W. K. Lis
Superstar
Leading and Lagging
Left Turn Signals Compared
From link. Best to use the link for the complete references.
There is a lot of confusion and controversy about left turn phasing. This page is intended to remove some of the confusion.
Terminology
Even the terminology about left turn phasing is confusing, because so many different terms have been used for the same things, and because some of the terms and abbreviations have been used for different things. A complete glossary is provided at the end of the article in Appendix A. For immediate use, here is a list of the unambiguous terms used in the rest of this article:
- Circular green permissive - Permissive left turns made during a circular green indication.
- Double-Split Lead-Lag - One leg on the street has leading turn, opposite leg has lagging left turn. Either phase can change first.
- Dual Lag - Both left turns on the street have simultaneous lagging turn phases.
- Dual Lead - Both left turns on the street have simultaneous leading turn phases.
- Dual Split Lag - Both left turns on the street have lagging phases. They can begin at different times.
- Dual Split Lead - Both left turns on the street have leading phases. They can end at different times.
- Exclusively/Permissive - No left turn phase. Left turns filter through gaps in oncoming traffic.
- Exclusively/Protected (E/P) - The left turn is protected during one phase, and is not allowed to proceed otherwise.
- Flashing Yellow Arrow - An indication where left turns are made through gaps in oncoming traffic.
- Flashing Yellow Arrow permissive - Left turns made through gaps in oncoming traffic during a flashing yellow arrow indication.
- Lagging left turn - A sequence where the green left turn arrow is next after the oncoming thru green.
- Leading left turn - A sequence where the oncoming thru green is next after the green left turn arrow.
- No-Split Lead-Lag - Left turn phasing where each leg on the street has an exclusive phase.
- Permissive left turn - An indication where left turns are allowed to filter through gaps in oncoming traffic.
- Prohibited left turn - An indication where left turns must wait until the indication is no longer displayed.
- Protected left turn - An indication with a green left turn arrow, where no other traffic can move in conflict with the turn.
- Protected/Permissive (P/P) - The left turn is protected during part of the cycle and permissive during another part.
- Single lag - A green left turn arrow is displayed in one direction next after the oncoming thru green.
- Single lead - The oncoming thru green is displayed next after a green left turn arrow for one leg.
- Single-Split Lead-Lag - One leg on the street has leading turn, opposite leg has lagging left turn. The leading turn must end first.
- Yellow trap - Left turns trapped in the intersection with a yellow or red light while oncoming traffic is still moving with a green.
Note the lack of refreshes to public transit.
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