# Safe Driving Distance and Speed for Collision Avoidance in Connected Vehicles
## 1 Introduction  
Road traffic accidents kill approximately 1.3 million people each year, according to the World Health Organization (WHO), prompting the United Nations General Assembly (UNGA) to set an ambitious target of halving the casualties of road traffic accidents worldwide between now and 2030. One of the primary causes of traffic accidents is tailgating--tailgating consists in having a vehicle following another vehicle too close to brake safely if the followed vehicle stops unexpectedly, as shown in Figure 1). To avoid tailgating, there is a need for a smart technique based on Intelligent Transportation System (ITS) and Internet of Vehicles (IoV) [1; 2; 3; 4; 5; 6; 7; 8] to determine the ideal inter-vehicle space referred to as \"Assured Clear Distance Ahead\" (ACDA) [9; 10] or Safe Driving Distance for connected vehicles on the smart roads and highways. Therefore, the existing ITS and Autonomous Emergency Braking (AEB) system [11; 12; 13] incorporated in smart vehicles require a good understanding of an ideal gap between connected vehicles [14] that will not result in a collision if the followed vehicle stops abruptly. Having a good knowledge of an ideal safe driving distance for connected vehicles will solve the following problems:  
1. Forward collision due to tailgating. 2. Traffic optimization: leaving too much gap between two connected vehicles will result in poor utilization of the roads and highways and consequently create unnecessary traffic on crowded roads. 3. Over speeding (for the following vehicle) and under speeding (for the followed vehicle) on a busy highway. The determination of ACDA by classical means is unsuitable for Intelligent Transportation Systems (ITS), smart cities, and smart roads [10]. ACDA depends on many variables, such as the vehicle's speed, the length of the two vehicles, the condition of the tires, the type of road, and the weather condition; this makes the determination of the ACDA using traditional means very difficult. IoV technology [1; 2; 3; 4; 5; 6; 7; 8], as leading-edge technology, has been advancing for human's need for a safe ITS, which provides a wealth of possibilities with most applications and services focusing on the safety of users and pedestrians [15; 16; 17]. IoV can be considered as the next generation ITS. Among the popular applications and services provided are automatic speed limit violation detection [18; 19; 20], traffic optimization and management [21; 22], road lighting control and monitoring [4; 23], transportation pollution monitoring [22; 23], and infotainment [24]. In IoV, an intelligent vehicle (V) has an On-Board Unit (OBU) which extends the vehicle's onboard computer's capabilities. The simplicity of conversion of a non-IoV vehicle to an IoV vehicle can be done by incorporating the OBU into a non-IoV vehicle; this simplicity of converting a non-IoV vehicle into an IoV vehicle makes the deployment and the transition toward the next generation ITS easy. IoV and Connected Vehicles (CV) consist of several entities [8; 24; 25]. The main entities are the intelligent vehicle (V), the road-side unit (RSU) (which forms the Infrastructure), the cloud servers (C), the smart grids (G) required to charge the Electric Vehicles (EV), the surrounding smart building (B), the home of the owner or the driver of the vehicle (H), the onboard sensors inside the vehicle (S), and the smart road traffic devices (R). The intelligent vehicle V may or may not be self-driving or driverless. There are several communications  
Figure 1: Vehicle tailgating. The red vehicle tailgates the blue one. scenarios in IoV, such as V2V, V2I, V2S, V2P, V2C, V2H, V2B, V2G, and V2R; these scenarios govern the communication between the vehicle V and the Infrastructure (interconnected RSUs), the onboard Sensors S, the Pedestrians P using IoV-enabled wearables, the Cloud servers C, the Home H, the surrounding buildings B, the smart grids G, and the road devices R respectively [14; 26].