From Castaic Junction north to Castaic the Ridge Route has been largely buried by the Ridge Route Alternate and Interstate 5. At Castaic the Ridge Route Alternate turned northwest from the old road, now the intersection of Castaic Road and Neely Street. The first piece of Ridge Route Road out of Castaic was realigned as recently as the late 1990s when the North Lake housing development was built. The road begins to climb after passing North Lake; a portion was straightened in 1924 and now is next to the southbound lanes of I-5. In this area, known as the Five-Mile Grade, the four-lane Ridge Route Alternate became the northbound lanes of I-5, while the added four-lane alignment, built to the east (next to the old Ridge Route), had lower grades and became the southbound (downhill) lanes to cut down on runaway trucks. Two bridges were built to allow traffic to cross to the left side. Near the north end of this area, the Ridge Route curves away from the newer bypass. The road enters the Angeles National Forest about south of Templin Highway, with the Forest Service road designation 8N04.