This means that oscillations which are introduced into a traffic flow – by braking and accelerating vehicles – may be amplified in the upstream direction.
CACC addresses this problem, and in either case may improve stability, by reducing the delay of the response to the preceding vehicle.
In human drivers this delay depends on reaction time and actions such as moving the foot from throttle to brake pedal.
In ACC this delay is reduced, but there still is a large phase delay because of the estimation algorithm needed to translate the discrete range measurements (supplied by radar or lidar) to a metric of change in range over time (i.e. acceleration and deceleration of the lead vehicle).
The communication stack was based on CALM FAST, using (by that time commercially available) IEEE 802.11p hardware in the 5.9 GHz range.