Stack interchange

A stack interchange is a highly efficient type of directional interchange. It is a grade-separated junction used to connect two controlled-access highways.

This design allows traffic to flow freely in all directions without stopping. Because the roads are separated by different levels, drivers do not have to weave between lanes. This system provides the highest vehicle capacity of any junction type. It also allows vehicles to travel shorter distances than other interchange designs.

The mechanism of a stack interchange relies on multiple layers of road surfaces. In a standard four-level stack, one major freeway crosses another using a viaduct. A viaduct is a long bridge-like structure that carries the road over another.

To connect the different highways, engineers build connector flyover ramps. These ramps exist on two additional levels to prevent roads from crossing at the same height. Some of these ramps are direct, meaning they are short and allow for high speeds. Other ramps are semi-direct, which causes the driver to deviate more from their intended direction.

There are several specific types of stack configurations used by engineers. The most common is the four-level stack. This setup creates two "inverse" dual-carriageways. In these sections, the turn ramps cross the middle so that traffic drives on the opposite side of oncoming traffic. Some engineers build three-level stacks instead. This happens when semi-directional ramps are spaced far enough apart that they do not need to cross at a single point. Other designs are hybrids. These combine a stack with a cloverleaf, using a 270-degree loop ramp for the least-used left turns.

The history of these massive structures began in the mid-20th century. The first directional interchange in the world was the Four Level Interchange. It opened to traffic in Los Angeles, California, in 1949.

This junction connects US Route 101 and State Route 110. It was later renamed the Bill Keene Memorial Interchange. In 1958, the first four-level stack in Texas was completed in Fort Worth. This junction at I-35W and I-30 cost $1,220,000 to build. Local residents gave it nicknames like "The Pretzel" or the "Mixmaster."

Stack interchanges are significant because they manage massive amounts of movement. In South Africa, the EB Cloete Interchange was chosen to handle high volumes of traffic. The N3 highway is the busiest in the country and serves as a major truck route. Similarly, the Mount Edgecombe Interchange was upgraded to a four-level stack in 2018. This change was necessary to handle increasing traffic in the uMhlanga and Mount Edgecombe areas. In the United States, the Springfield Interchange was rebuilt into a four-level stack. This was required because the original configuration could not handle the rerouting of I-95.

Many unique examples of these interchanges exist across the globe. In Canada, the interchange at Highway 400 in Vaughan, Ontario, is the only true four-level stack. In the Southern Hemisphere, the Light Horse Interchange in Sydney, Australia, is the largest.

Some stacks are even more complex, reaching five or six levels. The High Five Interchange in Dallas, Texas, is currently the tallest interchange in the world. In Shanghai, China, the Yan'an East Road Interchange reaches six levels.

This massive structure includes a central pillar known as the Nine-Dragon Pillar.

These engineering feats connect to broader systems of urban planning and logistics. Large stacks like the one in Doha, Qatar, help provide access to international airports. In Texas, five-level stacks often include frontage roads. These roads act like a grid of one-way streets that connect to nearby areas. By using multiple levels, engineers can manage the intersection of major interstates with local traffic. This complexity ensures that even the busiest cities can maintain constant movement across their highway networks.