Q: Context: The Arroyo Seco Parkway, formerly known as the Pasadena Freeway, is the first freeway in the Western United States. It connects Los Angeles with Pasadena alongside the Arroyo Seco seasonal river. It is notable not only for being the first, 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 It conformed to modern standards when it was built, but is now regarded as a narrow, outdated roadway.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.

Question: when was the first freeway built in los angeles?
Let's give stream of consciousness below
The relevant information to answer the above question is: It is notable not only for being the first, mostly opened in 1940, but for representing the transitional phase between early parkways and modern freeways. So the answer is 1940.