From Minnesota to California to England to China, let's explore the diverse world of interchange designs.
The interchange has long been the main way to move travelers from one roadway to another. The concept was first patented in 1912. Since then, as more and more people use our roads, increasingly complex systems have been invented to accommodate them.
To get a better understanding of this important transportation infrastructure, we connected with SEH interchange designer Scott Hotchkin, PE, who shared his insight on a variety of different service and system interchange designs.
We’ll start with the basics.
Before setting out, we should explain that there are, according to Hotchkin, two main types of interchanges: service interchanges and system interchanges.
Service interchanges exist between a freeway or controlled access facility (typically high-speed, high-traffic roadways) and a lower-class roadway such as an arterial or collector (lower speed roadways). System Interchanges exist between two or more freeways or controlled access facilities.
Let’s go.
The diamond interchange, and its iterations, are the prevailing service interchange design. We’ll begin in Minnesota with examples of each.
A diamond interchange involves four ramps, exiting and entering the highway. These designs are very economical because, compared to other options, they require less land and materials. Some of the first diamond interchanges were developed along the Pasadena Freeway in Los Angeles in 1941.
A tight diamond has the same general form as the conventional diamond. But, as the name suggests, the spacing of the design is tighter. The spacing between the two at-grade intersections (intersections directing travelers either onto or off of the arterial) is usually between 250 and 400 feet, says Hotchkin.
The single point diamond interchange is also known as a single point urban interchange (SPUI). As you can see, SPUIs have only one at-grade intersection on the minor road. However, according to Hotchkin, they are more expensive than traditional interchange options due to the need for a longer, wider bridge. The first SPUI sprang up in 1974 in Clearwater, Florida.
The double crossover diamond (DCD), or diverging diamond interchange (DDI), was first developed in France and brought to North America around 2002. It is the latest new interchange form developed in the past 30 years. Because of this, it comes with a learning curve for many drivers. The setup is initially confusing because drivers have to drive on the opposite side of the road as the roadways diverge. But, once learned, this design yields significant safety improvements for both drivers and pedestrians, says Hotchkin. Click here to see a visualization of how a DDI works.
Partial cloverleaf interchanges, also called “parclos,” use one, two or three loops for left turn movements. Parclos are highly adaptable and can accommodate high traffic volumes. According to Hotchkin, they are especially advantageous when one or more corners of the interchange must be avoided due to right of way or environmental restrictions.
A system interchange controls traffic where two freeways meet. Unlike the service interchanges we've seen so far, which exist at the intersection of a high-speed, high-traffic freeway and a less busy, lower traffic collector road. We’ll begin with more common examples before moving on to highly uncommon system interchanges across the world.
The cloverleaf design eliminates the need for traffic signals and keeps motorists moving. However, weaving is a problem that may lead to a breakdown in traffic operation and more accidents. The cloverleaf was the first interchange design constructed in the United States. It was built in Woodbridge, New Jersey in 1928.
The directional interchange design often requires less right of way than a cloverleaf design. The primary disadvantage is increased cost because of the need for multiple-level structures. Directional interchanges are often warranted in certain urban areas where traffic volumes are very high and high-speed maneuvering is desired.
The stack interchange design eliminates the need for looping and weaving, making for easier transitions. However, they tend to be costly and take up a lot of land to implement. The first four-level stack interchange was built in Los Angeles, around 1952.
Now that we’ve taken a look at more common examples of system interchanges, we’ll fly around the world for one-of-a-kind examples.
This three-level roundabout just outside Leeds, England, is perhaps most famous for how poorly it works, causing standstills during rush hour. The interchange was built in the 1970s, and has been unable to sustain traffic loads that pass through the area.
This stack interchange is considered a “complete interchange” because it allows drivers to exit in all possible directions. The interchange was featured in the movie “Speed,” where a bus jumped across an unfinished section of the freeway.
We end our tour at an interchange in Shanghai, China, that may make you feel like you’re on the world’s largest roller coaster. This bridge interchange spirals upward before providing an exit. These interchanges are often used on steep terrain, or when the approaching road terminates too far from the end of the bridge.
As we’ve seen, interchanges around the world are as varied as the challenges they solve. Different solutions are better in certain conditions. It’s through the ingenuity and vision of highway designers around the world that we're able to keep moving from one place to another.
Scott Hotchkin, PE, is a highway designer dedicated to safe and efficient interchange designs. Contact Scott
*Some images may contain unusual visual distortions resulting from Google's mapping software.
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