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1. Introduction
Nowadays, even though vehicle driving assistive technology has been assembled in the premium cars on a large scale, the concept of the self-driving car has constantly appeared in various news and reports (Ross, 2014; Ackerman, 2016a; Harris, 2016; Hassler, 2017; Computerworld, 2012). However, due to few literatures (Van Arem, 2014; Ibanez-Guzman et al., 2012) reviewing the key technology of self-driving car, many problems are ambiguous, i.e., what is the progress of self-driving car? Is the large-scale commercial use of self-driving car helpful to the human society? To address the above problems, this paper investigates the key technology of self-driving car, discusses its implementation obstacles and summarizes the whole picture of the technology progress, which are expected to be helpful for the reader to understand the commercial use of the self-driving car.

2. The key technology of a self-driving car
The automatic control, architecture, artificial intelligence, computer vision and many other technologies are integrated into the self-driving car, which is a product of the highly developed computer science, pattern recognition and intelligent control technology. From a different viewpoint, the technology of self-driving car represents the level of scientific research and industrial strength of a country. However, few papers have surveyed the technology process of a self-driving car due to its complexity. In view of this problem, this paper proposed a new classification, for the key technology of self-driving car according to the function implementation, which will make the description easy and clear.

Compared with manual driving, it is the key characteristic of a self-driving car that using automation equipment to replace the human driver. Based on this characteristic and functional requirement on driving and on-board equipment module, the core technology of self-driving car is classified into four key parts, which are known as car navigation system, path planning, environment perception and car control. The detailed description is provided in the following sections.

3. The main research institutions of self-driving car

The idea of self-driving car is same as of the vehicle invention. Until recent years, the self-driving car achieved the laboratory applications level with the development of sensor technology, computing technology and mobile internet. The self-driving cars that appear in the self-driving car trial represent the top level of this industry. The main research institutions can be found out by introducing several competitions.

The first challenge is the DARPA Grand Challenge, which was held in (DARPA, 2007)004, 2005 and 2007. In 2004, 15 teams joint the first challenge that was held in the Mojave Desert region of the USA. The goal of the challenge was passing through 240-km desert; however, no team could complete the task. Until 2005, five teams completed the task smoothly with the champing completion time of 6 hours and 54 minutes. Different from the previous two challenges, the third DARPA Challenge held in 2007 was an urban challenge, in which the test environment was an urban road. Six teams completed the competition requirements. The Stanford Racing team won the second good result. The teams from Virginia Tech, Blacksburg, Virginia and MIT, Cambridge, Massachusetts came in the third and fourth, respectively.

4. The trend and discussion of self-driving car

From the previous sections, we know that the experimental prototypes of the self-driving car have been developed, some typical self-driving cars have been tested on more than one million kilometers, and the test for driving license for a self-driving car has been issued by some states in USA. However, we should know that the realization of self-driving car is not only influenced by the self-driving car technology, but also by the vehicle cost, social habits, human psychology, law and so on; therefore, there will be a long way to go for the entire commercialization of self-driving car.

At present, self-driving car configures many sensors that are non-existent in a traditional car. For instance, the lasers and vision sensors used for environment perception are the typical key sensors. These sensors are expensive and have high requirement on using condition. Moreover, the service lifetime of these sensors will be greatly reduced when they are installed in the moving car. Meanwhile, the reliability is also worrying.

In addition, it needs time for the social habit of humans to accept the self-driving car, for instance, the share of self-driving car among the people, the development of automatic supporting facilities for self-driving car energy compensation, the human psychological acceptance degree for self-driving car, and the development of law to deal with the traffic accident on self-driving car.