I. Introduction
With increasing number of vehicles, societies have faced with significant challenges in congestion and traffic crashes. A recent report from World Health Organization[1] indicated that annually 1.24 million people die due to traffic crashes. China has over 59,000 people died in motor vehicle crashes in 2015 [2]. According to police-reported crashes, approximately 90% of them can be attributed to inattention, following too closely, or a direct result of inappropriate driving behavior [3]. Drivers may keep small headways because they maintain an acceptable level of risk in their driving behavior [4]. Even an alert driver may fail to maintain a safe headway from the car ahead because of a perceptual inadequacy in estimating headways [5]. Therefore, maintaining a safe headway while driving is considered one of the most key determinant of defensive driving.