In this paper, we discuss and analyze results from real-world vehicle-to-infrastructure measurements in an IEEE 802.11p-based vehicular ad hoc network. For our experiments we have used six roadside units (RSUs) mounted on highway gantries equipped with five different antenna types. We compare the performance of directional and omnidirectional antennas and analyze performance improvements in terms of coverage range and throughput, achievable by using directional antennas. Our results show that the use of directional antennas yields substantial performance improvements in IEEE 802.11p-based networks. However, for high-gain antennas even more careful RSU antenna positioning with respect to the lane geometry is required.