近年來，無線網路發展的成熟、行車安全問題的重視、全球定位系統的普及，以及車用網路(Vehicular Ad Hoc Networks, VANET)在研究上的前瞻性與未來的高應用價值，使得車用網路的發展也愈來愈受到各方的探討。然而，車用網路的拓撲變動性相當高且規模大，且屬於點對點(Ad-Hoc)的網路架構，因此，對於吞吐量(throughput)和碰撞率(collision)以及延遲時間(delay)皆有一定的需求。此外，由於車輛在道路上的分布容易造成訊號之間的碰撞和干擾，所以我們藉由修改RTS/CTS機制提出一個Detection-Based MAC，其機制可透過封包之間的訊息傳遞，偵測出網路的壅塞狀態，預測網路中鄰近節點的競爭情形，並依此來動態調整競爭視窗大小。最後，將此方法實作在IEEE 802.11p的媒體存取控制層(MAC)中，以解決車用網路節點互相干擾所造成的過多碰撞，並提升整體吞吐量。 / In recent years, the research on Vehicular Ad Hoc Networks (VANET) has been paid much attention due to the development of wireless network becoming mature, people think highly of traffic safety issues, the popularity of Global Positioning System (GPS), promising applications of VANET being exploited. However, VANET has the essential property of rapid change of large scale network topology. Besides, vehicles randomly distributed over the road usually causes serious signal collisions and interferences. Thus, it calls for special concerns to deal with the performances of throughput, collision, and delay. In this paper we propose a Detection-Based MAC mechanism by modifying RTS/CTS to detect network congestions through message exchange, and predict number of competing nodes. Both these information is used to dynamically adjust contention window sizes. Simulations are implemented on the MAC layer of IEEE 802.11p. The results show that the proposed method is able to effectively reduce collisions and increase overall throughput.