Left
Turn Signals
What is
the Justification for a Left-Turn Arrow?
Left-Turn Signal Phasing
Left-turn signal phases facilitate
left-turning traffic and may improve the safety of the intersection for
left-turning vehicles. However, this is done at the expense of the amount of
green time available for through traffic and will usually reduce the capacity
of the intersection. Left-turn arrows also result in longer cycle lengths,
which in turn have a detrimental effect by increasing stops and delays.
Pedestrian delays may be increased and due to the increased delay, pedestrians
may elect to ignore the pedestrian signals.
While phases for protected left-turning
vehicles are popular and commonly requested, other methods of handling
left-turn conflicts also need to be considered. Potential solutions may include
prohibiting left-turns and geometric improvements.
Left-Turn Phase Criteria
The three left-turn phase criteria presented
below are the result of considerable research and study. These three criteria
are used in numerous States, including Washington, as warrants for the
installation of separate left-turn phases at a signalized intersection.
1.
Traffic Volumes
Separate left-turn phasing may be considered
when:
- The product of the left-turning and opposing through volumes during
the peak hour exceed 100,000 on a four-lane street (2 approach lanes), or
50,000 on a two-lane street (1 approach lane); and,
- The left-turn volume is greater than two vehicles per cycle during the
same peak hour period.
Volumes meeting
these levels are only an indication that a left-turn phase may be justified.
Further study of the intersection is recommended, including review of other
potential solutions.
2.
Delay
Separate left-turn phasing may be considered
if:
- The total delay to all the left-turning vehicles on a crucial
approach equals or exceeds two vehicle hours during a peak hour; and,
- There is a minimum left-turn volume on the approach of greater than
two vehicles per cycle during the same peak hour; and,
- The average delay for all left-turning vehicles on the approach is
at least 35 seconds during that same peak hour.
3. Collision Experience
Separate left-turn phasing may be considered
if the critical number of reportable left-turn collisions has occurred. These
are:
- For one approach to the intersection, the critical number is five
left-turn collisions in one year, or seven in two years.
- For both approaches to an intersection, the critical number is
seven left-turn collisions in one year, or eleven in two years.
4. Air Quality
Vehicle delay is increased when protected left turn
movements are implemented. As the left turning vehicles are protected with a
green arrow, all other vehicles are idling in front of a red ball signal
indication. Due to the increased idling, air quality can be severely
compromised. Because of this, even if an intersection meets the other criteria,
if an air quality analysis shows unacceptable degradation, a protected left
turn may not be approved.
Protected/Permitted Left-Turn Phasing
Protected/Permitted left-turn phasing is a
left-turn movement of traffic at a signalized intersection having a separate
left-turn phase in the signal cycle to provide a protected green arrow
interval, as well as a non-protected circular green interval. Use of the
protected/permitted left-turn phasing technique is based on the assumption that
the need for a protected left-turn interval has been established. One of the
basic precepts of the protected/ permitted left-turn phasing is that the
protected arrow is displayed only when needed in a traffic demand condition. It
is therefore emphasized that the protected/permitted left-turn phasing
technique is an efficient concept opposed to a collision reduction concept,
although it will probably offer safer operation than permissive operation only.
Protected Only Left-Turn Phasing
When a separate interval is provided to
accommodate a left-turn without allowing any conflicting traffic a protected
only left-turn phasing occurs.
With a protected only left-turn, vehicles
are only permitted to turn left when the left-turn green arrow is activated and
turns are prohibited on the circular green indication.
Unprotected Left-Turn Phasing
Unprotected (permissive) left-turn phasing
occurs when there is no exclusive phase provided for left-turn vehicles.
Left-turns are permitted during a circular green when there are sufficient gaps
in opposing traffic flow to allow a left-turning vehicle to safely make the
turn. Exceptions would be at those locations where turns are restricted by
signs or other devices.
Separate left-turn lanes may or may not be
provided.
If you have any questions about Left Turn
Signals, please contact the City of Spokane, Traffic Operations Division at
(509) 232-8800.
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informational purposes only. Please
refer to the Citizens
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send any questions or concerns for the street department to:
Spokane
Streets
To contact the Street Department, call (509) 232-8800 during normal working
hours.
If you are in need of after hour assistance please contact us at (509)
625-7733.
or send correspondence to:
City of Spokane Street Department
901 N Nelson St.
Spokane WA 99202-3769
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