Energy conservation is one of the hot topics within the domain of traffic problems. It is well known that shortening the distance between vehicles reduces the aerodynamic drag of the lagging (or following) vehicle and leads to energy savings, which benefits the drivers. Recently, systems have been developed in which trucks or vehicles travel in a platoon with reduced headway from the preceding vehicle by using automated driving or driver assistance systems. The objective of the present study is to investigate how human factors, such as driving style, a driver's condition, or a driver's personal characteristics, influence the decision of a driver to close the gap with a preceding vehicle and obtain the benefit of aerodynamic drag reduction. We developed a realistic experimental paradigm for investigating the relationship between distance and several factors including the driver's personal characteristics and the size of preceding vehicle. Our experimental setup made use of real vehicles on a test track, as opposed to a vehicle simulator. We examined behavior of subjects that drove the following vehicle as well as subjects that sat in the passenger seat in the following vehicle. The experimental results demonstrate that all subjects attempted to reduce the distance to the preceding vehicle in order to gain the benefit. Based on the experimental and questionnaire results, we conclude that there are relationships between the category of subjects and subject's following distances.