will be problems though (fast trail runner or mountain biker?)…