  This paper demonstrates that the acceleration/deceleration limits in ACC
systems can make a string stable ACC amplify the speed perturbation in natural
driving. It is shown that the constrained acceleration/deceleration of the
following ACCs are likely to cause speed overshoot to compensate for an extra
large/small spacing. Additionally, we find that the constrained deceleration
limits can also jeopardize safety, as the limited braking power produces extra
small spacing or even crashes. The findings are validated through experiments
on real cars. The paper suggests that the ACC parameter space should be
extended to include the acceleration/deceleration limits considering their
significant role exposed here. Through numerical simulations of ACC platoons,
we show i) a marginal string stable ACC is preferable due to the smaller total
queue length and the shorter duration in congestion; ii) congestion waves in a
mixed ACC platoon largely depend on the sequence of vehicles provided different
acceleration/deceleration limits, and iii) the safety hazard caused by the
constrained deceleration limits is more significant in mixed ACC platoons when
string unstable ACCs exist.
