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A car-following assisting system named the rear window notification display (RWND) was developed, with the aim of improving a driver's manual car-following performance. The RWND presented lead-car acceleration and time headway (THW) (i.e., intervehicle distance divided by the speed of the following car) on the rear window of a lead car, which was driven automatically. A simulator-based experiment with 22 participants showed that the RWND reduced both the mean and standard deviation of THW but did not increase the occurrence of potentially unsafe headways of less than 1 s. The parameter estimation of a common linear car-following model showed that drivers accomplished the performance improvements by adopting higher control gains with respect to intervehicle distance, relative speed, and acceleration. A postexperiment questionnaire revealed that the display was generally not regarded as a distraction nor did participants think that it provided too much information, with means of 4.0 and 2.9, respectively, on a scale from one (completely disagree) to ten (completely agree). The results of this study suggest that the RWND can be used along with Cooperative Adaptive Cruise Control to increase traffic flow without degrading safety.