# Design and Implementation of a Smart Traffic Signal Control System for Smart City Applications
## 1 Introduction  
The population in the world has reached 7.7 billion in 2019, an increase of 1 billion since 2007, and the size of the global population would stand between 8.5 and 8.6 billions in 2030 [1]. Population growth causes an increase in the number of vehicles, and a lack of effective traffic management would lead to huge economic lost such as energy consumption, greenhouse gas emissions, and time lost. For example, the United States lost 5.5 billion hours of time due to traffic congestion from 2000 to 2010 [2]. With the continuous population growth, traffic congestion has become one of the biggest obstacles restricting the city economic development, it results in high consumptions of fuel, increases the cost of commutes, and also pollutes the environment.  
In order to reduce the traffic congestion and improve the public transport efficiency, infrastructures which enables smart transport services would be the key factor for the success of intelligent transportation systems (ITS). For example, day-1 and day-1.5 services proposed in the cooperative ITS (C-ITS) in H-2020 [3] demonstrate a trend in the development of ITS. The development of cooperative intelligent transportation systems, such as transit signal priority (TSP) for public transport, emergency vehicle signal preemption (EVSP), and adaptive traffic signal control (ATSC), is considered as the one of the important indexes in smart and sustainable cities. Among these services, the key technology isvehicular network, which enables vehicles equipped with an on-board unit (OBU) exchange real-time information with roadside unit (RSU) and other OBUs, achieving the communication scheme of vehicle-to-everything (V2X). Two vehicular network communication protocols are available including 802.11p [4] and LTE-V2X [5], which are defined by IEEE and 3 GPP associations, respectively.  
A traffic signal controller, as the most important part of infrastructure in smart city transportation, is the main coordinator for the urban traffic flows. Traditionally, three types of traffic control strategies are adopted including pre-time (predefined signal plan), actuated (triggered signal control), or ATSC (adaptive traffic signal control) depending on the traffic assignment strategy decided by traffic management authority. However, these approaches do not fulfil the intelligent transportation systems requirements in the smart city, such as V2X-based applications proposed in a C-ITS project.  
In this paper, an integrated RSU for smart traffic control system (STSC) is designed and implemented to support multi-modal V2X-based applications including emergency vehicle signal preemption, transit signal priority system, adaptive traffic signal control, and pre-time signal control, eco-driving supporting, road work, and incident alarm message broadcasting. The features of the STSC system are the integration of multi-modal applications, modular design, real-time traffic control algorithms, and traffic data collection. The proposed STSC system is designed by a unified system architecture with an RSU middleware which organized by modular functional design. Moreover, it is backward compatible with traditional traffic signal controller so that these applications can be fast and cost efficiently deployed without replacing legacy infrastructure.  
The remainder of this paper is organized as follows. Background knowledge of ITS applications and related works discussion are illustrated in Section 2. The details of the proposed STSC system and algorithms are presented in Section 3. System implementation and experiment results are illustrated in Section 4. Finally, the concluding remarks and some suggestions for future works are presented in Section 5.