Flash & Arrow

Protected-Permissive (or Permissive-Protected) Left Turn operation (PPLT) has been used by traffic signal engineers around the world for decades. When applied to the appropriate intersection conditions, PPLT operation offers improved operational efficiency and traffic progression, reduced vehicle delay, reduced fuel consumption and reduced air pollution. Many traffic engineers agree on the advantages of PPLT. However, the preferred display used to convey a clear message to the driver is a subject that has wide and diverse opinion.

The NCHRP 3-54 study, Evaluations of Traffic Signal Displays for Protected Permissive Left Turn Control, is considered the most comprehensive study of the PPLT display to date. The research project surveyed current practice, studied driver understanding of known permissive displays in the U.S., analyzed crash data and operational data, studied the implementation of an experimental permissive display and conducted a confirmation study using a full-scale driving simulator to study driver understanding of the most promising permissive displays. Unlike previous research, the NCHRP 3-54 study placed a strong focus on human factors and the techniques used to observe human reactions. The final report was published in 2003 as Report 493 and is available through the National Academies of Science.

Doghouse communities

Within the U.S., the Manual on Uniform Traffic Control Devices (MUTCD) is a federally mandated document that provides structure for uniformity in the application of traffic control devices. The MUTCD identifies the five-section cluster (also called the “doghouse”) display as the recommended display arrangement for PPLT control. The cluster display arrangement is normally located centered between the left-turn and through lanes. This particular display arrangement is used at approximately 65% of all intersection approaches in the U.S. with PPLT operation. There is a growing voice among traffic engineers that the standard circular green indication, which is used to communicate the permissive left-turn right-of-way, is inadequate because there are a large percentage of the drivers that don’t understand its meaning.

In an effort to convey a definitive message to the motorist about the intersection operation, traffic engineers in North America have been experimenting with innovative displays and phasing arrangements. For example, within the U.S. there are agencies that have implemented the flashing circular red, the flashing red arrow, the flashing circular yellow and the flashing yellow arrow indications to convey the permissive turn interval. There also have been some innovative advances in signal phase sequence, such as the “Dallas Display.”

Within Canada, the permissive interval is communicated exclusively by the circular green indication. However, the circular green indication also communicates other meanings. For example, the provinces of British Columbia, Alberta and Saskatchewan flash the green left-turn arrow indication for the protected left-turn phase, whether it is protected only or protected-permissive; and Ontario communicates a protected left-turn movement by flashing the circular green indication at some intersections and uses a solid green arrow indication (same as the U.S.) at other locations. The Transportation Association of Canada has efforts under way to gain consistency in the application of turn indications.

The flashing yellow arrow display has been used for PPLT phasing in Heidelberg, Germany, on the Neckarstaden at the Congress House and Stadhalle. The flashing yellow arrow indication also is used for the permissive interval in Bern, Switzerland. In Strasbourg, France, the flashing yellow indication is used for right-turning traffic to indicate “yield to pedestrian traffic.” And in Spain, there is some use of the flashing yellow arrow indication for the permissive left-turn interval. There has been a concerted effort by practicing European traffic engineers to develop a European standard for the use of the flashing yellow arrow indication, though efforts to implement nationwide across Switzerland were unsuccessful due to concerns expressed by the Swiss federal police department.

Uncovering alternatives for the protected

Through the overview presented above, one can quickly conclude that there is great diversity in the application of turn displays. In a focused effort to gain uniformity in the U.S., a comprehensive research project was funded by the National Cooperative Highway Research Program (NCHRP) to evaluate alternative displays for protected-permissive left-turn control. The research problem statement was originally prepared by the Signals Technical Committee of the National Committee on Uniform Traffic Control Devices (NCUTCD) and, based on the results of this research effort, this group is now actively considering possible amendments to the MUTCD regarding PPLT operation. Kittelson & Associates Inc. was the prime contractor for the NCHRP Project 3-54 in association with Siemens ITS, the University of Massachusetts (Amherst) and the Texas Transportation Institute. There are many significant findings related to the operation and design of PPLT control resulting from the various research activities. Specific to the project recommendations, the research team offers the following key findings:

• The flashing yellow arrow indication is equally understood by drivers when compared to the circular green indication (even though most drivers had never seen the flashing yellow arrow before);
• There was no significant difference in drivers correctly interpreting the meaning of the flashing yellow arrow indication compared to the circular green indication;
• The flashing yellow arrow showed fewer fail-critical responses (driver incorrectly assumes right-of-way);
• The conflict study showed that drivers interpret the meaning of the flashing yellow arrow display correctly;
• The flashing yellow arrow display was successfully implemented in the field in five geographic locations with little to no technical or political problems;
• The majority of practicing traffic engineers contacted during the study endorsed the flashing yellow arrow display; and
• Field data supports high understanding of the flashing yellow arrow display.

NCHRP 3-54 study recommendations

The research study included 14 individual study tasks, each of which was successful in gathering pertinent and useful data. Study task results and findings led the research team to identify a concise set of recommendations. Based on the findings of this project, the team made the following three key recommendations:

• Incorporate the flashing yellow arrow display into the MUTCD as an optional display to the circular green for PPLT operation;
• Conduct a follow-up study; and
• Restrict the use of the flashing red indications.

The research team recommended inclusion of the flashing yellow arrow as an optional display instead of a replacement of the current MUTCD standard recognizing the near-term replacement of the many existing PPLT displays would be cost prohibitive. Additionally, post-implementation crash statistics will become available in the coming years (second recommendation), and only after such data is analyzed and reported could such a wholesale replacement be considered.

Display recommendations

The research team expanded their recommendations to address specific display arrangements to be used. The four-section all arrow display was recommended in part because the arrow display identifies the directional movement for which it is intended (left-turn or right-turn application). Over the course of the research project, there were extensive discussions within traffic engineering forums suggesting that left-turning drivers need to know that there is a change in right-of-way permissions. It was proposed that a new display was needed that would change in color, mode, position and shape. When in operation, these separate changes would ensure that drivers would be alerted to new information on the signal face so they could align their movements with the correct right-of-way permissions. Each of these four criteria is incorporated into the proposed four-section flashing yellow arrow display (as it changes from protected to permissive) as follows:

• Indication color changes from green to yellow;
• Mode changes from steady to flashing;
• Shape changes from a circular green to a yellow arrow.

Only one indication is illuminated at any time, and no supplemental signing is needed. A three-section display face with a bimodal lens also could be allowed since it operates the same as the four-section display face. If the bimodal lens is used (bottom section for a horizontal face and right-most section for a vertical face), the yellow arrow flashes and the green arrow is steady. The research team recommended that the four-section all arrow display be located over the left-turn lane in an exclusive signal arrangement (a separate display face for the left-turn movement). This exclusive display face reinforces the association between the left-turn movement and its corresponding signal indications (display face).

Operational recommendations

With respect to operations, the research team recommended that the flashing yellow arrow permissive indication be logically tied to the opposing through movement green indication/display (rather than the adjacent movement through green). Logically tying a phase means that the traffic signal control software outputs the flashing yellow arrow indication (permissive turn) only during the green interval of the opposing through phase. Operationally, this recommendation assures the complete elimination of the yellow trap in all circumstances (including those associated with skipped phases, preemption sequences, conditional service and overlap green extensions). The green arrow is illuminated steady during the protected phase (e.g., phase 1), and a flashing yellow arrow is displayed during the permissive phase (e.g., phase 2). A steady yellow arrow is used to clear both the protected and permissive indications, except when the permissive phase is to be followed immediately by the protected phase (e.g., permissive-protected sequence), in which case the flashing yellow arrow indication continues during the change intervals and transitions directly to the green arrow. TME

This article presents a concise summary of the major findings and recommendations of the NCHRP 3-54 research effort. Interested parties will find a much more comprehensive report and individual working papers documenting the seven-year work effort (as well as ongoing activities) on the project website at www.projects.kittelson.com/pplt.