This paper deals with a safety of automatic platooning trucks in energy-saving intelligent transportation system (ITS) project. It needs to develop a platooning-specialized brake system for keeping safety of experimental development. A secondary braking system and an adjusting device of braking force have been developed. A virtual experimental platform was realized by integrating truck simulation software and platooning cruise control algorithm into a universal driving system. By using the experimental platform, on the assumption of emergent stop of preceding truck during automatic platooning, driver behaviors for emergent avoidance in following truck are studied. Especially, braking operations are analyzed in different maximum braking decelerations of the following trucks. The brake system was improved by this development. Moreover, the safety of the automatic platooning experiment was extremely improved.