Cajon Pass marks the separation between the San Gabriel and San Bernardino Mountains, and was created by the San Andreas Fault. It is an ideal location, in terms of ease of access (Geographically it is horrible), for a highway. The pass is an important gateway to Southern California and the Inland Empire. It was first paved in 1916 and widened over the years. Before Interstate 15 passed through the area, the Cajon Pass was also signed as US Highway 395, 91 and 66. Interestingly, the multiple realignments of the route have dropped the summit of Cajon Pass several hundred feet over the years.