Co-operative Adaptive Cruise Control (CACC) uses wireless communication between vehicles to form a vehicle platoon. It has been proven that CACC aids in the stability and efficiency of traffic flow, and also reduces the energy consumption of the vehicles by lowering air resistance of the following vehicles. However, communication delay is a natural uncertainty for a platoon under CACC mode. When the delay gets worse to a particular level, the string stability of the platoon will be destroyed, meaning spacing or speed errors will be amplified upstream from vehicle to vehicle in the platoon. Therefore, in this study, a numerical method is proposed to identify the communication delay boundary according to the vehicle dynamics and the parameter settings of the CACC controller. First, in the Laplace domain, a series of string stable bodies under different disturbance frequencies are created based on the string stability criteria. Considering the frequency characteristics of traffic flows, the string stable bodies are then combined into a uniform one. Lastly, with the increase of communication delay, its upper bound will be obtained until the area of string stability totally disappears. Two simulations were conducted to verify the correctness and accuracy of the proposed method. The results indicate that the string stability cannot be guaranteed once the communication delay just exceeds the upper bound. This study can provide an objective boundary for practitioners or engineers, thereby allowing them to determine whether wireless inter-vehicle communication is capable of maintaining a string-stable platoon.

To date, air pollution has caused over seven million deaths worldwide. Real-time monitoring is an efficient solution to prevent and mitigate air pollution. However, air quality monitoring networks are not currently dense enough to cover a large area because of the cost. With the development of low-cost electrochemical gas sensors, the current air quality monitoring network could be extended to a large area with a relatively high-resolution ratio. Nevertheless, electrochemical gas sensors are not accurate enough compared to standard sensors if they are not carefully calibrated. Therefore, we proposed an Environment-Adaptive Calibration (EAC) system dedicated to the implementation of a collaborative calibration technique for outdoor low-cost electrochemical gas sensors. The EAC system uses a new practical calibration algorithm operating on the Raptor IoT cloud platform that was developed as part of the H2020 Captor project. The system can identify the linear characteristics of gas sensors according to weather conditions, which is valuable prior knowledge for the calibration and the unique character of the EAC system. The outstanding performance of the EAC system is verified by evaluation of a long-term measurement campaign, in which various metrics are considered, e.g., the goodness-of-fit, target diagram and daily residuals.

Vehicle platooning is a perspective technique for intelligent transportation systems (ITS). Connected and automated vehicles (CAVs) use dedicated short-range communication (DSRC) to form a convoy, in which the following vehicles can receive the information from their preceding vehicles to achieve safe automated driving and maintain a short headway. Consequently, a vehicle platoon can improve traffic safety and efficiency, further reducing fuel consumption. However, emergency braking inevitably occurs when the platoon meets an accident or a sudden mechanical failure. It is more critical when the wireless communication got delays. Therefore, “how to predefine a minimum safe distance (MSD) considering communication delay” is a challenging issue. To this end, a series of field tests were carried out to measure the communication delay of IEEE 802.11p that is the underlying protocol of DSRC. Subsequently, MSD is modeled and analyzed when the platoon travels at accelerating, cruising, and decelerating states. More importantly, the results of field tests are applied in the models to investigate the impact of communication delay on MSD in practice. The simulation results verify that the proposed model can effectively maintain the platooning vehicles’ safety even if emergency braking happens with certain communication delays.

The worldwide economic cost of road crashes and injuries is estimated to be US$518 billion per year and the annual congestion cost in France is estimated to be €5.9 billion. Vehicular Ad hoc Networks (VANETs) are one solution to improve transport features such as traffic safety, traffic jam and infotainment on wheels, where a great number of event-driven messages need to be disseminated in a timely way in a region of interest. In comparison with traditional wireless networks, VANETs have to consider the highly dynamic network topology and lossy links due to node mobility. Inter-Vehicle Communication (IVC) protocols are the keystone of VANETs. According to our survey, most of the proposed IVC protocols focus on either highway or urban scenarios, but not on both. Furthermore, too few protocols, considering both scenarios, can achieve high performance. In this paper, an infrastructure-less Traffic Adaptive data Dissemination (TrAD) protocol which takes into account road traffic and network traffic status for both highway and urban scenarios will be presented. TrAD has double broadcast suppression techniques and is designed to adapt efficiently to the irregular road topology. The performance of the TrAD protocol was evaluated quantitatively by means of realistic simulations taking into account different real road maps, traffic routes and vehicular densities. The obtained simulation results show that TrAD is more efficient in terms of packet delivery ratio, number of transmissions and delay in comparison with the performance of three well-known reference protocols. Moreover, TrAD can also tolerate a reasonable degree of GPS drift and still achieve efficient data dissemination.

The current platoon control strategies of connected autonomous vehicles (CAVs) focus on controlling the fixed intervehicle distance, i.e., the string stability of the platoon system. Here, we aimed to design a CAV platoon control strategy based on a constraint-following approach to solve the problem of platoon starting. As the resistance of the vehicle during driving varies with time, this study regarded the CAV platoon system as a changing dynamic system and introduced the Udwadia–Kalaba (U–K) approach to simplify the solution. Apart from adding an equality constraint, unlike most other studies, this study imposed a bilateral inequality constraint on the intervehicle distance between successive CAVs to prevent collisions. Meanwhile, a diffeomorphism method was introduced to transform the bounded state into an unbounded state. The proposed control strategy could render each CAV compliant with both the original imposed bilateral inequality constraint and the equality constraint. The former avoids collisions, and the latter indicates the string stability of the designed CAV platoon system. The effectiveness of the proposed controller was verified by numerical experiments. The gap errors tend to converge to zero, which is not amplified by the propagation of traffic flow.