This paper presents a general traffic separation approach to transmit HSPA traffic in the existing ATM-based UMTS network, together with Release 99 (R99) traffic. The traffic separation technique enables QoS differentiations of HSPA and R99 traffic, while at the same time aims to achieve a maximum utilization of the transport resources in the radio access network. The potential benefit of applying traffic separation and its impact on the performance of the transport network as well as the end users are explored in this paper. The quantitative evaluations are provided by means of simulations. The results presented are obtained from a UMTS simulation model developed in this work which transmits both HSPA and R99 traffic, either with traffic separation enabled or disabled.