Innovative Interchanges

Connecting travel pathways from large to small and coast to coast, interchanges are the critical junctures that point us in the directions we need to go.

Every road journey is a series of connecting dots, each with varying distances between them, strung together, reaching toward one’s destination. These dots represent interchanges, the pivot points that rightly steer our course, seamlessly joining one path to another as we make our way down the road.

These junctures, which we often pass through so routinely we barely notice, seem largely unfantastic when we’re in them. But from a bird’s-eye view, the interlacing ramps and roadways, stacked one upon another, look incredibly fantastic. So much so that you sometimes quietly think, “Wow, I’ve been on that!”

From simple interchanges to complex “spaghetti bowls” or system-to-system interchanges, transportation networks in cities and rural areas across the nation are quite impressive to see. But more impressive is the incredible amount of engineering used to create them. These sophisticated structures present a multitude of considerations in every direction, and every detail is critical — from geometry to speed to vehicle clearance and so on, everything must be of precise design.

Whatever form interchanges eventually take, they all share the same purpose: to distribute traffic flows in the directions they need to go, and to do so safely and efficiently.

Interchange Design & Specifications

Many factors contribute to the design and function of interchanges, starting with the number of interconnecting roadways at a given juncture and the complexity those interchanges present. Sometimes, different types of roads are joined, and considerations of speed and grade come into play. Adding to these, engineers must also consider volume of traffic, type of vehicles, impact to surrounding human-made facilities or to the natural environment, and the amount of space available. After all these challenges are resolved, then comes the structure’s geometric configuration and the numerous calculations that inform its ultimate design.

There are many types of interchanges throughout the U.S. — steadily growing in number with the advent of their modified counterparts. But hitherto recent years, a handful of designs served as the standards: cloverleaf interchange (CI), partial cloverleaf interchange, T interchange, roundabout interchange, single-point urban interchange (SPUI), diamond interchange (DI), and diverging diamond interchange (DDI).

Each of these designs have their suitable applications, but with traffic volumes increasing, land availability decreasing, and constant pressure on Departments of Transportation (DOTs) to find more adaptive solutions, innovations are becoming increasingly necessary to help interchanges more fully execute their purpose.

More and more, a tide is turning in the design of interchanges, where usage and circumstance are now the primary drivers, as opposed to replication of traditional industry standards. Instead of drawing upon historically executed designs, engineers and DOTs are now posing the question, “What is the most practical design for this particular space, and what will be most beneficial to this interchange’s users?”

Enter: Modified Interchanges

A return to the cardinal rule of design, that form follows function, modified interchanges are becoming more and more prevalent, because they execute their functions so well. Derivatives of traditional interchange designs, modified structures are created with a forward-looking approach, dictated by the area’s specific needs, and balanced by its unique parameters and limitations. Modified interchanges can be thought of as “made to order,” with the objective of maximizing value and functionality.

All interchanges sort traffic and facilitate movement, but the magic of modified interchanges enters when traffic flow is strategically arranged or accommodated in non-traditional ways, based on what that traffic is and where it’s going. In other words, traditional interchanges facilitate movement that’s already happening, but modified interchanges are more innovative or “smarter,” so to speak, incorporating relevant variables into their design, then adding elements to accommodate whatever is needed most. These variables can be related to landscape, traffic trends, volume or specific location, and solutions can include things like added ramps, lanes, or signals, all the way up to large-scale reconfigurations. In short, modified interchanges augment tried-and-true designs and make them much better, while achieving a high level of design standards, as mandated by state and federal agencies.

Modified Making Sense

Because of their enhanced efficiency and practicality, modified interchange projects are rising in popularity with their innovative approach to meeting users’ needs. As DOTs have embraced this approach, Horrocks has been afforded some valuable project experience in recent years.

Among the firm’s most compelling redesigns is the I-15/US-93 Garnet Modified DDI, north of Las Vegas, NV. A high volume of truck traffic prompted the effort, where an almost-steady stream of traffic used this interchange to access US-93, Nevada’s busiest truck stop, and a nearby landfill. Building on the existing DI, where traffic is crossed to the opposite side of the roadway to allow uninhibited left and right turns, several free-flowing ramps were added to enhance efficiency, allowing trucks to enter and exit I-15 without stopping. The redesign also included multiple bridge replacements and access improvements, ending in significantly reduced travel times in this heavily used transport corridor.

Additional free-flowing ramps make the Garnet DDI a welcome sight for truck traffic.

In similar fashion, Horrocks led the redesign of a DI, located in St. George, UT. In what is perhaps the busiest interchange in the area, modifications were made to create a DDI to better accommodate heavy ramp traffic to nearby shopping malls, the city’s main thoroughfare, and Utah Tech University. This award-winning project was a critical upgrade for the area, made doubly impressive by its narrow footprint and extremely high traffic activity.

Limiting stops and signaling greatly enhances traffic flow in this busy St. George interchange.

In some circumstances, a complete redesign of an interchange is favored over upgrading certain features. Horrocks successfully converted the I-15 / West Mesquite DI to a free-flowing roundabout interchange, improving ramps and widening roadways to increase access and improve safety and traffic flow. Also of note, this project employed an Accelerated Bridge Construction (ABC) alternative delivery method, sliding two prefabricated I-15 bridges into place in just a few days, significantly reducing traffic disruption times.

The West Mesquite roundabout interchange keeps traffic on the move, both to and from I-15.

An interchange’s location can sometimes impose limitations on what designs can or cannot be used, and it can also require modifications to the shape or function of a design simply to make it fit in the space available.

Such was the case for a planned SPUI, also in St. George, UT, that Horrocks opted to modify to better integrate the structure with the landscape. The traditional SPUI design was altered to an hourglass shape, more effectively accommodating geometric and terrain considerations. The new design shifted ramps and bridges to span the Virgin River and an adjacent pedestrian underpass. Additionally, the northbound entrance ramp was altered to provide more comfortable space between it and the nearby Bluff Street interchange.

Navigating multiple obstacles in a tight corridor make this interchange uniquely functional.

It's a Mod, Mod World

As cities grow and transportation infrastructure continues to expand, the need for custom, innovative solutions becomes more and more imperative. More drivers on the road means interchange designs must continuously adapt, while always maintaining safety and efficiency. Additionally, constant competition for land and resources means engineers must continually find ways to circumvent and overcome challenges that will surely continue to arise.

Learn more about Horrocks and its full spectrum of civil design services, in addition to our focused efforts in Transportation Engineering.