A left-turn bay that does not have a sufficient length would be blocked by the adjacent through traffic. Left-turn vehicles can also spill back to block the through traffic when the left-turn demand is high. These two types of left-turn-related blockages have adverse effects on the operation of a signalized intersection. Left-turn bay blockage by through traffic will reduce the left-turn capacity, and left-turn bay spillback will affect the adjacent through capacity. Both problems are uncharted in the Highway Capacity Manual (HCM). This research studies these two blockage scenarios associated with left-turn bays under different left-turn signal operations. The capacity of the left-turn and through movements is modeled as a function of the length of the left-turn bay by using a probabilistic approach, taking into account potential blockage due to a short left-turn bay length. The developed capacity model will be validated on the basis of CORSIM simulations of a real-world signalized intersection. The research is aimed at estimating the left-turn and adjacent through capacity when left-turn bay blockage or spillback occurs, to complement the HCM method and highlighting the effect of the length of the left-turn bay and the left-turn signal strategy.