With the increase of teletraffic demands in mobile cellular system, hierarchical cellular systems (HCSs) have been adopted extensively for more efficient channel utilization and better GoS (Grade of Services). A practical issue related to HCS is to design a scheme for controlling and allocating call traffic to different layers. There are several strategies to deal with this problem, such as no call-overflow scheme, unidirectional call-overflow scheme and bidirectional call-overflow scheme. The objective of this paper is to investigate a bidirectional call-overflow scheme, based on the velocity of the mobile making the calls. To ensure that hand off calls are given higher priorities, it is assumed that guard channels are assigned in both macrocells and microcells. In order to evaluate the performance of the new scheme and compare the performance of several related schemes, two now models based on a one-dimensional Markov process are developed and analytical results are derived. Theoretical analysis and numerical evaluation show that the proposed scheme outperforms others in terms of average hew call blocking and hand off failure probability of the system. In addition, when the teletraffic to the HCS reaches a certain grade, the GoS is insensitive to the maximum velocity and the velocity threshold which is used to assign calls to different layers in our scheme.