The wireless access in vehicular environment (WAVE) architecture of intelligent transportation system (ITS) has been standardized in the IEEE 802.11p specification and it is going to be widely deployed in many roadway environments in order to provide prompt emergency information and internet services. A typical WAVE network consists of a number of WAVE devices, in which one is the road-side-unit (RSU) and the others are on-board-units (OBUs), and supports one control channel (CCH) and one or more service channels (SCH) for OBUs to access. The CCH is used to transport the emergency messages and service information of SCHs and any SCH is used to carry internet traffic of OBUs. However, the IEEE 802.11p contention-based medium access control protocol would downgrade transmission efficiency when the number of OBUs served by a RSU is large and all WAVE devices equipped with a single transceiver access to the same SCH. Synchronizing all WAVE devices to periodically and equally access the CCH and an SCH will cost as much as 50% channel resource wastage on SCH. As a solution, we propose an efficiency-improvement scheme, namely agent-based scheduling (ABS) scheme, which promotes the SCH throughput by means of choosing one OBU to be the agent to schedule the other OBUs contending the access opportunities on one SCH to access the other SCH served by RSU in a contention-free manner. Numerical results and simulation results demonstrate that the proposed scheme indeed improves the standard transmission efficiency.