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Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks

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Scenario 7–Stop on Red Light

Description

This scenario involves the subject vehicle stopping at a red light. Figure 55 shows the scenario diagram and provides additional details regarding the scenario. Briefly described, this scenario involves the subject driver approaching the intersection and seeing that the light is red, decelerating to a stop, waiting, and then proceeding through the intersection once the light turns green and the lead vehicle goes.

This scenario was divided into three segments (Approach, Stop, and Proceed Through Intersection). The primary bases for parsing the scenario into these segments were that each segment had a different overall driving goal and each had different speed characteristics (table 81).

The crash data related to this scenario indicate several characteristics relevant to the task analysis and configuration of this scenario. In particular, rear-end crashes made up 25.2 percent of all crashes in the 1991 GES data. GES data categorize this type of crash into two subtypes: lead-vehicle stationary (LVS) (16.1 percent of all rear-end crashes) and lead-vehicle moving (LVM) (9.2 percent of same).⁽²¹⁾ Both subtypes are relevant in the current scenario: the LVS subtype is involved when the lead vehicle decelerates to a stop before being struck by the subject vehicle, and the LVM subtype is involved when the lead vehicle becomes struck while it is still moving. The most common causal factors in rear-end crashes included driver inattention (56.7 percent) and tailgating/unsafe passing (26.5 percent). These findings suggest that the tasks associated with the Stop segment are primary sources of difficulty for drivers.

Figure 55 shows the Scenario 7 diagram and lists scenario details.

Just one assumption was made regarding the situational aspects of the scenario. The justification for making this assumption is summarized in figure 55 and more fully described in table 82.

Table 82. Scenario 7–Stop on Red Light assumptions and corresponding justifications.

Scenario Timeline

An approximate timeline showing the key temporal milestones for Scenario 7 was calculated based on vehicle kinematics (see figure 56). These milestones were used to make judgments about the pacing of tasks within segments and to provide a basis for the overall sequencing of certain tasks. Most segments included an interval with a variable time component, which represented intervals that were either long enough to effectively provide unlimited time to perform tasks or of a duration that was determined external to vehicle kinematic factors (e.g., waiting for lead vehicle to go).

Task Analysis Table

The results of the task analysis organized by scenario segment are shown in the task analysis table (table 83). The task analysis results are duplicated for individual segments in the segment analyses tables in the next sections, which also more fully discuss the organization and content of the tasks and information processing subtasks.

Table 83. Scenario 7–Stop on Red Light task analysis table.

Table 83. Scenario 7–Stop on Red Light task analysis table, continued.

Segment Analysis

Scenario 7, Segment 1, Approach

The Approach segment involves the subject vehicle traveling at or near full speed until the subject driver sees that the traffic light is red. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 84. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 57. Table 85 lists Approach segment details.

It is critical to note that the task 7.1.3 (maintain safe lane position) cognitive subtask was treated as involving the evaluation of a single dimension (workload estimate = 4), rather than as the evaluation of multiple dimensions (workload estimate = 6) based on relative speed and distance. This approach was taken because evidence suggests that drivers may evaluate time-to-arrival as a single integrated variable (tau) rather than as separate speed and distance components.⁽¹¹⁾Also, the deceleration that takes place in task 7.1.4 is not the same as the deceleration in the next segment, which stops the vehicle. The task 7.1.4 deceleration is just general deceleration that should be part of any approach to an intersection.⁽⁹⁾

Task Pacing and Timing - Task 3.1.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving, but the other tasks are self-paced.

Regarding the task ordering, tasks 7.1.4 through 7.1.6 are sequential and can essentially be performed in any order. The ordering chosen in this segment followed a logical sequence and was consistent with the ordering of similar tasks in the Approach segments in other driving scenarios included in this effort.

Scenario 7, Segment 2, Stop

The Stop segment involves the interval from when the subject vehicle begins to decelerate, until the vehicle comes to a complete stop behind the lead vehicle. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 86. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 58 and table 87.

* Difficulty in this subtask is increased by a value of 1 because the lead vehicle's speed is changing.

** Difficulty in this subtask is increased by a value of 1 because of degraded information.

Several points about the task analysis and workload estimation warrant discussion. The first is that deceleration (task 7.2.3) is initiated not based on the distance to the intersection, but according to when the lead vehicle begins stopping, which forces the subject vehicle to also begin stopping. In addition, although tasks 7.2.4 (observe vehicle stopping trajectory) and 7.2.8 (stop) seem to be redundant, they have different objectives. Specifically, the purpose of task 7.2.4 is to provide global oversight to the general process of slowing and making sure that the trajectory will remain relatively smooth, whereas the purpose of task 7.2.8 is to ensure the more fine-tuned stopping and positioning of the subject vehicle behind the stopped lead vehicle. Finally, task 7.2.8 (observe status of light) is included as a general check to see whether the traffic signal is still red. This task can occur at any time over the course of this segment.

Note also that this segment is made more difficult because the subject vehicle is required to maintain safe distances from lead and following vehicles. In particular, the workload value of the task 7.2.5 cognitive element was increased to reflect the increased difficulty of assessing the trajectory of the lead vehicle whose speed is changing. Also, the estimated workload value for the task 7.2.6 cognitive element was incremented by a value of 1, because determining the trajectory of the following vehicle is more difficult to do using the degraded indirect visual information from the rearview mirror.

Task Pacing and Timing - Task 7.2.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Task 7.2.3 (begin decelerating) is also forced-paced because the subject driver has to respond quickly to the lead vehicle braking. Similarly, task 7.2.8 (stop) is forced-paced because the subject vehicle has to stop within a certain distance behind the lead vehicle.

Regarding the task ordering, there is an obvious temporal sequence to 7.2.3 (begin decelerating), 7.2.4 (observe stopping trajectory), and 7.2.8 (stop), whereas the other tasks are essentially ongoing throughout the segment. The exception is task 7.2.7 (observe status of light), which is temporally discrete and can occur over a range of time.

Scenario 7, Segment 3, Proceed Through Intersection

The Proceed Through Intersection segment involves the subject vehicle accelerating through the intersection after the light turns green. The lead vehicle starts to move, and determines if it is safe to proceed. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 88. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 59 and table 89.

* Difficulty in this subtask is increased by a value of 1 because the lead vehicle's speed is changing.

Tasks 7.3.3 (make sure pedestrians/cyclists are not crossing or about to cross) and 7.3.4 (check for red-light-running cross traffic) are likely to be unnecessary because the lead vehicle also has to check for these hazards before proceeding. These tasks were included to cover any unexpected hazardous actions from cross traffic or pedestrians/cyclists.

This segment is made more difficult because the subject vehicle is required to maintain safe distances from the lead vehicle. Specifically, the workload value of the task 7.3.6 cognitive subtask was increased to reflect the increased difficulty of assessing the relative speed of the lead vehicle whose speed is changing.

Task Pacing and Timing - Task 7.3.2 is forced-paced because its timing is determined externally from the subject driver, but it has no practical effect on the pacing in the segment because the driver has control over when subsequent tasks are performed. The exception is task 7.3.7 (lane maintenance), which is forced-paced because it is part of the ongoing task of driving.

In the task ordering, the first four tasks occur sequentially in logical order, whereas the other tasks are essentially concurrent throughout the remainder of the segment.

Scenario-Wide Analysis

To help identify potential information processing bottlenecks in this scenario, workload estimates from all segments were combined into a single scenario-wide workload profile, which provides a general indication of where the areas of high workload demands are likely to be.

Figure 60 shows the summed workload estimates (separately for each information processing subtask) in each segment interval for the entire scenario. Also, the intervals in which key tasks are forced-paced are shaded in orange. As indicated by figure 60, the workload estimates peak for both the perceptual and cognitive elements during the Stop segment, with smaller peaks in the initial part of the Approach segment and the latter part of the Proceed Through Intersection segment.

Figure 61, which displays the average workload estimate of all the tasks in play during a particular segment interval, shows that the peak during the Stop segment in figure 60 arises largely from several tasks combined rather than just a few particularly difficult tasks. This is because the average workload values in this segment are not much greater than in the other segments. Figure 61 shows a broad peak for cognitive subtasks during the Stop segment, with other narrower peaks in the Approach and latter half of the Proceed Through Intersection segment. The biggest peak for the perceptual subtasks occurs during the Proceed Through Intersection segment; however, this peak is misleading because only a single task is being performed at that time. Otherwise, the average perceptual workload estimates indicate generally elevated levels throughout most of the Approach and Stop segments.

Intervals containing nonroutine forced-paced tasks are shaded in orange. This graph shows the overall level of workload associated with a segment.

Intervals containing nonroutine forced-paced tasks are shaded in orange. This graph generally represents the overall level of difficulty associated with the tasks in a segment.

Information Processing Bottlenecks

Information about the combined and average workload ratings, pacing of key tasks, and nature of bottlenecks for each segment is shown in table 90. Only information that represents potential problems is listed; blank cells indicate that no substantive issues occurred for a particular segment or cell. Following the table is a list of key information processing bottlenecks identified in each of the segments.

Approach nature of bottleneck: Visual demands.

• Moderate to high combined and average perceptual subtask workload involves some overlapping visual scanning tasks during the initial part of the segment. These effects are offset, however, by the self-pacing and routine, automatic nature of most of these tasks.

Stop nature of bottleneck: A relatively high number of concurrent tasks.

• Combined workload is high for perceptual and cognitive subtasks because several tasks are concurrent-some with either moderate to high perceptual or cognitive workload. These effects, however, are offset by the self-pacing and routine, automatic nature of most of these tasks.

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