Intelligent transport system (ITS) can induce improvement in road safety, as long as acceptability by user's population and adequacy with driving task demands have been checked. The Human-Centred Design process allows taking into account the diversified driver's population needs and functional abilities in addition to the complexity of the driving context, for setting up design recommendations and for the development of methodologies for safety assessment. Evaluation of the driver's mental workload is an important parameter, complementary to objective ones such as control of the vehicle and driver's visual strategies. This paper describes experiments conducted in the framework of the European project AIDE aiming at validating the DALI (Driving Activity Load Index), a tool set up to allow the evaluation of driver's mental workload while driving, with or without the support of in-vehicle systems. The main results and conclusion from the conducted analysis are presented.