What kind of sensor will detect a car outside the end of my driveway?
I think hall effect sensors may work (like those used to synch. traffic lights), laser/IR gates (such as 'garage-door' stop-close sensors) or a computer vision based 'car-detection' system, but just a background-suppression system will work. 

If you're good at programming, computer vision system may be better and most-effective (you can get easy notifications and also add a 'people detector' at little effort). Only things you need are: low-end computer (for simple BS), IR webcam and IR lamp.

But PIR may also do this job (it has to be adjusted) and is absolutely the cheaper.

... But if you like twitter notifications go for CV [Computer Vision].