A four level interchange, also known as a "stack" was first built in 1954 in Los Angeles as the interchange for US 101 and the Pasadena Freeway. It is a nice choice for two intersecting freeways. Both freeways have a direct connection to the other one, with no looping or weaving, and the ramps cross in a 4-level deck you can see for about a mile. If the ramps are two lanes wide, the interchange has quite high capacity and drivers with good tires probably won't even have to slow down. What's more, they're easy to navigate.

They aren't used as much, most likely because of the geometry, materials cost, and local opposition. To raise a ramp 60 feet or more requires a lot of concrete, or fill, or both, and sometimes conflict with nearby interchanges on the freeway as there might be no room to fit a stack between adjacent interchanges. Then there is the local opposition since towering interchanges can bring on resentment from having a freeway's shadow receding across the front lawn every afternoon.



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