  Cellular vehicular-to-everything (C-V2X) systems offer the potential for
improving road safety, in part through the exchange of periodic basic safety
messages (BSMs) between nearby vehicles. The reliability and latency of these
messages is a key metric. Hybrid automatic repeat request (HARQ)
retransmissions are one technique used to this end. However, HARQ may come at
the expense of consuming the limited available wireless resources, especially
in highly congested scenarios. This paper studies BSM transmission latency and
reliability when HARQ retransmissions are used with the semi-persistent
scheduling (SPS) in C-V2X transmission mode 4. We do so through extensive
system-level simulations that closely follow the SPS process. Furthermore, we
provide an analytical model for the tail behavior of the BSM latency
distribution with HARQ retransmissions that is a good approximation to the
simulation results. Our study reveals the impact of several deployment settings
(e.g., bandwidth configurations and vehicle density).
