Publisher Summary This paper describes the development of a driving-support system for reducing ride discomfort of passenger cars. It was demonstrated that it is possible to estimate ride discomfort caused by speed fluctuations of an automobile using longitudinal acceleration and jerk of it. Imitating human response to the speed fluctuations of an automobile, a ride discomfort index and a method to estimate the RDI from longitudinal acceleration and jerk in real time was proposed. Furthermore, a driving-support system to inform the driver of the ride discomfort index (RDI) in real time by visual and auditory means was developed. With the help of the system developed, a driver can modify his driving method in response to the RDI, thus reduce ride discomfort and provide a passenger-friendly driving. Using a microcomputer, the system evaluates the RDI and informs the driver of the RDI in real time. With the help of our system, the driver can improve his driving skills and modify his/her driving method to reduce ride discomfort. However, there still remain many topics about the improvements of the system.