Travel in the Arctic is nowadays predominantly by aircraft or by specifically designed overland vehicles, as few roads exist. Terrain damage resulting from off-road vehicular movement in arctic areas is potentially serious—particularly in the wetter, ice-rich permafrost terrain. Detailed examinations of vehicle trails made in the 1940s indicate that natural recovery and stabilization of these trails has been relatively slow. Several recent controlled tests using a variety of vehicles suggest that long-term impact of the vehicles on the terrain is a function of time of year, type of substrate, vegetation, soil moisture, ground-contact pressure, type of vehicle propulsion (i.e. tracks, air-cushion, etc.), and operator technique.