Left-Turn Signal Phasing
The Traffic Division receives numerous requests related to the type of left-turn traffic signal phasing utilized at particular intersections. In response to these requests, the Division has created an informational brochure that can be printed and distributed to the inquiring citizens. In order to view and/or print this brochure for yourself, please click on the following: Left-Turn Signal Phasing Brochure.
A protected left-turn signal provides the motorists a period of time where left-turns can be made without encountering conflicting vehicular and pedestrian movements. The current standard will include a red arrow on top, a yellow arrow in the middle, and a green arrow on the bottom arranged vertically and there will be no supplemental sign as with the image to the left.
When are they used?
- When left-turns are permitted from two lanes on one approach and there is an opposing through movement
- When the left-turn traffic queue frequently extends beyond the left-turn lane, thus blocking the through movement
- When a significant left-turn volume is present during peak traffic hours
- When intersection geometry creates a visibility problem which may be alleviated by a left-turn phase
- Where the speed of approaching traffic is sufficiently high to make driver judgment of gaps difficult
What are the issues?
Motorists naturally feel a higher degree of safety making a left-turn with an arrow. However, in many cases, the turning capacity provided by gaps in oncoming traffic plus the yellow signal phase can safely accommodate left-turns at an intersection.
The careful design of a phasing plan for a signalized intersection is essential to its safe and efficient operation. Unwarranted signal phases create undesirable effects in terms of stops, vehicular delay, and increased fuel consumption.
How does it work?
A five-section left-turn signal head was developed and implemented in the past at numerous intersections in the City. Under this new arrangement, left-turn signals provide the usual green arrow, which is followed by the yellow arrow. After the yellow arrow has terminated, left-turning motorists are faced with a solid green ball signal (no red ball). During the display of the solid green ball, left-turns can be made when there are adequate gaps in opposing traffic to complete left-turns safely. This type of left-turn phasing is designed to help minimize delays by allowing vehicles to turn on the green ball after opposing traffic has cleared. This allows motorists to avoid sitting and waiting to turn left when there is no opposing traffic, a situation that often occurs during periods of low traffic volumes. The signal still provides a green left-turn arrow during rush hours when traffic is heavy, but during off-peak hours, drivers are not delayed unnecessarily. Signal heads like those pictured to the left are still used at some locations where a protected turn is desired but there is not an exclusive left-turn lane. Recently, the City has begun to experiment with the provision of left-turn arrows on these five-section traffic signal heads only during the hours when it is needed. If it is not needed, it will only display a green ball which would indicate that the left-turn movement may be made assuming that there is no oncoming traffic in conflict with the desired left-turn movement.
Emerging technologies have replaced the five-section signal heads (that can display either a green ball, a green arrow, or both) utilized to provide protected/permissive left-turn phasing with a four-section signal head that includes a flashing yellow arrow. From top to bottom, this new 4-section signal head includes a solid red arrow, a solid yellow arrow, a flashing yellow arrow, and a solid green arrow like the image to the left. The protected portion of the left-turn phasing is accomplished with a solid green arrow. Meanwhile, the permissive portion of the left-turn signal phase is accomplished with a flashing yellow arrow. Clearance portions of the left-turn phase are accomplished with a solid yellow arrow and a red arrow. With a new Ordinance approved by Council in January, 2011, the City has completed the conversion process to utilize this new signal head at all locations where both protected and permissive left-turns are desired and an exclusive left-turn lane exists. An informational brochure was created for the flashing yellow arrows. To view this brochure, click on the following link to the Flashing Yellow Arrow Brochure.
Flashing Yellow Arrow Signal Videos
The City of Norman worked with the University of Oklahoma/Gaylord College Broadcasting and Media Arts Department to develop informational videos regarding the new flashing yellow arrows. Here are the three short, informative videos that were developed.
What are the advantages?
The advantages of this operation when compared to protected only phasing are:
- Reduces delays, as left-turn drivers may have an opportunity to make their left-turns during the green interval
- Allows the use of shorter cycle lengths in coordinated systems by reducing the time of the fully protected green interval for the left-turn movement
- Less chance of disturbing traffic in adjacent lanes as left-turn queues are less likely to exceed the length of the left-turn lane
What is the Best Alternative for Left-Turn Signal Phasing?
Either signal phasing option can accommodate left-turn movements, and both has its place. The objective is to delay the usually heavier through traffic as little as possible while serving left-turn movements promptly and safely. The phasing of left-turn movements will always require engineering judgment because the provision for left-turn signal green time detracts from through traffic green time, and this requires balancing between signal phases for the total intersection.
Typical data, which will help determine the best option for left-turn phasing, are:
- Left-turn volumes (hourly during the peak hour)
- Cycle length (total of all phases)
- Opposing through traffic during the peak hour of highest left-turn demand
- Number of opposing traffic lanes
- Speed of opposing traffic
- Available sight distance
- Crash history, including left-turn crashes
Additionally, a time-space diagram showing progression for adjacent traffic signals should be reviewed.
Why Not Use Protected/Permissive Phasing Everywhere?
The City will use protected/permissive left-turn signal phasing where drivers can turn left safely because there are gaps in approaching traffic and drivers can clearly see oncoming vehicles.
In order to provide for good signal coordination, among adjacent traffic signals, protected/permissive signals will not be available at all intersections. Special left-turn sequencing is used to improve signal coordination and provide smooth through traffic flow at selected intersections. At these locations, one left-turn arrow comes before the through movements receive their green indication and the opposing left-turn arrow comes up after the through movements receive their green indication. Therefore, protected/permissive operation would be dangerous for the drivers.
There are some intersections where the City could install protected/permissive left-turns. However, this might require a longer mast arm and installation of new traffic signal heads which may not be in the current budget. The City may use protected/permissive signals wherever possible as traffic signals are modernized and new signals are installed at locations meeting the recommended criteria.