Systems for detecting and notifying drivers of vehicles in adjacent lanes have attracted considerable interest as a means of reducing driver workload which tends to increase when changing lanes. This paper presents the results of a study that investigated the relationship between the timing for issuing a notification about a vehicle detected in an adjacent lane and the related effect on drivers’ trust in the system. The purpose of this study is to improve the perceived effect and value of such notification systems. Subjective evaluations were conducted using an experimental vehicle and a vehicle detection method employing a rear-mounted camera. Based on the evaluation results obtained, an investigation was made of a suitable timing for a system that notifies drivers about a rearward approaching vehicle when changing lanes.