State Route 99 ( commonly known as Old Highway 99 or just 99 , also future designated as Interstate 7 or Interstate 9 ) is a state highway in California , in the United States . California State Route 99 ( SR 99 ) , commonly known as Highway 99 or 99 , is a north -- south state highway in the U.S. state of California , stretching almost the entire length of the Central Valley .