The Cooperative Adaptive Cruise Control is an extension to the adaptive cruise control concept. CACC realises longitudinal automated vehicle control. In addition to the feedback loop used in the ACC, which uses Radar or LIDAR measurements to derive the range to the vehicle in front, the preceding vehicle's acceleration is used in a feed-forward loop. The preceding vehicle's acceleration is obtained from the Cooperative Awareness Messages it transmits using ETSI ITS-G5 or DSRC / WAVE technology. Generally, these messages are transmitted several times per second by future vehicles equipped with ITS capabilities.
Benefits over ACC
ACC systems, like human drivers, may not exhibit string stability. This means that oscillations which are introduced into a traffic flow – by braking and accelerating vehicles – may be amplified in the upstream direction. This leads to so-called phantom traffic jams or head-tail collisions. It has been shown that ACC systems designed to maintain a fixed following distance will not be string stable. ACC systems designed to maintain a fixed following time may or may not be string stable. 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 to a metric of change in range over time. CACC utilizes vehicle-to-vehicle communications so that the vehicle has information not just on the vehicle immediately in front, but also on a leading vehicle or vehicles further in front, through vehicle-to-vehicle communications of key parameters such as position, velocity, acceleration.
Implementations
The Dutch Connect&Drive project implemented CACC in seven Toyota Prius vehicles in 2009–2010. This project used a communication stack based on the reference architecture of the Car-2-Car Communication Consortium, using IEEE 802.11a hardware at the physical layer. The Grand Cooperative Driving Challenge in 2011 was an international challenge for teams from universities and industry to participate with a vehicle which could cooperatively drive several defined traffic scenarios. CACC was a large part of the challenge. The communication stack was based on CALM FAST, using IEEE 802.11p hardware in the 5.9 GHz range. The criteria on CACC performance included low platoon length, fast traveling time, platoon merging behaviour, and damping behaviour in strong acceleration and deceleration situations.
