An alert system is activated in response to an event by positioning a portable electronic device, a cellular telephone or PDA for example, such that a camera of the electronic device is orientated to capture images relating to a visual criterion by which the event is to be detected. Images relating to the visual criterion are processed to detect the event and an alert system is activated if an event is detected. In one application, the portable device is held in a holder in a vehicle and used to detect vehicle lane departure or driver drowsiness.