I-5 takes a neat course as it climbs out of the Los Angeles metro area. After passing the Santa Clarita area and the unincorporated hamlet of Castaic, the northbound side of the highway follows the old routing of the pre-interstate 4 lane US 99. This grade follows the west side of the Marple Canyon about 2/3rds of the way up the hill crossing over a ridge to the Violin Canyon for the rest of the trip to the 'Violin Summit' (top edge of the map). That this grade was somewhat steep (contour interval is 40 feet/12 meters) is seen by the remnants of a 'runaway truck ramp' just below the '1600 Feet/500 meter' line.
When I-5 came onto the scene, the existing US 99 grade was too narrow and steep to be practical for the full interstate highway. CalTrans instead decided to use the existing grade for the uphill (northbound) side of the new I-5 while constructing a new, far gentler grade for the downhill (southbound) side on the east side of the Marple Canyon. The interesting thing is that this created a 'retrograde' section of the new I-5 that is just over 7 km long (measured southbound). The two roadways are over 600 meters apart for about two kilometers on opposite side of the canyon during their ascent of that grade.
A neat entrance indeed to one of the world's great urban areas.
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Just follow this sign to make your OWN great signs!!!! |
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This page © Michael G. Koerner |
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