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In this article, we assess the string stability of seven 2018 model year adaptive cruise control (ACC) equipped vehicles that are widely available in the US market. Seven distinct vehicle models from two different vehicle makes are analyzed using data collected from more than 1,200 miles of driving in car-following experiments with ACC engaged by the follower vehicle. The resulting dataset is used to identify the parameters of a linear second order delay differential equation model that approximates the behavior of the black box ACC systems. The string stability of the data-fitted model associated with each vehicle is assessed, and the main finding is that all seven vehicle models have string unstable ACC systems. For one commonly available vehicle model that offers ACC as a standard feature on all trim levels, we validate the string stability finding with a multi-vehicle platoon experiment in which all vehicles are the same year, make, and model. In this test, an initial disturbance of 6 mph is amplified to a 25 mph disturbance, at which point the last vehicle in the platoon is observed to disengage the ACC. The data collected in the driving experiments is made available, representing the largest publicly available comparative driving dataset on ACC equipped vehicles. I. INTRODUCTIONAdaptive cruise control systems are now widely available as a standard or optional feature on many of the best-selling cars in the US and around the world. These vehicles represent the first wave of Society of Automotive Engineers (SAE) level 1 automated vehicle systems beginning to appear in the traffic flow. Changing the car following dynamics of a small fraction of vehicles in the traffic flow can fundamentally change the emergent properties of the flow, as experimentally demonstrated by

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