On September 22, 1953, the first four-level (or “stack”) interchange in the world opens in Los Angeles, California, at the intersection of the Harbor, Hollywood, Pasadena, and Santa Ana freeways. It was, as The Saturday Evening Post wrote, “a mad motorist’s dream”: 32 lanes of traffic weaving in eight directions at once. Today, although the four-level is justly celebrated as a civil engineering landmark, the interchange is complicated, frequently congested, and sometimes downright terrifying. (As its detractors are fond of pointing out, it’s probably no coincidence that this highway octopus straddles not only a fetid sulfur spring but also the former site of the town gallows.)
Before the L.A. four-level was built, American highway interchanges typically took the form of a cloverleaf, with four circular ramps designed to let motorists merge from one road to another without braking. But cloverleafs were dangerous, because people merging onto the highway and people merging off of the highway had to jockey for space in the same lane. Four-level interchanges, by contrast, eliminate this looping cross-traffic by stacking long arcs and straightaways on top of one another. As a result, each of their merges only goes in one direction—which means, at least in theory, that they are safer and more efficient.
