Left-Turn Phase Type Determination
Several factors can influence the left-turn phase type. They include: left-turn and opposing through traffic volume, opposing speed, number of opposing lanes, crash history, sight distance, and intersection geometry. There are also methods of evaluating protected-permissive versus protected-only phasing. FHWA Traffic Signal Timing Manual can be used to assist in determining the appropriate phase type. Additionally, local state DOT guidelines should be referenced to determine allowable phase types within the state.
Unprotected Left Turns – Left turns are permitted to cross opposing through-movement traffic during acceptable gaps. No special phase is included to accommodate left-turning movements.
Protected-Permissive Left Turns – A protected left-turn phase is provided, usually before the through movement phase in the same direction. During the through movement phase, left turns are permitted in acceptable gaps across opposing through movements.
Protected-Only Left Turns – Left-turning traffic can only move during an exclusive left turn phase. At all other times, left turns are prohibited.