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This study analyses the performance of a roadside two-way relaying system in which a roadside access point (AP) and a vehicle exchange messages with the aid of amplify-and-forward mobile relay (MR) based on partial relay selection. It has been shown that AP-MR-vehicle communications may experience severer channel fading than conventional cellular communications. Mixed Nakagami-m and `double' Nakagami-m fading is adopted to provide a realistic description of the involved AP-MR-vehicle channels. In this scenario, a tight closed-form lower bound and high signal-to-noise ratio approximate expression for the system outage probability are derived. By applying these results, the authors obtain the diversity and the coding gains and the average symbol error rate for the considered system. In particular, the optimum number of relays is provided, providing valuable guidelines for practical system design. It is shown that when the number of relays is greater than the optimum number, no performance gain would be further achieved. The simulation results highlight the authors theoretical analysis.