TRAFFIC IMPROVEMENT BOX SCORES

STATE
SCORE
FYA
OTHER
LT



























GB
DDI
=
DCD












































































































CFL


STATE
SCORE
FYA
OTHER
LT



























GB
DDI
=
DCD












































































































CFL

RANGES AND TOTALS OF STATES

STATE Flashing Yellow Arrows (FYA) Old Permissive Left Turn LT Diverging Diamond Interchanges DDI
.
 high
 
 REQ - Required for new sites  FYB - Flashing yellow ball  O - Open interchanges
 Y - YES  FRA - Flashing red arrow  U - Under construction
 Def - Defined  FRB - Flashing red ball  P - Planned
 low
 
 n - no, pro - prohibited*  DAL - Dallas phasing  S - Studied
 YES + REQ  FYG - Simultaneous FYA and GB  R - rejected
 no + pro + Def  GBL - All permissive turns green ball  CFL - contraflow left
actual  all but no and Pro  GB - Also green ball permissive  n - none
.
Overall:  No improvements at all (= 00)  Stopping yellow trap (FYA FYB FRA FRB DAL)  States all viable DDI

POINT VALUES FOR STATES

STATE Flashing Yellow Arrows (FYA) Old Permissive Left Turn Diverging Diamond Interchange
.
 max
 25
 10  REQ - Required for new sites  05  FYB - Flashing yellow ball  10  O - open
 08  Y - YES  04  FRA - Flashing red arrow  08  U - under construction
 02  Def - Defined  03  FRB - Flashing red ball  06  P - planned
 min
 00
 00  n - no, pro - prohibited  02  DAL - Dallas phasing  04  CFL - contraflow left open

If multiple items are
present, use highest one
for state score.
 01  FYG - Flashing yellow arrow & green ball  02  S - studied
 00  GBL - All permissive turns green ball  01  R - rejected
limits  00  GB - Also green ball permissive  00  n - none
.

There are 54 states listed because non-states are included, shown in this color. They are:
DC = District of Columbia, GU = Guam, PR = Puerto Rico, VI = Virgin Islands
The numbers used for the state scores are shown in this color. Each state appears only once in each column.
Aggregate totals applied to the above state figures are shown in this color.
The actual numbers of states using each particular left turn method are shown in this color. States appear multiple times.
The actual numbers of interchanges at each stage of development are shown in this color. States appear multiple times.

* At first approved, then required, and then prohibited

WHY THESE IMPROVEMENTS ARE NEEDED

These improvements have the following advantages over the old methods:

  1. FLASHING YELLOW ARROWS (FYA):
    flashing yellow arrow

    This improvement has the following advantages:

    • Improves the efficiency of signalized intersections.
    • Improves the safety of permissive left turns
    • Eliminates the yellow trap hazard.

    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 of 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, 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):
    diverging diamond interchange

    This improvement, also called a Double Crossover Diamond (DCD), increases the capacity of a diamond interchange, especially 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.

    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.

    To see a larger version of the image, right click and choose "View Image".

  4. CONTRAFLOW LEFT INTERCHANGE (CFL):
    contraflow left interchange

    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. It also 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.

    To see a larger version of the image, right click and choose "View Image".

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


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

Links: