VANETs inherit a conservative channel access control mechanism from IEEE 802.11 standard to mitigate the hidden terminal problem on the service channels, i.e., once a service channel is reserved by a node, all its neighbors in 2-hop range are prohibited from initiating the transmission simultaneously. To improve the channel utilization and increase the network capacity, we propose an optimistic distributed power control scheme, which relaxes the limitations on the 2-hop neighbor's data transmission and creates opportunities to start concurrent transmissions. The experimental results indicate that our scheme shows significant advantages over the fixed power scheme of IEEE 802.11p and Density-based Power Control (DBPC) [1] in terms of the average delay and throughput.