Driving simulator studies are usually designed to collect repeated measures on each participant. This design has different implications for the power of within and between-subject effects that needs to be recognized by researchers. The power for between-subject variables decreases when additional measures are collected on the within-subject variables. However, the power for the main and interaction effects of the within-subjects variables increases as more observations are collected on one participant. If the main interest of the experiment is a between-subject effect, such as age, then a completely randomized design can provide the same power with fewer participants. Through a case study, this paper demonstrates how the power changes between a repeated measures design and a completely randomized design.