As computers and other information technology move into cars and trucks, distraction-related crashes are likely to become an important problem. This paper begins to address this problem by examining how alert strategy (graded and single-stage) and alert modality (haptic and auditory) affect how well collision warning systems mitigate distraction and direct drivers attention to the car ahead when it unexpectedly brakes. We conducted two experiments in which drivers interacted with an in-vehicle email system and a collision warning system signaled a braking lead vehicle. The first experiment showed that graded alerts led to a greater safety margin and a lower rate of inappropriate responses to nuisance warnings. A second experiment focused on attitudes toward the collision warning system and found that graded alerts were more trusted than single stage alerts and that haptic alerts, a vibrating seat in these experiments, were perceived as less annoying and more appropriate. Graded haptic alerts offer a promising approach to developing context aware computing in a safety-critical application.