The Arroyo Seco Parkway, also known as the Pasadena Freeway, is one of the oldest freeways built in the United States. It connects Los Angeles with Pasadena alongside the Arroyo Seco seasonal river. It is notable not only for being an early freeway, mostly opened in 1940, but for representing the transitional phase between early parkways and modern freeways. It conformed to modern standards when it was built, but is now regarded as a narrow, outdated roadway. A 1953 extension brought the south end to the Four Level Interchange in downtown Los Angeles and a connection with the rest of the freeway system.