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.
The motorway has not undergone any significant changes since that time. The changes that have occurred include the covering of a section at the School of Saint Cyr and the widening of the roadway to 4 lanes in one direction and 3 in the other. It is on this motorway that one of the first automatic radars in Yvelines were installed.
There are plans to extend the A12 in the medium-term (2015–2020) up to Ablis and the A11. It is anticipated that the route will go around the town of Trappes. Beyond that point, the existing highway N10 will be reused.
This project, started back up in 2004 by the Transport Minister Gilles de Robien, caused significant concern among locals, such as the inhabitants of South of Yvelines. They object as they feel that this new section of highway will introduce transit of heavy lorries between the A10/A11 motorways and the western A86 tunnel (Rocquencourt), with traffic predictions as high as 100,000 vehicles per day. Others object to the extension crossing the Haute Vallée de Chevreuse Regional Park. A public consultation is expected to be organised in 2006.