Powered wheelchairs play a vital role in bringing independence to the severely mobility-impaired. Our robotic wheelchair aims to assist users in driving safely, without undermining their capabilities or curtailing the natural development of their skills. An important research question is to determine the conditions under which shared control is most beneficial. In this paper, we describe an experiment, where a distracting secondary task caused the majority of participants to crash the wheelchair when driving without assistance. However, when they were assisted by our collaborative controller, not only did they drive safely, but they also increased their performance in the secondary task. We demonstrate that a degree of shared control is beneficial even to proficient drivers under certain circumstances, for instance when they are under a heightened workload.