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 reduced by the opposing queue clearance time in the worksheet calculation of a left-turn adjustment factor that reduces the saturation flow rate of the left-turn lane group. A difficulty with the 1994 HCM is that it uses this saturation flow rate in a delay equation that allows vehicles to make “slow” left turns from the instant the green time begins. These vehicles actually make “no” left turns until the opposing traffic is less saturated, and then make “faster” left turns as acceptable gaps allow. This modified delay always exceeds the 1994 HCM delay for 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 lanes. Both delays are compared over a range of volumes for one example from the 1994 HCM.