Auxiliary through lanes (ATLs) are often implemented at congested signalized intersections where fiscal and right-of-way constraints do not allow for full roadway widening. Previous research has suggested that ATLs are used significantly less than the theoretical capacity of a continuous lane. However, analysts have had little empirical basis for evaluating ATL operations in the context of the 2010 Highway Capacity Manual (HCM). Two empirical models that predict the flow in the ATL as a function of congestion and total through-movement flow are presented. A method is described for incorporating the model's findings into the HCM procedure for estimating approach capacity. The study addresses the challenges of modifying the HCM lane utilization factor and accounting for the iterative effect of actuated signal control on ATL use. The study suggests that a cyclic process can be used to balance the ATL flow with the green-to-cycle-length ratio. A method is presented for determining the upstream ATL length needed for efficient ATL operations. Two example problems illustrate the use of the new models in the context of HCM 2010.