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Empirical study on the properties of adaptive cruise control systems and their impact on traffic flow and string stability

Abstract: 

Adaptive Cruise Control (ACC) systems appear as standard equipment in many commercially available vehicles. There are considered the first step of automation, and their market penetration rate is expected to rise, along with the interest of researchers worldwide to assess their impact in terms of traffic flow and stability. These properties are currently discussed mainly through microsimulation studies and empirical observations, with the first being the most common. Experimental observations can draw safer conclusions about the behavior of such systems but there are only a few in the literature. In this work, an experimental campaign with 5 vehicles equipped with ACC has been organized in the proving ground of AstaZero in Sweden to raise understanding on the properties of the ACC systems and their functionality under real driving conditions. The main parameters under investigation are the response time of controllers, the available time headway settings and the stability of the car-platoon. The results show that the response time range for the controllers is between 1.7s and 2.5s, significantly higher than the values reported in the literature. The time headway with the minimum setting and normal driving conditions is a bit over 1s for all the vehicles, while with the maximum setting it can reach 3.5s. Finally, imposed perturbations of variable magnitudes lead to instability for the car-platoon. Furthermore, instability appears even for slight perturbations derived by the variability in the road slope. Numerical differentiation on the altitude shows a negative correlation with the speed trajectory of the leading vehicle.