The Collaborative International Dictionary of English v.0.48:

slip road \slip road\ n.
   a short road giving access to an expressway [British] "in
   England they call an access road a slip road."

   Syn: access road, slip road.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

slip road
    n 1: a short road giving access to an expressway; "in Britain
         they call an access road a slip road" [syn: access road,
         slip road]