This work aims to contribute to the analysis of the powered wheelchair driving task. We use for this purpose the optimal preview control model OPCM. We study, in particular, the influence of the driving speed on the driver's visible distances measured through an eye tracking system. Experiments were carried out with a panel of healthy persons on a powered wheelchair driving simulator. A case study is given for some subjects whose visible distances were short thus inducing dropouts in their respective OPCM-computed trajectories.