State Highway 45 is a freeway loop around Austin, Texas, that exists in two open segments. The official designation of SH 45 is such to form a complete loop around Austin, a distance of roughly 80 miles (130 km).

Loop 1 is a freeway which provides access to the west side of Austin in the U.S. state of Texas. It is named Mopac Expressway after the Missouri Pacific Railroad. Local residents almost always use the name "MoPac" rather than calling the road by its number, which can cause much confusion, for few signs along the road use this name.