interstate 405 (i-405) is an interstate highway in the orange and los angeles counties of california. it is a spur route of the interstate 5. it goes from the i-5 near san fernando, through the west side of los angeles and long beach before ending back at the interstate 5 near irvine. it is the busiest highway in the united states, mainly because it is the only highway in the west side of los angeles that goes from north to south. it also passes important places such as los angeles international airport and the ports of los angeles and long beach.