This paper investigates the effects on response time of a forward collision event in a repeated-measures design. Repeated-measures designs are often used in forward collision warning (FCW) testing despite concerns that the first exposure creates expectancy effects that may dilute or bias future outcomes. For this evaluation, 32 participants were divided into groups of 8 for an AA, BB, AB, BA design (A= No Warning; B=FCW alert). They drove in a high-fidelity simulator with a visual distraction task. After driving 15 min in a nighttime rural highway environment, a forward collision threat arose during the distraction task (Period 1). A second drive was then run and the forward collision threat was repeated again after ∼10 min (Period 2). The response times from these consecutive events were analyzed. The results indicated there was differential carryover and significant Period 1 vs. 2 effects that diluted the magnitude of difference between FCW and No Warning treatments relative to first exposure only. Also, there was a non-significant trend suggesting that previous trials with FCW can slow responses on later trials with no FCW. The findings suggest that driver warning HMI evaluations should exercise care in using repeated exposures to low-probability critical events. Researchers should explore carryover effects when repeated-measures designs are used, or else avoid repeated exposures to low-probability, unexpected critical events altogether.