Continuous flow intersections are a safe bet for motorists

Posted in: Transportation

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The up-and-coming continuous flow intersection is slowly making its way through the transportation design community. Also known as a crossover displaced left-turn, this design is said to decrease mainline traffic congestion and increase motorist safety with its unconventional approach to left-turning traffic.

Technically speaking, a continuous flow intersection is an at-grade, high-capacity intersection design that diverts left-turning vehicles from crossing the main intersection. Its design is more technical than the description lets on: a CFI begins with a signalized four-way intersection, but left-turning vehicles make their turn a few hundred feet before the main intersection. This segment is called the “left-turn cross-over.”

The intersection also uses two-phased traffic signals at the main intersection and each cross-over point to create safe, unimpeded flow for travelers. When left-turning north-south drivers are crossing traffic, east-west motorists at the intersection can drive straight through the main intersection. Once the traffic signal phases switch, north-south motorists at the main intersection can drive through without the possibility of colliding with left-turn drivers. At the same time, the east-west left-turn drivers are safely crossing traffic hundreds of feet away from the intersection.

No turns are permitted in the intersection. All turns take place outside of the main traffic flow, and because of this restriction, CFIs reduce conflict points from 32 points at conventional intersections to 28 at a full CFI. Four less conflict points reduces the likelihood of accidents, which translates into significant safety improvements.

CFIs also help heavily congested intersections keep traffic moving, which is why they are called “continuous flow” intersections. While there aren’t enough of these intersections constructed to create a comprehensive, definitive study, independent research from recently-built intersections show increased throughput compared to conventional intersections. In a technical brief released by the Federal Highway Administration, this increase can reach up to 30 percent.

There aren’t many CFIs in the United States – less than 30 – but CFIs gained traction quickly within a short timeframe. There is a high cost associated with these intersections, but the cost is comparable to other more popular unconventional intersections that help with congestion. Additionally, there is a need for additional right-of-way. But the cost for acquisition would be similar to an at-grade intersection option.

For towns and cities with large, high-traffic intersections and limited space to expand, constructing a CFI may lower the probability for crashes while increasing mainline traffic flow. These new intersections are an interesting concept for traffic management, and their results look promising. I’m excited to see them gain traction across the United States.

Deb Weaver, PE

About the Author

Deb Weaver, P.E., has an in-depth knowledge of traffic regulations and the challenges facing communities. Specifically, she brings a wealth of expertise in traffic studies and safety, traffic signal design, and traffic analysis and modeling. Along her 30-year career, she has developed a deep commitment to traffic safety and a broad experience working collaboratively.

Read more posts by Deb Weaver, PE

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