Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.
For many of our studies, stop signs, traffic signals, and roundabouts supply the necessary capacity and acceptable operations. Sometimes, though, we handle projects that require innovative thinking and lead us toward consideration of alternative intersections. Alternative in this case being defined as not traditional (the aforementioned stop sign, traffic signal, or roundabout) in terms of geometry, traffic control, or both.
The key to using alternative intersections, or conventional ones for that matter, is understanding the basic issues trying to be solved.
- Do you have a capacity problem, with the whole intersection or with specific movements?
- Are there a lack of gaps for turning traffic?
- Is there a safety issue at the intersection or on an approach?
- How’s the sight distance on each approach and lane?
- Do you have right-of-way limitations?
Once you understand the basic issues and concerns, you can then determine whether a conventional intersection will provide the answer or if you need to explore alternative intersections. Here are some those alternatives that we have considered for our projects and sometimes recommended:
- Access Management (right-in/right-out only or 3/4-access). Restricting one or more movements, either permanently or time-based.
- Offset “T” Intersection. Separating a four-legged intersection into two “T” intersections.
- Indirect Left Turns. Shifting the left turn movements on one or more approaches to a location several hundred feet away from the primary intersection.
- Split Intersection. Separating the main road traffic directions to create a pair of one-way intersections, similar to interchange intersections although without a freeway.
- Continuous Flow Intersection or Displaced Left-Turn. Removing the left-turning movements from the primary intersection and directing them to a separate roadway running parallel to the main lines.
- Echelon Intersection. Splits the intersection into two levels, creating a pair of one-way street intersections essentially on top of each other.
- Hamburger or Through-About Intersection. A variation of a signalized roundabout with the mainline through movements allowed straight through the intersection and the side-street/turning movements occurring on a circulatory roadway around the intersection. (Image is of a Hamburger Intersection in Fairfax, VA)
- Synchronized Split-Phasing or Double-Crossover Intersection. The through and left turn movements on the mainline cross over to the opposite site before the primary intersection, similar to a Diverging Diamond Interchange.
A simple internet search will provide lots more detailed information on these and other types of designs to accommodate traffic. As mentioned, always keep in mind that your solution(s) should be trying to solve the problem. There’s no need to be fancy if it’s not needed.