WHY THESE IMPROVEMENTS ARE NEEDED

These improvements have the following advantages over the old methods:

1. FLASHING YELLOW ARROWS (FYA):
   

   This improvement has the following advantages:

   • Improves the efficiency of signalized intersections.
   • Improves the safety of permissive left turns
   • Eliminates the yellow trap hazard.
   • Allows signal progression on streets that could otherwise not have it.

   Yellow trap occurs when the following conditions occur together:

   • Left turns are allowed to filter through gaps in traffic when the light is circular green.
   • Both directions of travel on the same street are moving with circular green lights.
   • One direction remains green, while the other direction changes to yellow, and then red.
   • A driver was waiting to turn left from the direction that changed to yellow.
   • That driver thinks that traffic going the other way also has a yellow light, and turns across live opposing traffic that still has a green.
   • A collision is very likely.

   The Flashing Yellow Arrow continues to flash when this happens, even though the straight-ahead signals turn red. This keeps the driver looking for gaps in opposing traffic, until the opposing signal turns yellow. Then, the flashing yellow arrow ends with a steady yellow arrow, followed by a red arrow. This is the new 2009 MUTCD (Manual on Uniform Traffic Control Devices) standard for left turn faces.

2. DALLAS PHASING:
   

   This is a system that removes yellow trap by using a circular green on the left turn signal face, but controls it from the signals facing the opposite direction. The circular indications on the left turn signals must be louvered, so that straight-ahead traffic can't see them.

   This signal can stay green when the straight-ahead signals turn yellow, and then red (as in illustration), preventing yellow trap. But drivers are often confused by it. The new 2009 MUTCD no longer allows it, but already existing installations are allowed to remain until redesigned or replaced.

3. DIVERGING DIAMOND INTERCHANGE (DDI):
   

   This improvement, also called a Double Crossover Diamond (DCD), increases the capacity of a diamond interchange if a large portion of the traffic makes left turns. It reduces the number of signal phases to two. A regular diamond interchange needs two to six phases, increasing delays to drivers.

   Note that the straight-ahead capacity on the cross street of a DDI is less than the straight-ahead capacity of a regular diamond interchange.

   The Diverging Diamond Interchange accomplishes this by crossing the two directions of flow on the cross street. Traffic crossing the bridge (or driving under it) on the cross street drives on the left side of the road. The two traffic signals control the crossing of the two streams.

   Left turns do not have to wait for a special signal phase to turn across oncoming traffic. They crossed oncoming traffic at the other end of the interchange.

   The traffic turning onto the cross street can also be controlled by the same signal that controls the crossing of the two directions of traffic on the cross street, without extra signal phases.

   Existing diamond interchanges can be retrofit with this design.

   To see a larger version of the image, right click and choose "View Image". Then use 'ctrl +' to increase the size.

   One DDI in Utah was removed in 2019 because development increased the straight-ahead traffic on the cross street to the point where the DDI could not pass the traffic.

4. CONTRAFLOW LEFT INTERCHANGE (CFL):
   

   This is halfway to a Diverging Diamond. Only the left turns entering the freeway are crossed to the other side.

   There are three divider strips on or under the bridge for crossroad traffic. The left turns pass to the left of the left turns going the other way, but the straight ahead traffic stays on the right.

   This improvement increases the capacity of the interchange, and allows a longer signal cycle.

   This does not reduce the traffic signals to only two phases like the DDI does. It reduces the phases required from six to three, and allows more flexible signal operation.

   Note that this does not increase the capacity of the left turns on the exit ramps the way the Diverging Diamond can.

   There can be two left turn lanes if the entrance ramp has two lanes to take two streams to increase the capacity of the left turns.

   Existing diamond interchanges can be retrofit with this design if there is room for the dividers.

   To see a larger version of the image, right click and choose "View Image". Then use 'ctrl +' to increase the size.

Other traffic improvements used in all states over the past few years:

• TELEVISION DETECTORS:

  Using infrared TV cameras instead of magnetic loop detectors has the advantages of being able to cover large areas, speed up signal operation, and detect bicycles and pedestrians.

  But they have the disadvantages of being affected by sun reflections, shadows, bad weather, and wind moving the camera mount.

• LED TRAFFIC SIGNAL LAMPS:

  LED signal lamps have purer colors, last longer, and use much less power than conventional traffic signal lamps. But they can fail in very strange ways.

  One disadvantage of LED signals is that they do not get warm enough to melt snow off the lenses or out of the visors.

• ROUNDABOUTS:

  Now that the proper way of operating them is known, they increase the efficiency of many intersections.

When every state has flashing yellow arrows and diverging diamond interchanges, this page will be reduced to a minimal info page.

Links:

• All Yellow Trap Links are here:
  YELLOW TRAP INDEX PAGE
• Proper implementation of Flashing Yellow Arrows.
• Do you understand yellow-trap?
• Left Turn Signals - Then and Now - Includes most of the signals listed above.
• How they choose HIGHWAY INTERCHANGES
• Diamond Interchange Variations - Includes Diverging Diamond and Contraflow Left.
• Unusual Traffic Problems and Solutions