  Experimental measurements on commercial adaptive cruise control (ACC)
vehicles \RoundTwo{are} becoming increasingly available from around the world,
providing an unprecedented opportunity to study the traffic flow
characteristics that arise from this technology. This paper adds new
experimental evidence to this knowledge base and presents a comprehensive
empirical study on the ACC equilibrium behaviors via the resulting fundamental
diagrams. We find that like human-driven vehicles, ACC systems display a linear
equilibrium spacing-speed relationship (within the range of available data) but
the key parameters of these relationships can differ significantly from
human-driven traffic depending on input settings: At the minimum headway
setting, equilibrium capacities in excess of 3500 vehicles per hour are
observed, together with an extremely fast equilibrium wave speed of 100
kilometers per hour on average. These fast waves are unfamiliar to human
drivers, and may pose a safety risk. The results also suggest that ACC jam
spacing can be much larger than in human traffic, which reduces the network
storage capacity.
