This paper discusses how to reduce driver's over-reliance on a rear-end collision warning system when an ACC system is active.We propose “gdeceleration meter” which shows the maximum deceleration rate in the last several seconds.By looking at this meter after a relatively rapid deceleration, a driver may understand how the last deceleration was close to the deceleration limit.We conducted an experiment on a driving simulator and showed that driver's reaction to the rapid deceleration of the lead vehicle was not delayed by using the deceleration meter even when a warning was missed.This result suggests that the deceleration meter is effective to manage the risk of rear-end collisions due to over-reliance on the warning system.