In the future, road traffic will incorporate a random mix of manual vehicles and cooperative adaptive cruise control (CACC) vehicles, where a CACC vehicle will degrade to an adaptive cruise control (ACC) vehicle when vehicle-to-vehicle communications are not available. This paper proposes a generalized framework of the Lighthill-Whitham-Richards (LWR) model for such mixed vehicular flow under different CACC penetration rates. In this approach, the kinematic wave speed propagating through the mixed platoon was theoretically proven to be the slope of mixed fundamental diagram. In addition, the random degradation from CACC to ACC was captured in mathematical expectation for traffic scenarios where the CACC only monitors one vehicle ahead. Three concrete car-following models, the intelligent driver model (IDM) and CACC/ACC models validated by Partners for Advanced Transit and Highways (PATH) program, were selected as examples to investigate the propagation of small perturbations and shock waves. Numerical simulations were also performed based on the selected car-following models. Moreover, the derived mixed LWR model was applied to solve some traffic flow problems. It indicates that the proposed LWR model is able to describe the propagation properties of both small perturbations and shock waves. The mixed LWR model can also be used to solve some practical problems, such as the queue caused by a traffic accident and the impact of a moving bottleneck. More importantly, the proposed generalized framework admits other CACC/ACC/regular car-following models, including those developed from further experiments.

String stability is important for understanding traffic flow dynamics, while its analytical study for mixed traffic is deficient. We focus on an analytical framework on string stability of mixed traffic consisting of cooperative adaptive cruise control (CACC), adaptive cruise control (ACC), and human vehicles. The analytical framework was conducted based on generalized car-following model formulations of the three vehicular types. Then string stability criterions of one-class traffic flow, mixed traffic flow, and local CACC/ACC platoons were derived. Taking into account the mixed flow with partial degenerations of CACC and three concrete car-following models, an example application was studied. The example application analyzed the string stability from the perspectives of the homogeneous flow, the mixed traffic flow, and the local CACC/ACC platoon, respectively. The example application also studied balance between stability and capacity for the mixed traffic with CACC vehicles. Results show the usefulness of the proposed analytical framework, in term of not only analyzing string stability but also providing suggestions for dynamic regulations of CACC/ACC management strategy to balance string stability and traffic capacity for the mixed CACC-human flow.

This paper focuses on traffic scenarios randomly mixed with cooperative adaptive cruise control (CACC) vehicles and manual vehicles with vehicle-to-vehicle (V2V) communication. The analytical investigation of string stability of such mixed traffic flow is conducted under different CACC penetration rates. The parametric sensitivity analysis on CACC desired time-gap is also studied. Moreover, numerical simulation is conducted under both open and periodic boundary conditions. Research results indicate that increase of CACC vehicles is helpful for improving the mixed traffic flow string stability. The mixed flow will be stable for all possible velocities if CACC rate reaches to 0.64 when the CACC time-gap is considered as 0.6s. Larger CACC time-gap would have more stable regions in string stability charts with respect to equilibrium velocity and CACC penetration rate. This paper contributes to a new way for analytical investigation of string stability of the mixed CACC traffic flow.

The influence of (cooperative) adaptive cruise control (CACC/ACC) on rear-end crashes have been conducted by some previous studies. However, they usually did not consider the degeneration from stable CACC to unstable ACC when vehicle-to-vehicle (V2V) communication is not available. California PATH program validated the instability of ACC system by using real experimental data, which might have negative impacts on safety. Therefore, this paper evaluated the impacts of degeneration of CACC vehicles on rear-end crash risks. A micro-simulation based method was adopted to evaluate rear-end crash risks with the increase of the CACC market penetration rate (MPR). The results obtained were quite different from those in previous studies and showed that the rear-end crash risks of the full MDV flow increase before CACC MPR increases to a certain level. It was found that the instability of ACC vehicles might be the main factor for the negative results. The results of this paper provide helpful insight into the management of CACC vehicular flow in the V2V communication environment.

To enhance safety and traffic efficiency, stability of a mixed human and connected cruise control (CCC) system is studied. The authors consider individual vehicle platoons, in which the tail CCC vehicle receives feedback from multiple human‐driven vehicles ahead via vehicle‐to‐vehicle communications, with the objective of stability analysis and feedback control design. To deal with this, the transfer function theory is used. Simulations are also performed to evaluate impacts of the mixed human and CCC system on safety and traffic efficiency. Results show that the output bounds of the CCC feedback coefficients can be appropriately designed to keep local individual vehicle platoons stable for all possible vehicle speeds. The feedback coefficient has a larger design range and the required lower bound value decreases as the feedback length increases. Additionally, the system design would improve traffic safety and efficiency even at lower CCC vehicle penetration rates.