For critical vehicular communication services, such as traffic safety and traffic efficiency, it is advisable to design systems with robustness as the main criteria, possibly at the price of reduced peak performance and efficiency. We describe a simple, low-cost method for combining the output of L directional (i.e., not omnidirectional) antennas to the input of a single-port receiver with the aim to guarantee robustness, i.e., to minimize the probability that K consecutive packets arriving from the worst-case angle-of-arrival are decoded incorrectly. To minimize complexity, the combining network does not estimate or use channel state information. The combining network consists of L-1 analog phase shifters whose phases are affine functions of time. For a general Lłe K and when the packet error probability decays exponentially with the received SNR, the optimum slopes of the affine functions can be computed by solving an optimization problem that depends on the antenna far-field functions. We provide analytical solutions for the special case of L=2 and 3 antennas, which turns out to be independent of the antenna far-field functions and placement on a vehicle. In an experimental setup consisting of two monopole antennas mounted on the roof of a Volvo XC90, the proposed combining method is shown to give significant performance gains, compared to using any one of the antennas.