The IEEE 802.11p/1609 standards specify a suite of communication protocols for vehicular communication networks. One control channel and six service channels are used in such a network. Ideally, the data transmission load generated by all service providers and their user stations should be spread over all of the six service channels evenly to fully utilize the aggregate bandwidth of these service channels. However, the IEEE 802.11p/1609 standards do not define a way to achieve this load-balancing goal. As a result, many service providers may choose to use the same service channel and leave some service channels totally unused. This will cause heavy congestion on some service channels while wasting the bandwidth of unused service channels. To solve this problem, in this paper we propose a cooperative approach to fully utilizing the bandwidth of the six service channels in the IEEE 802.11p/1609 networks. In our approach, every service provider collects and computes the current load of the service channel that it is using and shares the computed load information with other service providers on the control channel via its WSA control packets. With this global knowledge, each service provider can choose to use an idle or lightly-used service channel to avoid heavy congestion. We used the EstiNet network simulator and emulator to conduct simulation studies. Our simulation results show that our approach significantly improves the utilization of the aggregate bandwidth of the six service channels by as high as 600%.