Congestion downstream of a freeway off-ramp often produces a traffic queue at the mainline and thus reduces the freeway capacity at the interchange area. To prevent such queue spillback, a proposed real-time control strategy provides a priority signal control to the off-ramp traffic when potential queue spillback is detected. Two priority strategies—unconditional and conditional controls—that quickly discharge the queuing vehicles on the off-ramp can contend with different congestion patterns at the freeway and local arterial. Based on field data from a freeway interchange in Zhubei, Taiwan, extensive simulation experiments demonstrate the effectiveness of the proposed strategies in preventing off-ramp queue spillbacks. The results show that the overall network performance can be significantly improved with the proposed strategy, even though some roadway segments of the local arterial network may experience negative impacts.