A12 autoroute

Autoroute A12, formerly known as the Autoroute de Bretagne (English: Motorway of Brittany), is a French autoroute located in Yvelines, near Paris. It branches off from A13 (also known as the Motorway of Normandy) at an exchange known as the Triangle de Roquencourt located in the town of Bailly and merges with RN12 and RN10 at Bois-d'Arcy. Its total length is 8.5 km (5.3 mi). Access is free.

It has not been lengthened since its construction, which makes it the oldest French motorway to have remained on its original route.

Its history started in 1934 with the publication of Plan Prost, which envisaged a network, already dense, of motorways in Île-de-France. The A12 was then known as the "Autoroute de Bretagne", which is a reference to its proximity to the nearby RN12 route which extends to Brittany. Its construction began in 1936 with the first deforestation on the site of the Triangle de Rocquencourt. Work ceased at the outbreak of war in 1939, with only earthworks having been executed. Work began again in the post-war period and lasted until 1950.