  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.
