This method is contained in Appendix G of Chapter 16 of the 2000 HCM. The back of queue is the number of vehicles that are queued depending on arrival patterns of vehicles and vehicles that do not clear the intersection during a given green phase. The queue values produced by this estimation procedure may be higher than those from other procedures, especially at high degrees of saturation or high percentiles.
Users should consult the Appendix G of Chapter 16 of the 2000 Highway Capacity Manual for details of this queue calculation methodology.