Numerous aspects of driving performance seem compromised in people with Parkinson's disease (PD). Measures of cognitive impairment consistently correlate with poor driving simulator performance in this population; however, the effects of specific cognitive difficulties on discrete aspects of driving behavior have not been investigated thoroughly. Previous studies have demonstrated that people with PD exhibit difficulties internally cueing cognitive processes. This study examined the impact of impaired internal cueing on specific driving behaviors. A simulator measured the driving behavior of 18 current drivers in the mild-to-moderate stages of PD and 18 matched controls. Participants navigated through different driving conditions where the opportunity to use internal and external cues was manipulated. People with PD exhibited difficulties using internal cues to regulate driving behavior around traffic signals and curves. Instead of using internal cues, participants with PD were more reliant on external cues to regulate driving behavior. They were also less able to adapt their driving behavior to suit driving conditions. Because all participants with PD were current drivers in the mild-to-moderate stages of the disease, findings challenge the widely-held assumption that cognitive difficulties only impact on driving performance in the moderate-to-severe stages of PD.