  The evolution of oscillations is a very important issue in traffic flow
studies. A recent car-following experiment (Experiment-I) showed that the speed
standard deviation grows in a concave way along a platoon of vehicles following
one another. This finding indicates that the traditional traffic instability
mechanism is debatable, in which the speed standard deviation initially grows
in a convex way. This paper has investigated the growth pattern of traffic
oscillations in another car-following experiment (Experiment-II) and compared
it with that in Experiment-I. It is shown that the speed standard deviation
also exhibits concave growth characteristics in Experiment-II. The
paired-sample t-test and the Mann-Kendall (MK) trend test showed that there is
no significant difference between the two datasets. However, the acceleration
standard deviation was remarkably larger in Experiment-II since drivers were
asked to follow closely. Furthermore, a comparison experiment has been
performed which indicates that the set of experiments on a circular track can
be considered equivalent to that on a straight track. Our study is expected to
shed light not only on traffic flow dynamics itself but also on the future
design of the experiment scheme.
