October 20th, 2011
DDI AND APPLE PIEOptimize Mobility, by Catherine Higgins.
UDOT’s new Diverging Diamond Interchange is sweet when it comes to mobility and safety.
A new DDI at Bangerter Highway and State Route 201 is in operation, and drivers should notice less traffic delay right away. The intersection has safety improvements too. A DDI is more efficient and safer than a regular intersection because of fewer signal phases and conflict points.
When it comes to efficiency, understanding traffic signal cycles and phases within those cycles is the key. “Traffic signal timing is like a yummy apple pie – think of the entire pie as the total green time for the intersection in all directions,” says Taylor. Fewer phases within a cycle are comparable to getting a bigger piece of pie.
At a regular intersection, signals need phases to allow for left turns so “we have to divide up the pieces of the pie into 4 pieces,” explains Taylor. Those “pieces” or phases include the primary street through traffic, primary street left turns, secondary street through traffic and secondary street left turns.
On some busy city streets, no one would be able to turn left without protected left-turn phases. Adding phases, like protected left turns, makes the cycle length longer. For good traffic progression, signal operators try to make the cycle length just right – not too short, not too long. A too-short cycle can create delay when too few cars can pass through. Long cycles reduce traffic speed and can make progression sluggish.
“DDIs are more efficient because of fewer phases,” explains Taylor. Eliminating left turns mean that the green time that you would normally give to the left turn can be given to other movements.” A DDI eliminates left turns on the primary street movement, making the pie pieces bigger for the other competing movements.
Taylor adds that left turns are necessary but generally inefficient. If the capacity volume of a left turn is low compared to the through movements, any green time that can be taken from a left turn movement adds to the overall efficiency of the intersection. “Slicing the pie piece larger for the through movements is much more efficient. The left turn equivalency factor is commonly between 2 and 3, meaning that a left turner looks like 2 to 3 through passenger cars.”
DDI’s are safer because of fewer conflict points. A conflict point occurs when two cars have the potential of being in the same place at the same time if every direction had a green light. According to Taylor, “a diamond interchange contains 30 conflict points; a Single Point Urban Interchange has 24; a DDI has only 18. Fewer conflict points are safer.”