# A Hierarchical Control System for Autonomous Driving towards Urban Challenges
## 1 Introduction  
According to a survey reported in 2018 [1], the number of on-road accidents is more than 90% caused by personal inadvertences. Self Driving Cars (SDCs) are a prominent method for the human to solve collisions, frustrating situations, and decrease energy expenditure. Although SDCs have breakthroughs in trial stages, the governments are still hesitant to commercialize due to the worry about safety in public transport. Recently, SDCs technology has been developed with a large number of studies, especially in the epoch of deep learning. Besides, the biggest challenge of SDC technology is that it needs to handle all circumstances on the road like expert drivers. Within 2016, the Society of Automotive Engineers (SAE) defined various criteria of the autonomy level of SDCs [2]. Firstly, it starts at level 0 with comprehensive human handling. Level 1 can support the driver by operating either lane-keeping assist systems or adaptive cruise control. Level 2, which assists human drivers in steering and acceleration control, operates similarly to level 1 but is more intelligent in specific driving scenarios. At level 3, the autonomous module can conduct perception tasks like object detection and tracking to intervene in the control tasks, this level requires frequent human override for vehicle control. At level 4, the operation can manage most of the circumstances on the road; nevertheless, human override is an option to handle unexpected scenarios. To the best of our knowledge, at this level, an exclusively self-driving company named Waymo is ready to deploy inpublic transportation over numerous million miles on city roads in fall 2018 [3]. Level 5 is designated to a fully autonomous vehicle. The Defense Advanced Research Projects Agency (DARPA) started an autonomous driving challenge in 2002 [4]. This challenge can be considered as the first well-known experiment for SDCs, and it opened the imagination of humans about what the autonomous vehicle can do. The first competition was held and awarded one million dollars for the winner on March 13, 2004 [4]. However, in the first contest, no team could complete the challenge. The second DARPA grand challenge was held on October 8, 2005 [4]. Five teams of twenty-three could accomplish the autonomous driving mode. [PERSON] robot of Stanford won the competition followed by [PERSON] of Carnegie Mellon. Later, on November 3, 2007, the DARPA established a competition called DARPA Urban Challenge, which operated on busy roads and needed much more intelligent motion planning and perception [4; 5]. This competition also required vehicles to drive automatically in several locations with GPS-denied environments. The winning team was [PERSON] of Carnegie Mellon University, and the Stanford Junior team came second. After the DARPA competition, many autonomous vehicle challenges and tests have been organized. The European Land-Robot Trial (ELROB) [6] has been conducted from 2006 to the present. From 2009 to 2013, the Intelligent Vehicle Future Challenge in China was held [7]. In South Korea, the autonomous vehicle challenge was firstly announced by Hyundai cooperation in August 2008 [8]. The most recent competition was announced in March 2018 and organized in July 2019. The organizer built a testing ground called K-city to mimic the urban area. From that time of the final DARPA Urban Challenge, many successful stories in the industry of autonomous driving in urban environments have been reported. Specifically, Google's self-driving car had driven automatically over 140 thousand miles in California in 2010 [9]. About four years later, Google successfully developed an autonomous vehicle that can be driven up to 40 kilometers per hour in the urban area. In 2016, Google's autonomous driving vehicle was renamed to Waymo [3]. From that moment, Waymo has been getting many achievements and leading the race to get autonomous cars into public transportation. Around the end of 2015, the Tesla company induced an autonomous driving vehicle called Autopilot to market [10].