Peaks in workload while driving might have immediate safety implications. The Peripheral Detection Task (PDT) has shown to be a sensitive objective workload measure. Drivers have to respond to the onset of a peripherally presented simple visual stimulus (red square or LED) by pressing a finger switch, on average, once every 4 seconds. The more demanding the task, the more cues are missed and the longer the response times to the PDT. This chapter focuses on measuring driver’s cognitive workload by the PDT and shows the method’s sensitivity for measuring variations in workload dependent on road and driving conditions or driving while interacting with in-vehicle devices. These examples include both driving simulator and on-the-road studies and their results favor the ‘cognitive tunneling’ hypothesis (in contrast to perceptual tunneling), viz. that the PDT especially measures the (cognitive) selectivity of attention.