Hard shoulder

A shoulder, or hard shoulder is an emergency stopping lane by the verge of a road or motorway, on the right in countries which drive on the right, or on the left side in Japan, the UK, Australia, and other left-side driving countries. Many wider U.S. as well as Swedish freeways have shoulders on both sides of each directional carriageway, in the median as well as at the outer edges of the road, for additional safety. Shoulders are not intended for use by through traffic, although there are exceptions (see below).

Shoulders have multiple uses, including:

The shoulder is usually slightly narrower than a full traffic lane. In some cases, particularly on older rural roadways, shoulders that initially existed were hardened with gravel rather than being paved with asphalt or concrete. In Britain, motorway shoulders are now paved, but are still known as 'hard shoulders.' Older, gravel shoulders have sometimes been termed soft shoulders by comparison. Because the paved surface ends at that point, they are less safe if they need to be used for emergency maneuvers. Notably, the section of Ontario Highway 401 between Windsor and London had soft shoulders with a sharp slope which was blamed for facilitating vehicle rollovers, if drivers accidentally drifted off the paved section of the road and then overreacted after hitting the gravel. Modern practice is to build a continuous paved shoulder whenever possible.

The U.S. Federal Highway Administration encourages the placement of a Safety Edge — a 30° compacted taper on the end of the pavement dropoff — to ensure that any driver running off the edge of the roadway is better able to maintain control while trying to steer back onto the pavement. The Safety Edge is effective on roads where the shoulder is narrow or nonexistent.

To save money, the shoulder was often not paved to the same thickness as the through lanes, so if vehicles were to attempt to use it as a through lane regularly, it would rapidly deteriorate. In Britain, shoulder running can occur during roadworks, and full depth construction is now standard. In some metro areas, road authorities also allow shoulders to be used as lanes at peak periods. However, rural shoulders often collects various bits of road debris that can make driving there less safe.

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Wikipedia