MODIFIED DELAY EQUATIONS FOR PERMITTED LEFT TURNS

For permitted left turns at signalized intersections from exclusive or shared lane groups with multiple opposing lanes, this paper proposes modifications to the delay equation used in the 1994 Highway Capacity Manual (HCM). In the 1994 HCM, the green time for left turns is reduce d by the opposing queue clearance time in the worksheet calculation of a left-turn adjustment factor that reduces the saturation flow rate o f the left-turn lane group. A difficulty with the 1994 HCM is that it uses this saturation how rate in a delay equation that allows vehicles to make ''slow'' left turns from the instant the green time begins. T hese vehicles actually make ''no'' left turns until the opposing traff ic is less saturated, and then make ''faster'' left turns as acceptabl e gaps allow. This modified delay always exceeds the 1994 HCM delay fo r exclusive left-turn lanes (by more than 50% over total delay in some cases) but may either increase or decrease the HCM delay for shared l anes. Both delays are compared over a range of volumes for one example from the 1994 HCM.