ACC stands for Adaptive Cruise Control. Adaptive Cruise Control is a system which maintains a speed set by the driver (like a conventional cruise control), but also keeps a certain distance to its predecessor. If the ACC system detects a predecessor, it measures the distance and calculates the difference in speed. If necessary, the speed will be corrected to obtain a safe following distance. Research about ACC has been conducted for years, resulting into its availability on exclusive cars nowadays.
The objective is: "an analysis of the effects of different kinds of ACC on platoon stability." As a preparation for the research a literature survey was conducted, to give insight in the working of ACC and the effects on driving behaviour and the traffic stream.
ACC has a promising future, because of the expected positive effect on traffic safety. Due to its fast and adequate reaction, disturbances in the traffic stream are absorbed more easily. ACC is seen by experts as a first step towards fully automatic driving in the future.
For technical reasons the working range of the ACC is limited. It cannot utilize the full braking capabilities of the car. The driver remains responsible for driving the car and must be prepared to overrule the ACC system in case of emergency.
Trough tests and simulator studies is had been showed that subjects get used to ACC very quickly. They find driving with ACC switched on enjoyable and comfortable. The researchers questions although, if ACC may lead to less situation awareness.
To be able to answer the question stated by the objective of this survey, two conceptions had to be made operational: the stability and the car following model. The stability of a platoon is measured with the standard deviation of the acceleration for each vehicle (STDEVA). The STDEVA is not allowed to increase from a vehicle to the next one.
The stability of a traffic stream and traffic safety are related to each other. A way of measuring safety is the time-to-collision (TTC). The TTC is defined as the time that remains before a collision of two preceding vehicles take place, if speed and heading are maintained. Mathematically this means that the following distance is divided by the difference in speed.
The car following model being used is the Linear / Helly model, as being used in the MIXIC model from TNO. Both manually driven vehicles and ACC type vehicles are part of this model. The default ACC parameters are used to make two extra types: a "kind-hearted" one and an aggressive type of ACC.
Different platoons of 8 vehicles are formed with these three types of ACC and manually vehicles. These platoons are subjected to two scenarios. One is an aggressive one with severe braking; the other one was conducted from real life data and reflects a calm highway travelling scenario.
The two scenarios points to the same conclusion: the higher the rate of ACC vehicles in the platoon, the better are disturbances taken care of. Besides that ACC results into higher TTC rates, which is a good thing for traffic safety.