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Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks

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SECTION 3. RESULTS

This section describes the results of the task analysis. The results are divided into the seven scenarios analyzed, which are based on the following maneuvers at signalized intersections:

• Scenario 1–Left Turn on Green Light.
• Scenario 2–Left Turn on Yellow Light.
• Scenario 3–Straight on Yellow Light.
• Scenario 4–Straight on Green Light.
• Scenario 5–Right Turn on Green Light.
• Scenario 6–Right Turn on Red Light.
• Scenario 7–Stop on Red Light.

Each scenario can be divided into six segments, based on the action:

• Approach
• Prepare for Lane Change
• Deceleration/Stop
• Decision to proceed
• Intersection Entry
• Prepare for Turn

The following sections describe the scenarios.

Scenario 1–Left Turn on Green Light

Description

This scenario involves the subject vehicle making a left turn on a green light. Figure 4 shows the scenario diagram and provides additional details regarding the scenario. Briefly described, this scenario involves the subject driver identifying the intersection as the turn location, then decelerating to a stop. Following the stop, the subject vehicle advances into the intersection and waits for an appropriate gap in oncoming traffic before making the turn.

This scenario is divided into five segments (Approach, Deceleration, Intersection Entry, Prepare for Turn, and Execute Turn). The primary reasons this scenario was parsed into these particular segments were that each segment had a different overall driving goal and different speed characteristics (table 3).

The crash data related to this scenario indicate several characteristics that are relevant to the task analysis for this scenario. In particular, the most common type of crash-occurring 76 percent of the time-in this scenario involves turning vehicles being struck by an oncoming vehicle.⁽²⁰⁾ This fact suggests that the activities preceding the turn (see Prepare for Turn segment in this scenario) are the most challenging for drivers. The most common causal crash factors include, in order of prevalence misjudging the gap, looking but not seeing oncoming traffic, view obstructed by intervening vehicles, and other vehicle violations.^(20,21) If only incidents in which the subject vehicle stops before turning are considered (same as the current scenario), then the same factors are implicated; however, view obstructed by intervening vehicles becomes the most common cause for crashes. Figure 4 shows the scenario diagram and gives details and assumptions.

Several assumptions were made about the aspects of the scenario situation. The justifications for these are summarized in figure 4 and more fully described in table 4.

Scenario Timeline

An approximate timeline showing the key temporal milestones for Scenario 1 was calculated based on vehicle kinematics (figure 5). These milestones were used to make judgments about the pacing of tasks within segments, and they also provide a basis for the overall sequencing of certain tasks. Most segments included an interval with a variable time component, which represented intervals that either were 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 turn).

Task Analysis Table

The results of the task analysis organized by scenario segment are shown in table 5, the task analysis table. The 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 5. Scenario 1–Left Turn on Green Light task analysis table.

Table 5. Scenario 1–Left Turn on Green Light task analysis table, continued.

Table 5. Scenario 1–Left Turn on Green Light task analysis table, continued.

Table 5. Scenario 1–Left Turn on Green Light task analysis table, continued.

Segment Analysis

Scenario 1, Segment 1, Approach

The Approach segment involves the subject vehicle traveling at full speed until the intersection is identified as the turn intersection and lasts until it is time to begin decelerating. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 6. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 6 and table 7.

The workload estimates shown in table 6 and all subsequent segment task analysis tables are based on the workload estimation chart shown in table 2.

Several aspects about the task analysis and workload estimation warrant discussion. The first is that the deceleration that takes place in task 1.1.3 (decelerate) is not the same as the deceleration in the following segment which stops the vehicle. Instead, the task 1.1.3 deceleration is just general deceleration that should be part of any approach to an intersection.⁽⁹⁾ Note also that task 1.1.7 (identify intersection as turn intersection) is a new task that is not indicated or suggested by any of the task analysis sources used,⁽⁹⁾ but it is included because it is required by the scenario assumptions.

Task Pacing and Timing - Task 1.1.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Task 1.1.7 (identify intersection as turn location) is forced-paced because it is constrained by the fact that for an unfamiliar intersection, this task cannot be performed until the street signs are readable-yet it has to be done before it is too late to decelerate safely. Similarly, task 1.1.8 (activate signal) is also forced-paced because it must follow task 1.1.7, but precede deceleration (the next segment).

Regarding the task ordering, tasks 1.1.7 and 1.1.8 are confined to the latter parts of the segment because the subject driver has to be close enough to the intersection to read the street signs. In addition, because tasks 1.1.3 through 1.1.6 are self-paced, there are no barriers to performing them in advance of task 1.1.7 as long as the intersection is visible. Also, these are tasks that the subject driver must perform regardless of whether he or she is turning at this intersection or continuing straight; therefore, it makes sense that they would be done early. Finally, task 1.1.5 (observe status of light) can logically occur over a range of time.

Scenario 1, Segment 2, Deceleration

The Deceleration segment spans the interval from where the subject vehicle begins to decelerate to when the vehicle comes to a complete stop at the stop line. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 8. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 7 and table 9.

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

It is important to note that the task 1.2.5 (maintain safe distance from decelerating following vehicle) 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 estimated workload value for the cognitive element of this task was incremented by a value of 1. This was done because determining the closing trajectory of the following vehicle is more difficult to accomplish using the degraded indirect visual information available from the rearview mirror. Note that this task is further complicated because the following vehicle's speed is also changing; however, the difficulty level was incremented only once to reflect these factors.

It is also noteworthy that although the subject driver may have determined in the previous segment that the light will not change to yellow soon, it is still necessary to check the traffic signal sometime during this segment. Doing so ensures that the driver did not misjudge the light duration or that the light status is not about to change.

Task Pacing and Timing - Task 1.2.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Also, task 1.2.3 (begin decelerating) is forced-paced because the subject driver has a limited amount of time to decide to decelerate before the vehicle is too close to the stop line to permit safe or comfortable deceleration.

Regarding the task ordering, task 1.2.3 must come first because it initiates deceleration (the segment driving objective), while the other tasks are essentially ongoing throughout the segment. The exception is task 1.2.6 (observe status of light), which is temporally discrete and can occur over a range of time.

Scenario 1, Segment 3, Intersection Entry

The Intersection Entry segment involves the subject vehicle entering the intersection from the stop line after the lead vehicle has turned. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 10, and the scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 8 and table 11.

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

Several points about the task analysis warrant discussion. The first is that task 1.3.1 (wait for lead vehicle to turn) is not directly specified in the task analysis references used for this analysis.⁽⁹⁾ This task is included because it is logically a necessary part of this segment. Also, although the subject driver probably has enough space to enter the intersection, the driver is assumed to wait at the stop line until the lead vehicle turns. This assumption was made for two reasons: to reduce the number of steps in this segment and have a default course of action if the intersection is not large enough for following drivers to establish themselves in the intersection. (Note that the workload is the same either way because the steps are sequential.)

Another assumption is that cross traffic is moving and in the process of slowing to a stop, which makes task 1.3.4 (check for red-light-running cross traffic) more difficult because it requires the subject driver to determine if the slowing vehicles are stopping rather than simply checking if the vehicles are stopped at the intersection. Finally, the estimated workload value for the task 1.3.3 (check for conflicts with following vehicle) cognitive element was incremented by a value of 1 because determining the closing trajectory of the following vehicle is more difficult to do using the degraded indirect visual information from the rearview mirror.

Task Pacing and Timing - All tasks in this segment are self-paced. Although they are inherently limited by the duration of the green light phase, a prior assumption in this segment is that the light will remain green long enough for this to not be a significant constraint.

Regarding the task ordering, the subject driver must wait until the lead vehicle turns (task 1.3.1) before completing other tasks, except for tasks 1.3.2 and 1.3.3, which require periodic checking for the duration of the segment.

Scenario 1, Segment 4, Prepare for Turn

The Prepare for Turn segment spans the time interval from when the subject vehicle stops after establishing itself in the intersection as the next turn vehicle to when the driver makes the decision that there is enough of a gap in oncoming traffic to safely make the turn. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 12. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 9 and table 13.

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

Note that task 1.4.1 (look for gap in traffic) is likely to be the most common source of errors in this scenario because "misjudging the gap" and "looked but did not see oncoming traffic" are among the most commonly cited driver errors in this situation.^{(20, 21)} Not only is the gap judgment task particularly difficult, but drivers also must continuously cycle their visual gaze among additional sources of key information (e.g., the traffic signal) throughout the environment while they are assessing the suitability of gaps in traffic. Note that with the subject vehicle stopped and the following vehicle likely traveling at slow speeds, task 1.4.4 may be unnecessary; however, it was included because the consequences of being rear-ended and possibly pushed into oncoming traffic are relatively severe. Also, the estimated workload value for the task 1.4.4 (check for conflicts with following vehicle) cognitive element was incremented by a value of 1, because determining the closing trajectory of the following vehicle is more difficult to do using degraded indirect visual information from the rearview mirror.

In this segment, no specific task is allocated for the decision to turn; rather, this decision is the result of the process of correctly identifying a suitable gap (task 1.4.1) and confirming that the turn path is free from hazards (task 1.4.3).

Scenario 1, Segment 4 Diagram Blue dotted outlines indicate general distribution of primary information in key perceptual tasks.  Note: Illustration dimensions and vehicle positions are not to scale.  Figure 9. Scenario 1–Left Turn on Green Light Prepare for Turn segment scenario diagram.

Table 13. Scenario 1–Left Turn on Green Light Prepare for Turn segment relative timing and duration of segment tasks and summary of key findings. Potential contributions to high workload and information processing bottlenecks: Concurrent and continuous conduct of 1.4.1, 1.4.2, 1.4.3, and 1.4.4 under high time pressure. The majority of the time in this segment is likely to be taken by tasks that have either a relatively high cognitive workload (1.4.1, 1.4.4) or high perceptual workload (1.4.3). Task 1.4.1 represents a high-stress decision with possible severe consequences if done inaccurately. The information sources are widely distributed throughout the environment. Mitigating factors: None.

Task Pacing and Timing - All tasks in this segment are forced-paced because the subject driver is limited in time because new gaps are continuously appearing and requiring evaluation. This situation forces the driver back to task 1.4.1, a task that is repeated until a safe gap is finally identified, and then the driver is limited in time because the gap is rapidly approaching and the decision to turn must be completed before the lead safe-gap vehicle arrives at the intersection.

Regarding the task ordering, the tasks are all concurrent because the driver must repeatedly cycle between evaluating new gaps as they become visible (task 1.4.1) and maintaining a current assessment of the situation (tasks 1.4.2 through 1.4.4).

Scenario 1, Segment 5, Execute Turn

The Execute Turn segment involves the actions related to initiating and completing the left turn. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 14 and the scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 10 and table 15.

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

Note that the perceptual and cognitive subtasks associated with task 1.5.1 (accelerate to initiate turn)likely represent a worst case situation for accelerating into the turn. More specifically, if the actual gap in oncoming traffic is quite large, drivers may not need to verify that they will clear the oncoming traffic. In contrast, task 1.5.1 probably represents a situation where the gap may be in the uncomfortable range of what drivers are willing to accept, which forces them to accelerate a little quicker than usual and confirm that they are going fast enough to clear the oncoming traffic. Also, task 1.5.4 (continue accelerating up to speed) is simply a continuation of task 1.5.1 in the new lane, except that the difficulty associated with crossing oncoming traffic is gone. Finally, the workload value of the task 1.5.5 (maintain safe distance from accelerating lead vehicle) cognitive element was increased to reflect the increased difficulty of assessing the relative speed of the lead vehicle whose speed is changing.

Scenario 1, Segment 5 Diagram Blue dotted outlines indicate general distribution of primary information in key perceptual tasks.  Note: Illustration dimensions and vehicle positions are not to scale. Figure 10. Scenario 1–Left Turn on Green Light Execute Turn segment scenario diagram.

Table 15. Scenario 1–Left Turn on Green Light Execute Turn segment relative timing and duration of segment tasks and summary of key findings. Potential contributions to high workload and information processing bottlenecks: Concurrent and continuous conduct of 1.5.1, 1.5.2, and 1.5.3 involve high cognitive (1.5.1 and 1.5.2) or perceptual (1.5.3) workload. Some of the more difficult tasks are also forced-paced. The initial turning phase of this segment is a high-stress situation with possible severe consequences if conducted improperly. Mitigating factors: Several tasks are routine, automatic activities.

Task Pacing and Timing - The tasks associated with accelerating and making the left turn (tasks 1.5.1, 1.5.2, and 1.5.3) are forced-paced because they must be accomplished during the brief time that the vehicle is turning and oncoming vehicles are rapidly approaching the exposed subject vehicle. Also, task 1.5.5 (lane maintenance) is forced-paced because it is part of the ongoing task of driving.

Regarding the task ordering, tasks 1.51 through 1.5.3 are concurrent, as are tasks 1.5.4 through 1.5.7.

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 that provides a general indication of where the areas of high workload demands are likely to be.

Figure 11 shows the summed workload estimates (shown 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 11, the workload peaks for the perceptual subtasks occur during the Approach, Deceleration, and Prepare to Turn segments, whereas the peaks for the cognitive subtasks occur during the Deceleration and Prepare to Turn segments. The highest psychomotor workload demands also occur during the Deceleration segment.

Figure 12 indicates that for perceptual subtasks, relative peaks occur during the Approach and Execute Turn segments, whereas for the cognitive subtasks, the average workload estimates are slightly higher during the Prepare to Turn and Execute Turn segments.

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

Note: 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 provided in table 16. Only information about 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.

Table 16. Scenario 1–Left Turn on Green Light combined and average workload ratings, pacing of key tasks, and nature of bottlenecks that indicate potential problems for each scenario segment.

Approach nature of bottleneck: Visual demands:

• Moderate combined and high average perceptual subtask workload involves some overlapping visual scanning and reading tasks. The difficulty of the activities in this segment is increased by the forced-pacing of the task that involves identifying the intersection as the correct turn intersection.

Deceleration nature of bottleneck: Several concurrent tasks:

• Combined workload is moderate for perceptual and cognitive subtasks; however, this level of workload is offset by the self-pacing of most of these tasks.

Prepare for Turn nature of bottleneck: Concurrent high workload, high-stress tasks under time pressure:

• There is moderate combined and high average perceptual and cognitive subtask workload involving continuous and concurrent tasks of moderate and high workload. These are also high-stress tasks that must be performed under time pressure, and they require information sampling that is widely distributed in the environment.

Execute Turn nature of bottleneck: Concurrent high workload, high-stress tasks under time pressure:

• The initial phase of this segment is somewhat of a continuation of the previous segment. Moderate combined and high average perceptual and cognitive subtask workload involves continuous and concurrent tasks of moderate and high workload. In addition, these initial tasks are high stress and must be performed under time pressure.

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