Slumgullion Pass is a high mountain pass at an elevation of 3.514m (11,530ft) above the sea level, located in San Juan Mountains, in Colorado, USA. It claims the steepest grades of any fully paved and maintained road in Colorado.
The road over the pass is asphalted. It’s called Colorado State Highway 149, also known as Silver Thread Scenic Byway. The north side has the steepest grade of any continuously paved road in Colorado (9%). It has a few switchbacks and tight spots, but other than that, most travelers will find it an easy, scenic route. It's one of the highest mountain roads of Colorado.
This narrow two lane road road is usually open all year because snowplows clear the route regularly during this season. To reach the summit, starting from Lake City, the ascent is 11.3 km long. Over this distance, the elevation gain is 783 meters. The average percentage is 6.9 %. The climb starts out immediately with near 10% grades and other than false flat in the middle for around 1K the grade never really lets up.