# Review of Autonomous Intelligent Vehicles for Urban Driving and Parking
## 3 The Concepts of Autonomous Driving
### On the Road  
While traveling on the road, there are several actions that an autonomous vehicle would perform, such as lane keeping, lane changing, overtaking, and obeying traffic rules. For fully autonomous driving, lane keeping is the evolution of a lane-departure warning system. While the idea of detecting whether a vehicle has drifted into another lane unintentionally is the same for both systems, the lane-keeping system corrects the vehicle's direction to keep it within its lane [78; 79]. Early systems corrected vehicle direction by differential braking. Still, current systems actively control steering to maintain lane centering [78] by taking the dynamic and kinematics model of the vehicle into consideration to control the lateral motion of the vehicle [79; 80; 81; 82; 83; 84]. Such a system is a great tool to prevent off-the-road crashes [85]. Studies have shown a significant reduction in driver injury crashes with cars equipped with such a system compared to cars without this system. A lane change is described as a maneuver that involves a deliberate and substantial shift in the lateral position when traveling in the same direction associated with simple lane change, merge, exit, pass, and weave maneuvers [86]. Such actions will require a suitable time gap to avoid a frontal collision crash [87]. A lane change is usually carried out due to the following: (1) to travel at a faster lane and (2) to obstacle avoidance. For an autonomous vehicle, other than assessing the lane change risk by checking surrounding vehicles, it should control the vehicle to complete the intended lane change if there is no risk involved [88]. Information such as the position, velocity, and acceleration of surrounding vehicles to generate the risk analysis can be based on computer vision systems, such as those used in lane-departure warning systems, a precise measurement system (inertial navigation aided by a global navigation satellite system (GNSS) in conjunction with high-resolution maps) [89] or a V2V communication system [90]. For normal lane changing,the collision avoidance problem is formulated as an optimization problem where the time for completing the lane changing maneuver is minimized by selecting the appropriate lateral and longitudinal control inputs. After the collection of this information, the vehicle then computes the trajectory of each neighboring vehicle then compares this with its own trajectory to determine if any adjacent vehicle poses a safety risk [89]. In an emergency lane-changing situation such as obstacle avoidance, lateral and longitudinal control inputs are calculated to minimize the longitudinal distance between the vehicle and the obstacle [91]. In such a scenario, geometric characteristics of obstacles are also taken into consideration for the planning of trajectory [92]. Finally, steering control would perform the maneuvering action based on the planned course as closely as possible [92; 93]. Such a system will potentially reduce human errors such as inaccurate estimation of the surrounding traffic or illegal maneuvers. The autonomous overtaking procedure consists of a combination of lane keeping and lane changing. Three consecutive maneuvers could begin with lane changing followed by traveling on a straight path (lane keeping) parallel to the vehicle to overtake and, again, a lane change, which has to be planned and coordinated [94]. This procedure can be divided into two stages. The procedure begins with checking if overtaking can be carried out [95]. Essential factors to consider when performing the maneuvers include the following: a safe distance to the vehicle to be overtaken; an adequate period for each lane-change maneuver, accounting for varying road widths; a smooth and comfortable lane-change trajectory; and safely returning to the original lane or maintaining a safe distance from a vehicle ahead when the overtaking maneuver cannot be executed [96]. Information of surrounding vehicles can be obtained as discussed earlier. The second stage is the performing of the maneuver.