This work contributes to the understanding of the behavior of the upstream end of a high-occupancy vehicle (HOV) lane. Indeed, just upstream of this lane, vehicles may change lanes to be sure they are allowed to use the downstream lane (either HOV or general-purpose lane(s)). This upstream end is therefore the place of lane changes that may reduce the capacity. To estimate this drop, we adapt an existing weaving section analytical model to this upstream interface. We assess this model by comparison with the maximal theoretical supply and with simulation results. Our model adaptation indeed reproduces a capacity drop. Although with a less important drop, simulation results are coherent, except for cases with 0 or 100% of the vehicles being highly occupied. Further work should focus on treating the successive lane changes when considering more than two upstream lanes, and on comparing the results with the ones of other simulators.