The Collaborative International Dictionary of English v.0.48:

Roadbed \Road"bed`\, n.
   In railroads, the bed or foundation on which the
   superstructure (ties, rails, etc.) rests; in common roads,
   the whole material laid in place and ready for travel.
   [1913 Webster]

WordNet (r) 3.0 (2006):

roadbed
    n 1: a bed supporting a road