Traffic impacts of commercial Adaptive Cruise Control time-gap policy

Two field experiments have been carried out with a commercially available Adaptive Cruise Control system, one in an urban environment and one in the highway. The time-gap policy is mapped for the whole speed range. It is shown that while the system follows a constant time-gap policy for highway speeds, for lower, urban speeds, the time-gap increases as the speed decreases. A new time-gap policy is also provided by a recent UN regulation for level 3 Automated Lane Keeping Systems. The three different options are compared by drawing the density-flow diagram. They are further investigated for possible dynamic phenomena using microscopic traffic simulation. The results show that the observed time-gap policy can induce phenomena such as capacity drop and increase string instability. The time-gap policy coming from the regulation is producing stable but lower traffic flow.