We investigated the effects of secondary working memory tasks that loaded either visuo-spatial working memory or verbal working memory (phonological loop) and which required either rehearsal or executive processes involving stimulus manipulation. The effects of the secondary tasks on driver look-out behavior and driving performance were assessed. Preliminary studies were conducted to select tasks that resulted in similar levels of accuracy and perceived difficulty across modalities (visuo-spatial, verbal, rehearse, and manipulate). Piloting and the preliminary studies were also used to evaluate different visual tasks and to select a visual task that could not be encoded verbally. Results of the study reveal that driving performance is significantly impaired while performing a secondary manipulation task than performing a rehearsal task of equivalent difficulty. The study finds that visuo-spatial and verbal secondary tasks produce the same level of interference with overall driving performance.