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---Mental Workload is a demand placed upon humans.
Defined in term of experienced load
---With experienced load, workload is not only task-specific, it is also person-specific (Rouse et al.1993). Not only individual capabilities, but also motivation to perform a task, strategies applied in task performance, as well as mood and operator state, will affect experienced load.
Parkes (1991) defines the primary task of the driver as " safe control of the vehicle within the traffic environment".
Car driving is a dynamic control activity in a continuously changing environment. The driving task is not only influenced by the drivers themselves, but also by the behavior of other traffic participants . It is not an easy task to model driver behavior.
A useful model offered by Michon (1971, 1985) and Janssen (1979) takes more into account than just “safe control”. In their model, car driving is described as a complex task with processes at a minimum of three hierarchical levels .
A model of the main task of the driver is useful in mental workload research in driving
Three hierarchical levels in Michon and Janssen’s model
Top level- --strategic level
strategic decisions are made, such as the choice of means of transport, setting of a route goal, and route-choice while driving
Intermediate level- --the maneuvering level.
reactions to local situations including reactions to the behavior of other traffic participants, take place
Lowest level- -- the control level.
the basic vehicle-control processes occur, such as lateral-position control. At this level automatic processes occur, while a level higher controlled processing is required.
Driver-performance measures can be connected to the three levels.
A highly diagnostic measure is only sensitive to variations in workload in specific computational processes. Therefore diagnosticity restricts sensitivity . Also, diagnosticity presupposes selectivity .
Interaction between characteristics can be expected to be particularly high in the case of secondary tasks . For example, a diagnostic measure that is sensitive to secondary-task performance can only be reliable if primary-task intrusion of the secondary task is low.
The most desirable characteristics of measures of mental workload are high sensitivity, preferably in a wide bandwidth, high reliability and low primary task intrusion. Diagnosticity can also be of major importance, in particular, if a certain stage of information processing is suspected to be affected.
the number of errors made, speed of performance or reaction time measures are frequently used as primary-task performance measures.
Most frequently used as secondary tasks are choice reaction-time tasks, time estimation or time-interval production, memory-search tasks and mental arithmetic (see O'Donnell & Eggemeier, 1986, Eggemeier & Wilson, 1991 and Wickens, 1992, for overviews)
Reference tasks are standardised tasks that are performed before and after the task under evaluation and they mainly serve as a checking instrument for trend effects. Changes in performance on reference tasks can indicate effects of mental load of the primary task.
Traffic studies performed by Dick de Waard (1964)
2. Studies in which driver state is affected
Studies divided into two categories: Triprolidine, motorway Triprolidine, ringroad On-the-road Anti- histamine Alcohol, motorway Alcohol, ringroad Fatigue, motorway Fatigue, ringroad On-the-road DREAM selected load condition(s) test environment study
Properties of self-report measures high high operator acceptance low low implementation requirements low low primary-task intrusion high (?) high Reliability (?) prob. high selectivity low low diagnosticity D, (B-C) (D-)A1, A3-B sensitivity (Region) Activation RSME Property Measure Summary of properties of self-report workload measures .
Properties of primary-task performance measures high high operator acceptance low high implementation requirements none none primary-task intrusion high high Reliability (low) (low) selectivity low low diagnosticity D, B D, B sensitivity (Region) SDSTW SDLP Property Measure Summary of properties of primary-task workload measures .
Properties of secondary-task workload measures high high high operator acceptance high high high implementation requirements low low low-moderate primary-task intrusion high ? high Reliability moderate (?) ? moderate (?) selectivity moderate-high moderate (?) low-moderate diagnosticity A3-B ? D-A1, A3-B sensitivity (Region) Mirror checking (frequency) Mirror checking (duration) Delay in car following Property Measure Summary of properties of secondary-task workload measures .
Properties of physiological workload measures high-moderate 1 high high high moderate operator acceptance high high moderate moderate high implementation requirements 1 low low low low primary-task intrusion ? high high high moderate-high Reliability ? moderate-high low-moderate low low selectivity high low low low low diagnosticity A3(?) A3 D, B D, B D-A2 sensitivity (Region) Eye movements (fixations/minute) ECG: .10 Hz ECG: HRV ECG: HR EEG Background Property Measure
Conclusions related to driver workload measures
The most important characteristic of a measure is its sensitivity to workload. Workload reflecting the individual reaction to task demand.
A highly diagnostic measure is selectively sensitive, in particular, diagnosticity has strong links to the multiple-resource theory ,on the other hand, diagnosticity restricts sensitivity to a certain bandwidth
Of primary importance in mental workload research is the region of performance . Optimal performance with low mental workload is obtained in region A2 . Not only driver state, but also the complexity of a task or of the driving environment may cause an increase in mental workload.
Characteristics of measures Workload in 6 regions and sensitivity of different measures to driver mental workload
Conclusions related to driver workload measures
Optimal performance of driver is in region A2. If the traffic environment becomes more complex, drivers have to exert (task-related) effort to maintain performance.
Once the driver is no longer able to successfully act against detrimental effects of increased task demands by means of effort compensation, the level of performance will drop to be in Region B .
With further increases in demand, performance will further drop until a minimum level is reached and Region C is entered. In this region, performance measures are insensitive, nor will measures of heart rate reflect workload. Only scores on the activation scale may indicate overload.
None of the measures alone is sufficient to reflect mental workload, Only in combination with self-reports and/or physiological parameters can a conclusion about workload level be made.
Mayser, C., Piechulla, W., Weiss, K.-E., & König, W. (2003). Driver workload monitoring . In H. Strasser, K. Kluth, H. Rausch, & H. Bubb (Eds.). Quality of Work and Products in Enterprises of the Future. Proceedings of the 50th-Anniversary Conference of the GfA and the XVII Annual ISOES Conference in Munich, May 07th-09th, 2003 (pp. 41-44). Stuttgart: Ergonomia.
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KAREL A. BROOKHUIS* and DICK DE WAARD(2002), On the assessment of (mental) workload and other subjective qualifications , ERGONOMICS, 2002, VOL. 45, NO. 14, 1026 ± 1030
De Waard, D. (1996). The measurement of drivers' mental workload . PhD thesis, University of Groningen. Haren, The Netherlands: University of Groningen, Traffic Research Centre.