In multihop cellular networks, mobile users can communicate with the base stations via relay stations (RSs), and handoff between different RSs. This type of handoffs is referred to as inter-relay handoffs. In networks with highly mobile users, the inter-relay handoffs can occur very frequently. Making intelligent inter-relay handoff decisions is important in order to improve the network performance. In this article, we study the inter-relay handoff decision problem in a two-hop cellular network with highly mobile vehicles using a semi-Markov decision process. The objective is to maximize the total reward, which is defined by taking into consideration the transmission rate of the user's link, the overheads for performing inter-relay handoffs, and the moving speed of the user. Numerical results are shown to demonstrate the effectiveness of the handoff decisions.