This dissertation presents different methods for indirect assessment of driver’s cognitive load, where the performance of a secondary task is considered as an indicator of changes in cognitive load. It is centred on the standardised Detection-Response Task (DRT) method, a method for the assessment of cognitive load and its effect on driver’s attention, which has gained in popularity in recent years mainly due to its relatively simple implementation. The DRT is based on the detection of visual or tactile stimuli, where the response time and success hit rates are considered as indicators of changes in cognitive load. However, since driving is a visual-manual task, it relies mostly on the vision and touch senses. Therefore, the use of an alternative sense, such as hearing, could be more appropriate. This dissertation presents an analysis of the sensitivity of all three DRT stimuli modalities (visual, tactile and auditory) to changes in cognitive load. It also presents the development and validation of a new auditory version of the DRT method, which is based on the detection of sound stimuli. Since the DRT method is based on the performance of a secondary task while driving, the intrusiveness of the method and its effect on the driver and driving performance are also explored.