This paper proposes an analytical throughput model for IEEE 802.11-based vehicle-to-infrastructure networks with omni-directional antennas (OA) and directional antennas (DA) applied in three different ways. The model considers the antenna gain pattern, multipath effect, co-channel interference, variable link rates and mobility effects. When DAs are applied, the deafness and hidden terminal problems are considered. With the model, the paper compares throughputs of an 802.11g V2I network where (i) all-omni: all nodes (its access point and mobile stations) always employ OAs, (ii) half-omni: all nodes use OAs for RTS/CTS and DAs for DATA/ACK transmissions, and (iii) all-directional: all nodes always employ DAs for all transmissions. The mobility effect is modeled as a throughput scaling factor considering the antenna beam coverage. Numerical evaluation of the models reveals consistently best performance with the half-omni scheme due to the higher link rate and effective avoidance of interference caused by deafness and hidden terminal problems.