The Collaborative International Dictionary of English v.0.48:

highjacking \highjacking\ n.
   The seizure of control of a vehicle while it is in transit,
   either to rob it or divert it to an alternate destination.

   Syn: hijacking.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

highjacking
    n 1: robbery of a traveller or vehicle in transit or seizing
         control of a vehicle by the use of force [syn:
         highjacking, hijacking]