The growing level of traffic on streets and highways has led to the development of innovative means to control traffic. Protected/permitted left turn (PPLT) traffic controls have been developed to increase the left-turn capacity and reduce delay at intersections by providing an exclusive turn phase for left turns as well as a permissive phase during which left-turns can be made as gaps in opposing through traffic will allow. These left-turn controls can precede (lead) or follow (lag) the through phase.

  Protected/permitted left-turn traffic controls have been implemented in a variety of ways, because the Manual of Uniform Traffic Control Devices (MUTCD) initially provided limited guidance. The Federal Highway Administration (FHWA) recommends a five-section signal head tied to the adjacent through lanes for left-turn control. Many states have adopted the five-section cluster or "doghouse" signal located between the through and turning lanes, which provides a green arrow for the protected phase and a circular green to indicate the permissive phase. At least four variations of the display to indicate the permissive phase are known to exist, including the flashing circular red, the flashing circular yellow, the flashing red arrow, and the flashing yellow arrow. Other variations also exist in phasing, signal displays, arrangement, signal placement, and use of supplemental signs.

  Problems with protected/permitted left-turn traffic controls have been identified but have not been resolved. A major issue is the permissive display. Many argue that the circular green (circular green may be used exchangeably with green ball) is adequate while others argue that a "unique" display is needed because motorists in a left-turn bay may interpret the green ball as a protected display. Another problem that occurs with the standard protected/permitted left-turn operation, utilizing lagging left-turn operation--allowed by the current version of the MUTCD--is that, during the signal change from permitted movements in both directions to a lagging protected movement in one direction, a "yellow trap" is presented to the left-turning motorist whose permissive left-turn phase is terminating. To avoid the hazardous yellow trap situation using standard MUTCD displays, simultaneous lagging left turns have been used. This practice precludes the use of a lagging operation with phase overlaps, with a resulting potential to decrease capacity and increase delay.

 An innovative operation known as "Dallas Phasing" eliminates the potentially unsafe "yellow trap" situation by allowing a continued permissive left turn during the opposite approach lagging protected left-turn phase. Research has shown that this operation reduces delay and improves safety. The animation below illustrates the "yellow trap" when the east-bound driver in the yellow car makes a permitted left turn on a yellow assuming that opposing traffic is also yellow. A separate animation shows how PPLT ("Dallas Display") solves the "yellow trap".

  The research under NCHRP Project 3-54 (Phase I) found that a variety of PPLT traffic controls (seven basic forms) exist and that there has been limited research devoted to understanding the relative effectiveness of these forms of control. There have been concerns that some may be confusing to motorists, and the need exists to validate the relative operational advantages and safety aspects of the various left turn controls. Specifically, there is need for research to address the yellow trap and driver confusion problems. In addressing these problems, it will be necessary to consider the safety implications of increased signal efficiency and the difficulty in establishing uniformity between states.