A rut is a depression or groove worn into a road or path by the travel of wheels or skis. Ruts can be formed by wear, as from studded snow tires common in cold climate areas, or they can form through the deformation of the asphalt concrete pavement or subbase material. Rut-like depressions can be formed on gravel roads by the erosion from flowing water.
Ruts prevent rainwater from flowing to the side of the road into ditches or gutters. Rainwater trapped in ruts is a common contributing factor to hydroplaning crashes. Severe ruts can impede steering if a vehicle has difficulty steering out of the rut. If it proves impossible to steer out of a rut, though forward and backward progress can be made by the vehicle, it is referred to as being stuck in the rut.
Ruts in gravel roads can be removed by grading the road surface. Ruts in asphalt pavement can be filled with asphalt, then overlaid with another layer of asphalt, but better results can usually be achieved by grinding off the surface to restore the proper cross slope, then resurfacing. If the ruts are formed due to deformation of the subbase below the pavement, the only long-term repair is usually full-depth reconstruction of the road.
Typically rutting is reported in terms of rut depth. Rutting is measured at highway speeds with a laser/inertial profilograph.
The term stuck in a rut can be used figuratively to refer to a situation in which, as time progresses, the situation is unable to be changed or steered in a desired way.
Rutting can also be intentional. The ancient Assyrians, Babylonians, Persians and Greeks constructed roads with artificial wheel-ruts deliberately cut into rock. The ruts were spaced apart from each other the same distance as the wheelspan of an ordinary carriage, and thus constituted grooves that guided the carriages on the rutway. Such ancient stone rutways connected major cities with sacred sites, such as Athens to Eleusis, Sparta to Ayklia, or Elis to Olympia. The gauge of these stone grooves was 138 to 144 cm (4 ft 6 in to 4 ft 9 in). The largest number of preserved stone trackways, over 150, are found on Malta.
Some of these ancient stone rutways were very ambitious. Around 600 BC the citizens of ancient Corinth constructed the Diolkos, which some consider the world's first railway. The Diolkos was a hard poros limestone road with grooved tracks (wheel-ruts), along which large wooden flatbed cars carrying entire ships and their cargo were pulled by slaves or draft animals. The grooves were at 1.67 m (5 ft 6 in) centres.