VANET (vehicular adhoc network) based on 802.11p/WAVE is a comprehensive and systematic field of study which involves many subjects such as intelligent traffic system, wireless communication and self-organizing system. Roadside unit (RSU) works as IEEE 802.11p access points (APs) to open up services to clients from traveling vehicles for anytime and anywhere connection to Internet, thus making the modeling of downlink performance of RSU a key challenge. By introducing the vehicle density probability on freeway, the channel access contention scenarios of CSMA/DCF-based MAC layer well adapted to the time-varying vehicle arrivals are analyzed in detail. Then on that basis, the throughput performance model of VANET, including both the data downlink and the uplink through RSU for travelling vehicles, is presented. To address the issue of intermittent connectivity caused by RSU's inability to cover all the highway areas, VANET cooperative downloading scheme (VCoDS) is constructed to extend the downlink output of RSU. Simulations also verify the effectiveness of the proposed VCoDS scheme.