Controlled-access highway

A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway, and expressway. Other similar terms include throughway or thruway and parkway. Some of these may be limited-access highways, although this term can also refer to a class of highways with somewhat less isolation from other traffic.

In countries following the Vienna convention, the motorway qualification implies that walking and parking are forbidden.

A fully controlled-access highway provides an unhindered flow of traffic, with no traffic signals, intersections or property access. They are free of any at-grade crossings with other roads, railways, or pedestrian paths, which are instead carried by overpasses and underpasses. Entrances and exits to the highway are provided at interchanges by slip roads (ramps), which allow for speed changes between the highway and arterials and collector roads. On the controlled-access highway, opposing directions of travel are generally separated by a median strip or central reservation containing a traffic barrier or grass. Elimination of conflicts with other directions of traffic dramatically improves safety, while increasing traffic capacity and speed.

Controlled-access highways evolved during the first half of the 20th century. Italy was the first country in the world to connect cities with controlled-access highways reserved for fast traffic and for motor vehicles only. Italy opened its first autostrada in 1924, A8, connecting Milan to Varese. Germany since 1907 planned and in 1913 had started to built the AVUS in Berlin, but this 'Automobile traffic and training road' was a closed circuit with two long straights where vehicles could reach their top speed twice before returning back to the entrance. Opened in 1921 and mainly used for testing and racing for two decades, the AVUS was connected in 1940 to the nationwide network of Reichsautobahn roads that was proposed in the 1920s by various initiatives, delayed by the Great Depression and built in the 1930s with tax money. In 1932, the first controlled-access autobahn without speed limits was completed between the Western German cities of Cologne and Bonn. The 30 kilometres [19 mi] on what is now A555 was then referred to as a dual highway. With plans having been made earlier, Germany rapidly constructed the first nationwide system of such roads. The first North American freeways (known as parkways) opened in the New York City area in the 1920s. Britain, heavily influenced by the railways, did not build its first motorway, the Preston By-pass (M6), until 1958.

Most technologically advanced nations feature an extensive network of freeways or motorways to provide high-capacity urban travel, or high-speed rural travel, or both. Many have a national-level or even international-level (e.g. European E route) system of route numbering.

There are several international standards that give some definitions of words such as motorways, but there is no formal definition of the English language words such as freeway, motorway, and expressway, or of the equivalent words in other languages such as autoroute, Autobahn, autostrada, autocesta, autoput, that are accepted worldwide—in most cases these words are defined by local statute or design standards or regional international treaties. Descriptions that are widely used include:

One green or blue symbol (like or ) appears at motorway entry in countries that follow the Vienna Convention. Exits are marked with another symbol: or .

The definitions of "motorway" from the OECD and PIARC are almost identical.