Die Bundesautobahn 81 (A 81) is a motorway in Germany, branching off the A 3 at the Würzburg-West triangle and ends near the border with Switzerland. Its oldest part between the Weinsberg intersection (A 6) near Heilbronn and the Leonberg triangle (A 8) near Stuttgart was finished in the years 1938 to 1940. This section included the first tunnel built for an Autobahn, the 300 m long Engelberg tunnel. When Weinsberg-Leonberg was upgraded to 3+3 lanes in the 1970s, the tunnel with its 2 lanes each and steep grades (up to 6%) became something of a bottleneck. In 1999, the new 2,530 m long Engelberg tunnel was opened.