L.A. as Subject's most recent contribution to KCET.org looks at the 58-year history of the Four Level interchange, the junction of the 101 and 110 freeways near downtown Los Angeles:
Fifty-eight years ago today, the Four Level interchange first opened to traffic. This iconic concrete ribbon that binds the 101 and 110 freeways is an almost inescapable feature of many Southern Californians' commute. Admired by some and feared by others, the Four Level was—like many other highway innovations in Los Angeles—the first of its kind and destined to be copied elsewhere.
The Four Level, also known as the Stack, gets its name from its multi-tiered structure that separates traffic heading in each direction into dedicated lanes. On the bottom level are curved ramps for those changing from the 110 freeway to the 101. One level above is the main trunk of the 110 freeway, named the Arroyo Seco Parkway north of the interchange and the Harbor Freeway south of it. On the third level are the arcing flyover ramps carrying traffic from the 101 freeway to the 110. Finally, on the fourth and top level is the main trunk of the 101 freeway, named the Hollywood Freeway to the west and the Santa Ana Freeway to the east.
This design, now the basis of freeway interchanges around the world, was a marked improvement over the previous model. Older cloverleaf interchanges were less expensive and kept a lower profile, but they also tended to slow traffic and were more dangerous. They required motorists both entering and exiting a freeway to merge into one lane. (The 405 freeway's interchange with Wilshire Boulevard in Westwood is an example.) Stack interchanges, on the other hand, kept the eight directions of traffic separate until the final merge.