Microscopic parameters can influence each vehicle’s driving behaviors such that it has an impact on SSM performance. We concentrated on controller feedback gains and communication delays that determine car-following behavior and equilibrium condition parameters that have a close relationship with the desired car-following state.
4.1.1. Controller Dynamic Characteristics
Feedback gains for the CAV (AV) controller should be carefully designed within a rational range because improper selection of the feedback gains can cause inappropriate driving behaviors. According to reference [
27],
ranges from 0.3 to 0.5, and
ranges from 1.4 to 1.6.
fluctuates within a certain range however does not have an obvious effect on the results. Therefore, we set
as 0.3, 0.4, 0.5,
as 1.4, 1.5, 1.6, and
as −0.64 for gain impact analysis. Nine combinations were generated. Other than the above, a combination of 1, 2.5, −0.5 for
,
, and
was also included for comparison.
In addition, stability can be categorized into local stability and string stability. It is worth noting that local stability is necessary for CAV (AV) controller because it should be always ensured in real driving process. Hence, the feedback gain value candidates of the platoon should primarily check the conditions that have been proved by [
29] for local stability. On the other hand, disturbance (e.g., proceeding vehicle acceleration/deceleration) can be the main reason of system string instability and by the process of a rear-end crash, string stability describes how the disturbances are attenuated through vehicle string. The string stability is usually measured by a norm function; we used the disturbance energy norm, i.e.,
-norm to measure string stability and to give insights in safety performance. Note that
denotes
-norm of
, which can be approximated by the Riemann sum. According to reference [
27], the sufficient and necessary condition for a CAV system to be string stable is
. Moreover, to quantify the disturbance attenuation and better interpret the development trend of platoon’s string stability performance, we defined the damping ratio (
) for each following vehicle
and platoon average damping ratio (ADR) as below:
where
denotes the acceleration of the leading vehicle,
is the acceleration of the following vehicle
i,
means the product of damping ratio sequence, and
represents the
Nth root.
To analyze the performance for each individual vehicle microscopically, and meanwhile show the trend of TIT through vehicle string, our experiment was implemented on a platoon consisting of 10 CAV followers whose initial states were at the equilibrium point (0,0,0). To eliminate the specialty of a single trajectory that may cause bias in conclusion, the trajectory of the leading vehicle applied the data from a randomly generated dataset including 20 trajectories. The simulation ran for 20 different trials and then the average value was taken. The sampling period was every one-tenth of a second and the total experiment time duration was 45 s, which is usually sufficient to cover a whole ‘stop and go’ for a single vehicle during a traffic oscillation. Note that because the CAVs usually have better capability to conduct car following tasks, we enhanced the default TTC threshold to 5 s to better show the trend of TIT through vehicle strings. A sensitivity analysis on the TTC threshold value is given in a later section. Furthermore, to circumvent over-aggressiveness and over-conservativeness, the controller’s default desired time headway was selected to be 1.2 s. We calculated the platoon’s TIT value and damping ratio under ten different combinations of feedback gains, respectively. The results are shown in
Figure 3.
In
Figure 3a, it can be seen that some feedback gain combinations have zero-value TIT from the second vehicle whilst others are from the third vehicle, showing the different beginning locations of being totally safe in the platoon. The results imply that SSMs and string stability generally follows a correlative relationship. Besides, we also plot the platoon damping ratio in
Figure 3b, in which all candidate combinations except for the comparison are string stable for the monotonous decreasing trend. When the platoon is string stable, the magnitudes of
and
can determine different performances of platoon. Specifically, we found that smaller
and larger
(0.3/1.6/−0.64) performs better in terms of platoon TIT, whereas, larger
and smaller
(0.5/1.4/−0.64) tended to have better performance with respect to damping ratio. The potential reason for this phenomenon could be a larger setting on
which may lead the controller to overacting on velocity difference. On the contrary, when
is relatively large, the speeds of two vehicles tend to be consistent, such that TTC value becomes infinity, inferring an absolute safe state.
Without loss of generality, we chose the combination of 0.3/1.5/−0.64 for CAV and AV in the following experiments. As an example,
Figure 4 plots the trajectory information used to compute SSMs and stability measures with the selected feedback gains. The leading HDV of the platoon is numbered 1861 whose driving data were extracted from the NGSIM database [
30]. A proper filter has been applied on origin trajectory data to acquire clean data without sudden acceleration change or other wrong information.
- 2.
CAV controller communication delay
In this subpart, a pure CAV platoon with 15 followers were established. According to reference [
31], communication delay is usually assumed to be 0.2 s or 0.4 s for the CAV controller. As a comparison, the controller without any delay was also studied as an ideal case, due to the increasing maturity of 5G and even 6G technology. Furthermore, TTC threshold values normally range from 1 s to 4 s based on some previous references, e.g., reference [
18]. It is worth noting that for a more insightful analysis and strict safety requirement, the critical threshold was extended up to 5 s in this paper.
was set as 1.2 s. The leading vehicle’s trajectory was also applied from the aforementioned 20-trajectory dataset. The results are shown in
Table 2.
A negative effect on TIT is shown as communication delay increases. The main reason can be derived from larger communication delays resulting in more acceleration information lag such that the information may be outdated. Besides, we found that further increment of communication delay can lead to string instability and extremely large TIT values. This part is omitted due to the limited space. We set for the following experiments.