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Access to current workload is regarded as crucial for new driver assistance systems that aim at optimising drivers' workload in addition to their specific functionalities (e.g. coordinate, prioritise information). As stress factors change dynamically within the driving context, workload assessment should be dynamic and continuous rather than discrete. Here, the effects of two dynamically changing stress factors (traffic density and different demands within the manoeuvre 'entering the motorway') on workload are examined. The aim of the study is to analyse whether the influence of these stress factors on workload can be measured using a continuous subjective rating method. The results demonstrate that the use of the subjective rating method allowed the assessment of detailed information on the influence of varying stress factors on subjective strain. Furthermore, t.he significant interaction between the stress factors demonstrated the importance of precise description of the relations among different dynamically changing stress factors and resulting subjective driver strain. By integrating the current findings with results from the previous studies, the wider aim of the research, the development of a qualitative and quantitative model of stress and strain, is discussed.