Many people don't understand the purpose of the diverging diamond interchange. Here are some questions and answers to
help you understand this better:
What is a diverging diamond interchange (DDI)?
This is a special form of the standard diamond interchange where the thru lanes of the cross street cross each other twice.
This solves some traffic problems a standard diamond interchange cannot handle very well.
What is a double crossover diamond (DCD)?
This is the same thing as a diverging diamond interchange. DDI was coined by the man who developed it in the US. DCD was
coined by the US government, but is not as well known as DDI.
How does this solve traffic problems?
- The driver turning left onto either freeway entrance ramp does not have to wait for oncoming traffic to clear before
can make his turn. Since he is on the left of the oncoming traffic, he can just turn into the ramp.
- The driver turning left from either freeway exit ramp does not have to wait for crossroad traffic from the left. He
turns at the same time they move.
- There is no green arrow on either traffic signal, with its separate traffic phase needing extra signal time.
- Left turns from opposite directions do not cross each other's paths the way they do on a standard diamond.
Do drivers need special instruction to use a DDI?
No. Just read the signs and follow the lane markings.
Several drivers have reported that they did not realize that they had just driven through a DDI, until after
they had passed through the interchange. Only then did they have that "what just happened?" moment.
What are the advantages of a DDI?
This new interchange provides the following advantages:
- Left turns from opposite directions do not cross each other's paths.
- The DDI handles large volumes of left turning vehicles without extra lanes for left turns.
- Left turns do not have to wait for oncoming vehicles to pass.
- Turns from the ramps can be signal controlled or yield controlled.
- The traffic signals are reduced to two-phase operation. No third phase is needed for left turns.
- Drivers will not be stopped by both signals unless they are turning left from an exit ramp.
- The existing bridge can be used to convert a standard diamond to a DDI.
- Pedestrian paths across the cross street lanes are protected. There are no permissive turns across them.
- Pedestrian paths across ramp roadways can be made either protected (if ramp lanes are signalized) or permissive
(if the ramps have yield signs).
- On or under the bridge, pedestrians can be routed either through the central median, or along the outside edges of
- The DDI nicely fits into the land footprint of the existing standard diamond it replaces. It is much smaller than
a parclo (partial cloverleaf) interchange.
- The DDI operates well at an unfavorable location in a signal progression system, where platoons alternate in arrival
times. Other signals are usually not suited for these locations.
- Because there are fewer stops, gasoline is saved.
What are the disadvantages of a DDI?
This new interchange has the following disadvantages:
- The DDI does not handle heavy straight ahead traffic in both directions simultaneously. The maximum capacity for
balanced operation is 900 vehicles per hour per lane.
- The DDI does not work well at the meeting point of platoons in a progressive system.
- A driver who took the wrong exit can not re-enter in the same direction without leaving the interchange and turning
around somewhere on the cross road.
- The DDI does not work well where frontage roads parallel the freeway, unless the frontage road intersections are
far from the interchange, or the frontage roads are grade-separated from the crossroad.
- The DDI cannot be used as the major interchange part of a rebound interchange without collector/distributor (C/D)
roads parallel to the freeway. It can be used on the extended ramp part.
- Liberal journalists can't see the benefits, and think the concept is dangerous. They unnecessarily frighten others
with their scary predictions.
Is this the same thing as a single point urban interchange (SPUI)?
No. It looks similar from the air, but there are several major differences:
- The bridge for a SPUI is either longer or wider, to make room for the ramps, so a SPUI is more expensive than a
standard diamond. The bridge for a DDI can be smaller than that for a standard diamond, because fewer lanes are needed
on the cross road, so a DDI is cheaper to build than a standard diamond.
- The SPUI has one large intersection in the center of the bridge (or under it). The DDI has two intersections of
one-way roads, one at each end of the interchange.
- The SPUI needs one 3-phase or 5-phase signal to control all of the left turns. The DDI needs two 2-phase signals.
A standard diamond usually needs two 4-phase signals.
- A driver who took the wrong exit at either a SPUI or a DDI can not re-enter the freeway in the same direction
without leaving the interchange and turning around somewhere else on the cross road.
- Neither kind of interchange works with frontage roads that merge with the ramps. But frontage roads can be closer
to the SPUI than they can be to the DDI.
- Neither kind of interchange can be used as the major interchange part of a rebound interchange unless the freeway
has C/D roads. Either can be used on the extended ramp part.
- The SPUI is better when the cross street has high straight ahead traffic. The DDI works better when left turns are
- The SPUI is best at the point where progression platoons from opposite directions arrive at the same time.
The DDI is best at the point where progression platoons from opposite directions alternate in arrival time.
What's wrong with the standard diamond?
- Left turns from opposite directions cross each other's paths twice on the standard diamond. This can be fixed
with a SPUI, a DDI, or a Contraflow Left Interchange (CFL).
- The standard diamond needs more lanes to achieve the capacity of either the SPUI or the DDI.
- The standard diamond causes more delay to traffic than either the SPUI or the DDI.
- The standard diamond causes more traffic to be backed up through other intersections than either the SPUI or
How is signal progression affected by each kind of interchange:
- A standard diamond with lagging left turns works best at platoon meeting points, but has low bandwidth.
- A standard diamond with leading left turns works best at platoon alternating points, but has low bandwidth.
- A standard diamond with special clockwise sequence, best at platoon alternating points, with high bandwidth.
- A SPUI (single point urban interchange) works best at platoon meeting points, with high bandwidth.
- A CFL (contraflow Left) with lagging left turns works best at platoon meeting points, with high bandwidth.
- A CFL (contraflow Left) with leading left turns works best at platoon alternating points, with medium bandwidth.
- A DDI (diverging diamond interchange) turns works best at platoon alternating points, with high bandwidth.
- A roundabout interchange destroys progression platoons at high traffic volumes.
Why do traffic engineers need to use a DDI?
For the following reasons:
- Reducing congestion
- Handling high left turn volumes
- Utilizing signal green time more effectively
- Not having to build a new overpass
How do I keep from ending up going the wrong way?
Obey the signs, signals, islands, and road markings.
- When the traffic control devices are obeyed, most drivers don't notice that anything is unusual at the time.
- The best advice is to forget what is in the newspaper about the interchange, and pay attention to the road.
- Also forget any habits from the old interchange. Most wrong-way incursions are the results of people turning at
traffic signals, the way they did at the old interchange.
Why are they wasting money fixing something that isn't broke?
It is broke. The intersection or interchange is overloaded. If it were not overloaded, there would be no project.
Have there been any problems with the diverging diamond interchange?
- Most of the problems actually have had to do with construction activities restricting traffic flow.
- A few incidents occurred where drivers misunderstood the paths designated by traffic cones, drums, and
barricades. In most of these cases, not enough devices were used, or the correct paths were not discernible at
night. In one case at night, the driver stopped because the entire roadscape ahead of him was reflecting bright
orange. He could not see any openings in the rows of orange cones because the correct path was curved. There
were no arrow signs to guide the driver.
- There have been a few instances of drivers turning into traffic flowing the other way. Most of these turned
right at the first signal, as was done before the interchange was rebuilt.
- There have been a few cases where accidents occurred due to factors other than the properties of the diverging
diamond. In one of these cases, a truck ran off the exit ramp before reaching the cross road.
Are there locations where the diverging diamond will not work?
Yes, there are. A diverging diamond cannot be used where any of the following conditions exist:
- Straight ahead traffic volume on the cross road is too high
- Intersections too close to the interchange
- Continuous frontage roads along the freeway
- The interchange is the main interchange of a rebound interchange.
How many DDIs are now open? How many more are being built?
The answer can be found here: The Box Scores
How many states are now using the diverging diamond interchange? Just a few?
The answer can be found here: The Box Scores
How much more do diverging diamonds cost, compared to standard diamond interchanges?
Because the bridge does not have to be as large, a new diverging diamond should cost less than either a standard
diamond or a single point urban interchange (SPUI).
What is the shortest time it ever took to convert a standard diamond into a diverging diamond?
With the cross street totally closed for the construction period, 8 days. This happened in Henderson NV in
from 01/17/2015 to 01/25/2015.
What should a bicycle rider do to safely cross the interchange?
Obey the signs, signals, islands, and markings.
What should a pedestrian do to safely cross the interchange?
Obey the signs, signals, islands, and markings. The crosswalks are usually adequately marked.
I don't like the diverging diamond interchange. What can be done to stop it?
The federal government has already approved it. It is an effective tool for reducing congestion. It should not be