How do people interleave attention when multitasking? One dominant account is that the completion of a subtask serves as a cue to switch tasks (e.g., Salvucci, 2005). But what happens if switching solely at subtask boundaries led to poor performance? In this paper, we investigate how drivers allocate their attention to a secondary phone dialing task while driving. We use a computational model to explain why we expect a particular pattern of task interleaving. These predictions are collaborated with empirical data of how participants dialed a UK-style number while driving.