The East Los Angeles Interchange complex is one of the busiest freeway interchanges in the United States, if not the world. At the time of its construction in the early 1960s it was considered a civil engineering marvel. Located about one mile east of downtown Los Angeles, California along the east 'bank' of the Los Angeles River, the interchange is comprised of six freeway segments (i.e. there are six freeway 'paths' of travel into the complex). Note that the actual number of numbered highways intersecting at this interchange is four - these freeways are:

• Santa Monica Freeway/San Bernardino Freeway (Interstate 10, I-10)
• Hollywood Freeway (U.S. Route 101, U.S. 101)
• Santa Ana Freeway/Golden State Freeway (Interstate 5, I-5)
• Pomona Freeway (California State Highway 60, CA/SR-60)

The primary reason why the complex is so 'complex' is that the intersecting freeways 'shift' alignments and directions. For example, Interstate 10 is not contiguous through the interchange; heading westbound into the complex on the San Bernardino Freeway (I-10), the primary road (or trunk) changes its number assignment and name to the Hollywood Freeway (U.S. 101). In order to follow the I-10 alignment, one must exit the trunk road and follow a connector, or transition road, that then connects to the Santa Monica Freeway (I-10); note also the change in the freeway's name.

It should also be noted that there is not complete freedom of movement within the interchange either. Traffic flowing into it on certain freeways cannot leave it on all of the others. For example, there is no direct connector between the westbound Pomona Freeway (CA/SR-60) and the southbound Santa Ana Freeway (I-5); travelers wanting to make this transition must exit at the Pomona Freeway's interchange with the Long Beach Freeway (CA/SR-710) located three miles to the east, head southbound, and then transition to the Santa Ana Freeway at the interchange between those two freeways. Naturally, travelers from outside the Los Angeles area may not know this and will find this confusing.

Further complication is added by the varying designs of each intersecting freeway and their related transition roads. Some have four lanes and are relatively straight and wide, while others have one lane, are narrow, and/or have curves with tighter radii or cambers. Thus, traffic congestion is exacerbated as vehicles on the newer freeways moving at high rates of speed 'transition' to the older freeways and encounter slower moving vehicles, or vice versa.