  Cooperative adaptive cruise control(CACC) system provides a great promise to
significantly reduce traffic congestion while maintaining a high level of
safety. Recent years have seen an increase of using formal methods in the
analysis and design of cooperative adaptive cruise control systems. However,
most existing results using formal methods usually assumed an ideal
inter-vehicle communication, which is far from the real world situation. Hence,
we are motivated to close the gap by explicitly considering non-deterministic
time delay and packet dropout due to unreliable inter-vehicle communications.
In particular, we consider a passive safety property, which requests a vehicle
to avoid any collisions that can be considered as its fault. Under the
assumption that the communication delay is bounded and we know the upper bound,
we then formally verify the passivity safety of a class of hybrid CACC systems.
This result allows us to define a safe control envelope that will guide the
synthesis of control signals. Vehicles under the CACC within the safe control
envelope are guaranteed to avoid active collisions.
