  Decentralized vehicle-to-everything (V2X) networks (i.e., Mode-4 C-V2X and
Mode 2a NR-V2X), rely on periodic Basic Safety Messages (BSMs) to disseminate
time-sensitive information (e.g., vehicle position) and has the potential to
improve on-road safety. For BSM scheduling, decentralized V2X networks utilize
sensing-based semi-persistent scheduling (SPS), where vehicles sense radio
resources and select suitable resources for BSM transmissions at prespecified
periodic intervals termed as Resource Reservation Interval (RRI). In this
paper, we show that such a BSM scheduling (with a fixed RRI) suffers from
severe under- and over- utilization of radio resources under varying vehicle
traffic scenarios; which severely compromises timely dissemination of BSMs,
which in turn leads to increased collision risks. To address this, we extend
SPS to accommodate an adaptive RRI, termed as SPS++. Specifically, SPS++ allows
each vehicle -- (i) to dynamically adjust RRI based on the channel resource
availability (by accounting for various vehicle traffic scenarios), and then,
(ii) select suitable transmission opportunities for timely BSM transmissions at
the chosen RRI. Our experiments based on Mode-4 C-V2X standard implemented
using the ns-3 simulator show that SPS++ outperforms SPS by at least $50\%$ in
terms of improved on-road safety performance, in all considered simulation
scenarios.
