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Adirt road ordirt track is a type ofunpaved road not paved with asphalt, concrete, brick, or stone;[1] made from the native material of the land surface through which it passes, known to highway engineers assubgrade material.[citation needed]
Terms similar todirt road aredry-weather road,earth road, or the "Class Four Highway" designation used inChina. Atrack,dirt track, orearth track would normally be similar but less suitable for larger vehicles—the distinction is not well-defined.Laterite and murram roads, depending on material used, may be dirt roads or improved roads.[citation needed]
Unpaved roads with a harder surface made by the addition of material such asgravel andaggregate (stones), might be referred to as dirt roads incommon usage but are distinguished asimproved roads by highway engineers. Improved unpaved roads includegravel roads andmacadamized roads.[citation needed]
Compared to a gravel road, a dirt road is not usuallygraded regularly to produce an enhanced camber to encourage rainwater to drain off the road, and drainage ditches at the sides may be absent. They are unlikely to haveembankments through low-lying areas. This leads to greater waterlogging and erosion, and after heavy rain the road may be impassable even tooff-road vehicles. For this reason, in some countries, such asAustralia andNew Zealand andFinland, they are known asdry-weather roads.[citation needed]
Dirt roads take on different characteristics according to thesoils andgeology where they pass, and may be sandy, stony, rocky or have a bare earth surface, which could be extremely muddy and slippery when wet, and baked hard when dry. They are likely to become impassable after rain. They are common inrural areas of many countries, often very narrow and infrequently used, and are also found inmetropolitan areas of many developing countries, where they may also be used as major highways and have considerable width.[citation needed]
Dirt roads almost always form a washboard-like surface with ridges. The reason for this is that dirt roads have tiny irregularities; a wheel hitting a bump pushes it forward, making it bigger, while a wheel pushing over a bump pushes dirt into the next bump. However, the surface can remain flat for velocities less than 5 mph (8 km/h).[2]
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