OK,
Pepper is right, you can have fully controlable vehicles in CS.... There are a few extra entities added to the later .fgd's that allow you to have func_vehicle and func_vehiclecontrols. That is how they do it.
This does not cross over to HL. Unless you build in CS and ignore all the errors when you load HL.. Not sure that would actually work though. The only way you can achieve this in HL is with Spirit.

So if you want to make a map where you drive the vehicle, then you should do it using CS.
(can't see the point really, as you can't shoot and drive at the same time...)