Not sure if it's the longest, but a long and winding road nonetheless
The Stelvio Pass is one of the highest Alpine Passes of Europe with its 2758 m. There are 48 hairpin turns on the northern side of the pass. The original road was built in 1820-1825 by the Austrian Empire to connect the former Austrian province of Lombardia with the rest of Austria, covering a climb of 1871 m. Since then, the route has changed very little. The 60 hairpin bends, 48 of them on the northern side numbered with stones, are a challenge to motorists.