Based　on　the　survey　data　of　three　signalized　intersections　with　exclusive　triple　left-turn　lane　and　with　outside　trap　lane　in　Beijing,　traffic　flow　characteristics　of　left-turns　are　studied　by　using　statistical　methods.　It　shows　that　the　discharge　headway　and　saturation　flow　rate　of　exclusive　triple　left-turn　lane　with　outside　trap　lane　divides　into　two　stages　when　the　discharge　time　of　vehicles　in　the　left-turn　bay　is　less　than　the　left-turn　green　time.　This　problem　is　uncharted　in　the　Highway　Capacity　Manual　(HCM).　This　research　analyzes　the　effects　of　relationship　between　the　length　of　left-turn　bay　and　green　time　on　the　left-turn　capacity.　A　theoretic　model　for　left-turn　capacity　is　developed　by　considering　this　characteristic.　The　developed　capacity　model　will　be　validated　on　the　basis　of　VISSIM　simulations　of　a　real-world　signalized　intersection.　And　the　results　demonstrate　that　the　proposed　capacity　models　well　estimate　the　left-turn　capacity　at　signalized　intersections　in　the　case,　which　the　length　of　left-turn　bay　and　green　time　is　not　well　optimized　and　well　reflect　the　capacity　loss　due　to　the　transitional　area　bottleneck.　©　2012　American　Society　of　Civil　Engineering.