This study proposes a method to reduce traffic accidents caused by distracted drivers by investigating the optimal deceleration intensity and initiation timing of gentle deceleration for alerting, differing from conventional systems such as FCW and AEB. Experiments with expert drivers revealed that a deceleration of over 0.075G redirects a distracted driver's attention, while under 0.125G is tolerable for following vehicles. Additionally, the reaction time for around 0.1G deceleration is mostly within 1.6 seconds. Therefore, it is effective to start gentle deceleration of 0.1G from TTC of 3.4 seconds to alert a distracted driver while considering the impact on the following vehicle.