Though US 101 remains a major coastal north–south link along the Pacific coast north of San Francisco, it has been replaced in overall importance for transport through the West Coast states by Interstate 5 (I-5), which is more modern in its physical design, goes through more major cities, and has more direct routing due to significantly easier geography over much of the route. US 101 is a major parallel route between Los Angeles and San Francisco, and is an alternative to the Interstate for most of its length. In 1964, California truncated US 101's southern terminus in Los Angeles, as I-5 replaced it. The old road is known as County Route S21 (CR S21) or Historic Route 101 in northern San Diego County.