# Urban Traffic Signal Control under Mixed Traffic Flows: Literature Review
## 6 Impact of Mixed Traffic Flows on Traffic Signal Control  
The introduction of CVs alters traffic flow dynamics, increasing the effective road capacity as the penetration rate of CVs rises. Driven by advanced sensing and computing capabilities, and by utilizing V2X communication CVs can percept the environment. They also share onboard data (speed, position, and heading) in real-time with the environment through V2X communication. Thanks to sensing technology and real-time communication, vehicles are becoming also a good basis for developing new ITS-related applications and services. Exchanging traffic information CVs will have timely information about the traffic state and can recommend the driver's ideal vehicle speed or alternative route. Relying on computation capabilities, CAVs will be part of a multi-agent system, where each vehicle will make decisions based on available data, which means CVs will affect traffic flow characteristics, increasing throughput and reducing delays. CVs are the vehicles with more information and recommendations, while CAVs are the vehicles with more information and orders. It is important to emphasize drivers' compliance in CVs as they receive recommendations from the TSC contrary to CAVs that receive the orders. To enhance the performance of CV applications at low penetration rates, the authors of [60] proposed a new method to estimate the speeds and positions of CVs and non-connected vehicles. The proposed method utilizes the information collected from CVs and the speeds and flows of conventional vehicles collected from loop detectors. Thus, the proposed method estimates the forward movement of conventional vehicles based on collected data from CVs and sensors. Obtained results showed that estimation error increases as the CVs penetration rate decreases. Furthermore, estimation error decreases as the traffic demand decreases. For CVs penetration rates above 20%, the CVs-based TSC strategy outperformed the commercial EPICS adaptive control in terms of minimizing travel time delay and the number of stops. EPICS is an adaptive control system for individual intersection optimization developed by PTV Group. It calculates signal program parameters every second based on real-time traffic conditions, and depending on current conditions system decides whether the phase needs to be adjusted [61]. Results presented in [62] also indicated that the penetration rate of CVs above 20% improves traffic operations compared to existing approaches. The authors developed algorithms for mixed flow traffic state estimation, where the mixed traffic flow is contained of CVs and conventional vehicles without communication capabilities (both types of vehicles are HDVs). The CVs and sensors provided information about traffic, and the proposed methodology adjusted signal timing accordingly. Testing in a simulated environment on five scenarios indicated that even at the 10% penetration rate of CVs, the number of completed trips increased by 3.2%. From the above mentioned facts, it can be observed that the penetration ratio of CVs affects the traffic flow dynamics, although the CVs only provide onboard data. Being able to operate independently of the driver suggests that the CAVs could be used as the actuators in mixed traffic flows executing orders obtained from the TSC. Results from [21] indicated that the increase in road capacity happens gradually before the penetration rate of CAVs reaches 30%. Road capacity growth rate is mainly determined by the CAV capability on the desired time gap when the CAV penetration rate is over 30%. The traffic system performance improves as the percentage of CVs and CAVs increases. Thus, because their current market penetration rate is low, improving TSC is still challenging for traffic engineers to open new research/application areas. The use of CVs and CAVs is expected to have advantages such as increased road capacity, traffic safety, and efficiency. As mentioned above, acting as mobile sensors, CVs will provide rich real-time information to the TSC system.