We have the good fortune (or misfortune depending upon your point of view) of handling a lot of projects that require innovative thinking. The low hanging fruit of solutions have already been picked and it’s up to us to review the information and come up with new ideas. Most of these projects involve a roadway junctions, which leads us to consider alternative intersections.
We define alternative in comparison to conventional intersections. Side-street or all-way stop, traffic signal, and roundabout control all generally fall into this category of conventional intersections for our purposes (roundabouts are borderline depending upon the community and their prior use of roundabouts). That leaves many ways to control traffic, including:
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. 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.