The driving task is basically a feedback control activity performed by the driver [24]. Driver, vehicle, and environment (DVE) form a compound and interact with each other. Perception, processing and interpretation of driving related information from the driver’s environment leads to vehicle control actions as a response to this information. Since the permanent movement of the ego-vehicle as well as of other road users result in a permanent changing constellation of sensory information [44], a permanent re-assessment of the current situation is necessary. Typical sources of immediate driving-related information are for instance the surrounding traffic, road conditions, traffic lights, pedestrians, and also the state of the ego-vehicle, e.g. speed, position within the lane, etc. Also the driver’s wishes (looking for a parking spot, upcoming left turn, etc.) play an important role in this compound and influence the driver’s actions. Since driving is a complex activity influenced by many factors, it makes sense to subdivide the driving task into more basic building blocks. Geiser introduced a categorisation of driving inprimary,secondaryandtertiary tasks [25, 58]:
– Primary TasksAll actions that are directly necessary for vehicle positioning. This com- prises mainly longitudinal (accelerating, braking) and lateral (steering) guidance with re- spect to the surrounding environment.
– Secondary Tasks All actions that occur in the process of the primary tasks in order to
ascertain safe vehicle guidance under varying environmental conditions. This includes shifting gears, operating windscreen wipers, operating headlights, but also the interaction with other traffic participants, e.g. operating turn signals, hazard lights, horn, etc.
– Tertiary TasksAll actions that are not directly involved in the safe operation of the vehicle,
but serve the satisfaction of diverse driver needs, e.g. for comfort (climate control), enter- tainment (radio, music), or information (traffic information, navigation). Usually these tasks are performed simultaneously to the primary and secondary tasks, i.e. during driving.
Note: Very often, the primary and secondary tasks are aggregated simply to "tasks related to driving" or the "primary task" and the tertiary tasks then become "secondary tasks", or tasks that are not driving-related. In the course of the work we will heavily use the termsecondary task which is to understand in the sense of tertiary tasks.
The primary taskis often described using the 3-Level-Model (also calledHierarchic vehicle
guidance model) by Bernotat [9]. This model has been frequently cited and extended, e.g. by
Donges [43], Michon [108] or Geiser [58]. The model comprises the following levels (cf. 2.1, right):
Navigation On the highest or most abstract level is the navigation task which comprises the selection of an appropriate route with respect to the available network of passable roads. Unexpected events, for example traffic congestion, accidents or incorrect driving actions (e.g. wrong turn) and their implications on the estimated time of arrival, potentially influence the driver’s route choice. The navigation task is typically fulfilled locally or temporally discrete with the driver monitoring the adherence to the chosen route by distinctive points along the road. Michon [108] calls this levelstrategic level where general plans with low action frequency are made. This level requires the most conscious mental activity. Navigation systems aim to support the driver on this level by taking over the task of selecting a route.
Manoeuvring Subordinated to the navigation level is the manoeuvring task which imple- ments the chosen navigational route into concrete driving manoeuvres. The driver must extract and interpret the relevant information from the current traffic situation and derive the appropriate reference variables regarding lane position and velocity. The driver anticipates the progression of these variables and intervenes appropriately in the sense of an open loop control in order to achieve the least possible deviation between the intended and actual course. Typical tasks on this level are lane changes, overtaking, or keeping constant headway. Michon identifies controlled action patterns on this level with a time constant in the dimension of seconds [108].
Stabilisation On the lowest level lies the stabilisation of the vehicle, that means the operation of pedals and steering wheel in order to implement the intended driving manoeuvres. The driver is part of a closed control loop, in which deviations from the normative values must be compensated by appropriate actions. Michon calls this level control level with automatic action patterns and an action frequency in the dimension of milliseconds [108].
Driver Skill-based Behaviour Knowledge-based Behaviour Rule-based Behaviour Identification Decision of task Planning Recognition Association state / task Stored rules for task Feature formation Automated sensori-motor patterns
Sensory input Signals Actions
Rasmussen, 1983 Environment Vehicle Navigation Manoeuvring Stabilization Longitudinal and lateral dynamics Road surface Road network Driver space (road and traffic) Alternative routes Range of safe command variables
Current lane and speed
Donges, 1982 Transportation Task Chosen route, temporal course Chosen command variables: desired lane, speed
Figure 2.1: The 3-Level-models from Rasmussen [132] and Donges [43] related to each other (translated and redrawn from [44]).
As mentioned, this model represents a hierarchic order. The tasks on the upper levels can only be executed if the lower levels do not require mental resources in an exceeding degree, since the cognitive demands increase with each higher level. The more automated the tasks are through practice, the less mental resources are required. If the stabilisation level requires a lot of mental effort as it is the case with driving beginners, manoeuvring and navigation are very difficult. Rasmussen developed a 3-level model describing the performance of a skilled human operator [132, 133], cf. 2.1, left:
Skill-based Skill-based behaviour "represents sensory-motor performance during acts or activities which [. . . ] take place without conscious control" [132], p. 2. This level is charac- terised by highly automatised stimulus-response mechanisms, resulting from long-term training processes. Those skills are most efficient, since they are often anchored in the cerebellum, and do therefore not require conscious attention. Examples are all kinds of basic vehicle operation activities, e.g. steering, clutch use and manual gear shifting, etc.
Rule-based Rule-based behaviour can be described by an "if-then-relation". In a certain traffic situation (e.g. overtaking a slower vehicle) the driver retrieves stored behavioural patterns as formerly experienced in similar situations that have been successfully solved. Those patterns can only be applied, if the appropriate conditions are met (e.g. no oncoming vehicle in the opposite lane, no vehicle overtaking from behind). Thenthe appropriate procedures are selected and executed (e.g. check mirrors, set indicator, accelerate, pass).
Knowledge-based In situations where no known rules can be applied, an action must be consciously chosen. Often there are a number of alternative options to choose from and to weigh against each other. On this level the highest mental activity is necessary. The execution, however, is usually a sequence of rules applied to this unknown situation.
A descriptive example for Rasmussen’s model is driving in a country where the traffic drives on the opposite road side with a car having the steering wheel on the other side as usual. Then skills and rules can often be not applied anymore and driving takes place on the knowledge-based level. Even experienced drivers must then carefully think about every action, e.g. what lane to take after turning. The models of Donges [43] and Rasmussen [132] are closely related, so that they can be condensed into one. Fig. 2.1 shows the relation of the two models.
The perception of the driving scene and the extraction of relevant information is a crucial point in all presented models. The primary perception channel hereby is undoubtedly the visual channel [23]. Through the visual channel, information about the position, velocity and heading direction of the ego as well as other road users can be perceived which is most important for safe driving. Table 2.1 shows an overview of the most relevant variables [162]. Accordingly, Rockwell [138, 150] attributes 90% of all perceived information during driving to the visual channel. However, multimodal information presentation, in particular in case of warnings, has been shown to be superior to single modal information (e.g. [151]).