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

Moby Thesaurus II by Grady Ward, 1.0:

117 Moby Thesaurus words for "roadbed":
   Autobahn, L, US highway, alley, alleyway, arterial,
   arterial highway, arterial street, artery, autoroute, autostrada,
   avenue, belt highway, blind alley, boulevard, branch, bypass,
   byway, cable railway, camino real, carriageway, causeway, causey,
   chaussee, circumferential, close, cog railway, corduroy road,
   county road, court, crescent, cul-de-sac, dead-end street, dike,
   dirt road, drive, driveway, el, electric railway, elevated,
   elevated railway, embankment, expressway, feeder, feeder line,
   freeway, gravel road, gravity-operated railway, highroad, highway,
   highways and byways, horse railway, interstate highway, junction,
   lane, light railroad, line, local road, main drag, main line,
   main road, metro, mews, monorail, motorway, parkway, pave,
   paved road, pike, place, plank road, primary highway, private road,
   rack railway, rack-and-pinion railway, rail, rail line, railroad,
   railway, right-of-way, ring road, road, roadway, route nationale,
   row, royal road, secondary road, sidetrack, siding, speedway,
   state highway, street, street railway, streetcar line, subway,
   superhighway, switchback, terminal, terminus, terrace,
   thoroughfare, through street, thruway, toll road, township road,
   track, tram, tramline, trestle, trolley line, trunk, trunk line,
   tube, turnout, turnpike, underground, wynd