  In Release 14, the 3rd Generation Partnership Project (3GPP) introduced
Cellular Vehicle--to--Everything (C-V2X) \textit{mode-4} as a novel disruptive
technology to support sidelink vehicular communications in out--of--coverage
scenarios. C-V2X \textit{mode-4} has been engineered to operate in a
distributed manner, wherein vehicles autonomously monitor the received power
across sidelink subchannels before selecting one for utilization. By means of
such an strategy, vehicles attempt to $(i)$ discover and $(ii)$ reserve
subchannels with low interference that may have the potential to maximize the
reception likelihood of their own broadcasted safety messages. However, due to
dynamicity of the vehicular environment, the subchannels optimality may
fluctuate rapidly over time. As a consequence, vehicles are required to make a
new selection every few hundreds of milliseconds. In consonance with 3GPP, the
subchannel selection phase relies on the linear average of the perceived power
intensities on each of the subchannels during a monitoring window. However, in
this paper we propose a nonlinear power averaging phase, where the most
up--to--date measurements are assigned higher priority via exponential
weighting. We show through simulations that the overall system performance can
be leveraged in both urban and freeway scenarios. Furthermore, the linear
averaging can be considered as a special case of the exponentially-weighted
moving average, ensuring backward compatibility with the standardized method.
Finally, the 3GPP \textit{mode-4} scheduling approach is described in detail.
