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Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks

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Scenario 5–Right Turn on Green Light

Description

This scenario involves the subject vehicle making a right turn on a green light. Figure 37 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 turn speed. Following this action, the subject driver determines that it is safe to go, advances into the intersection, and executes the turn while at the same time watching out for conflicts with an oncoming vehicle making a left turn into the adjacent lane.

This scenario was divided into four segments (Approach, Deceleration, Intersection Entry, and Execute Turn). The primary bases for parsing the scenario into these particular segments were that each segment had a different overall driving goal and each had different speed characteristics (table 53).

Table 53. Scenario 5–Right Turn on Green Light driving objectives and speed characteristics for each scenario segment used as a basis for the scenario partitioning.

The crash data related to this scenario indicate some characteristics that are relevant to the task analysis and configuration of this scenario. According to 1998 GES data, 5.7 percent of crossing path crashes involved right turns, 20 percent of which occurred at signalized intersections.⁽²⁶⁾The most common violations charged in right-turn crashes at signalized intersections were "failure to yield right of way" (19.2 percent ), followed by "other violation" (15.9 percent), and "running a traffic signal" (4.1 percent).⁽²⁶⁾Another notable finding from these same researchers⁽²⁶⁾ is that right-turning drivers had higher rates of reported involvement of driver distraction in crashes at signalized intersections (5.5 percent) compared to drivers making left turns [3.1 percent left turn across path-opposite direction (LTAP/OD) conflict; 1.5 percent left turn across path-lateral direction (LTAP/LD) conflict] or going straight (2.7 percent straight crossing paths) . In addition, the same report indicates that vision obstruction was not reported as a factor in any right-turn crashes at signalized intersections in the 1998 GES data sample used. This finding suggests that the cognitive subtasks in the Intersection Entry segment are likely to be primary sources of difficulty.

Figure 37 shows the Scenario 5 diagram, and table 54 lists Scenario 5 details.

Several assumptions were made regarding the situational aspects of the scenario. The justifications for these are summarized in figure 37 and more fully described in table 54.

Table 54. Scenario 5–Right Turn on Green Light assumptions and corresponding justifications.

Scenario Timeline

An approximate timeline showing the key temporal milestones for Scenario 5 was calculated based on vehicle kinematics (figure 38). These milestones were used to make judgments about the pacing of tasks within segments and provide a basis for the overall sequencing of certain tasks. Some 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).

Figure 38. Scenario 5–Right Turn on Green Light timeline of key segment phases duration and task milestones.

Task Analysis Table

The results of the task analysis organized by scenario segment are shown in the task analysis table (table 55). The task analysis results are duplicated for individual segments in the segment analyses tables in the next sections, which also discuss more fully the organization and content of the tasks and information processing subtasks.

Table 55. Scenario 5–Right Turn on Green Light task analysis table.

Table 55. Scenario 5–Right Turn on Green Light task analysis table, continued.

Table 55. Scenario 5–Right Turn on Green Light task analysis table, continued.

Segment Analysis

Scenario 5, Segment 1, Approach

The Approach segment involves the subject vehicle traveling at full speed until the intersection is identified as the turn intersection and it is the appropriate time to begin decelerating. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 56. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 39 and table 57.

Several points about the task analysis and workload estimates warrant discussion. The first is that the deceleration that takes place in task 5.1.3 (decelerate) is not the same as the deceleration in the next segment, which slows the vehicle to turning speed. Rather, the task 5.1.3 deceleration is just general deceleration that should be part of any approach to an intersection.⁽⁹⁾ Another point is that task 5.1.7 (identify intersection as turn intersection) is a new task that is not indicated or suggested by any of the task analysis sources used (e.g., McKnight and Adams, 1970),⁽⁹⁾ but instead is included because it is required by the scenario assumptions. Finally, task 5.1.6 (determine if the light is about to change) can range in difficulty based on the degree to which direct information is or is not available. For example, pedestrian walk signals can make the perceptual task into a simple inspection task (workload demand = 3) rather than a potentially complicated visual search task (workload demand = 7). The more demanding version was selected in keeping with the overall goals of addressing the worst-case scenarios.

Task Pacing and Timing - Task 5.1.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Task 5.1.7 (identify intersection as turn location) is forced-paced because it is constrained by the fact that when an intersection is unfamiliar to the driver, this task cannot be performed until the street signs are readable, yet the task must be performed before it is too late to decelerate safely. Similarly, task 5.1.8 (activate signal) is also forced-paced because it must follow task 5.1.7, but precede deceleration.

In the task ordering, tasks 5.1.7 and 5.1.8 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 5.1.3 through 5.1.6 are self-paced, there are no barriers to performing them in advance of task 5.1.7 as long as the intersection is visible. Also, these are tasks that the subject driver would have to perform regardless of whether the driver was turning at this intersection or continuing straight. Thus, it makes sense that they would be done early. Finally, task 5.1.5 (observe status of light) can logically occur over a range of time.

Scenario 5, Segment 2, Deceleration

The Deceleration segment involves the interval from where the subject vehicle begins to decelerate until the vehicle slows to near turning speed before entering the intersection. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 58. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 40 and table 59.

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

It is noteworthy that task 5.2.5 (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. The reason is that 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, 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.

Note also 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 to ensure that the driver did not misjudge the light duration or that the light status is not about to change.

Task Pacing and Timing - Task 5.2.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Also, task 5.2.3 (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 5.2.3 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 5.2.6 (observe status of light), which is temporally discrete and can occur over a range of time.

Scenario 5, Segment 3, Intersection Entry

The Intersection Entry segment involves the subject vehicle decelerating to turning speed and entering the intersection in preparation for turning. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 60. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 41 and table 61.

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

The decision regarding whether the turn can be made without coming to a complete stop is made during this segment. Because the subject vehicle has the green light and right of way for turning into the closest lane, this decision mainly involves confirming that no hazards are in the way (tasks 5.3.5 through 5.3.7) and that the light remains green. Consequently, there is no separate decision task; the decision follows from the outcome of tasks 5.3.5 through 5.3.7. Note also that there is no continue into intersection task as in other scenarios. This task was omitted because the subject vehicle does not have to go as far into the intersection. Omitting this task also reduces the number of tasks in this segment.

Several assumptions make this decision process more complicated. One assumption is that cross traffic is moving and in the process of slowing to a stop, which makes task 5.3.6 (check for red-light-running cross traffic) more difficult because it requires the subject driver to determine whether the slowing vehicles are stopping rather than simply checking if the vehicles are stopped at the intersection. Another complicating aspect is the assumption that an oncoming vehicle is making a left turn into the inside right-going lane. This situation requires that the subject driver monitor the path of the turning vehicle to ensure that the oncoming turning vehicle will not encroach into the subject vehicle's turn path (task 5.3.7). Finally, the estimated workload value for task 5.3.4 (maintain safe distance from decelerating following vehicle) cognitive subtask was incremented by a value of 1, because this task is made more difficult with the degraded indirect visual information from the rearview mirror.

Task Pacing and Timing - Task 5.3.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Also, tasks 5.3.6 through 5.3.8 are forced-paced because the rapidly approaching intersection and crosswalk limit the time available to check for these hazards in time to take evasive action.

Regarding the task ordering, tasks 5.3.1 through 5.3.4 are concurrent and ongoing throughout the segment. Although task 5.3.5 (observe status of light) is self-paced and can occur over a range of time, this task was positioned near the start of the segment by default, because the time available to perform the other tasks is limited by the rapidly approaching intersection. The other tasks (5.3.5 through 5.3.8) are sequential, and they are depicted in the order that they are most likely to be performed.

Scenario 5, Segment 4, Execute Turn

The Execute Turn segment involves the actions related to initiating and completing the right turn. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 62. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 42 and table 63.

There are several noteworthy points in this segment. The first is that task 5.4.3 (check for hazards in turn path) is duplicated from the previous segment, which occurs shortly before. The reason for this is that in the previous segment this task was part of the criterion for deciding whether or not it was safe to continue directly into the turn, whereas in the present segment this task acts as a safety check for hazards. Another point is that task 5.4.5 (continue accelerating up to speed) is simply a continuation of task 5.4.1 in the new lane.

One aspect that makes this segment more complicated than others is that an oncoming vehicle is in the process of making a left turn into the inside right-going lane. This action requires that the subject driver monitor the path of the turning vehicle to make sure that the oncoming turning vehicle will not encroach into the subject vehicle's turn path (task 5.4.4).

Task Pacing and Timing - Tasks 5.4.1 (accelerate to initiate turn), 5.4.2 (steer into turn), 5.4.5 (continue accelerating), and 5.4.6 (lane maintenance) are forced-paced because they are part of the ongoing task of driving. Task 5.4.3 (check for hazards in the turn path) is forced-paced because drivers only have a brief time to perform this task before the turn is complete.

In the task ordering, the tasks are mostly concurrent, except for the division between the actual turn maneuver and getting up to speed after the subject vehicle is in the new lane. The one exception is task 5.4.4 (check for conflicts with left-turning vehicle). This task overlaps between the two stages because of the possibility that the left-turning driver may be unaware of the subject vehicle (e.g., if it is in the blind spot), and will try to change into the subject vehicle's lane.

Scenario-Wide Analysis

To help identify potential information processing bottlenecks in this scenario, workload estimates from all the 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 43 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 43, the workload peaks at moderate levels for both the perceptual and cognitive elements during the Intersection Entry segment. Relative to other scenarios, there are also moderate levels of combined perceptual demands in all the segments and moderate levels of cognitive workload in the Deceleration and Execute Turn segments.

Figure 44, which displays the average workload estimate of all the tasks in play during a particular segment interval, shows that the peaks during the Intersection Entry segment in figure 43 arise from several tasks combined rather than just a few difficult tasks, although both the perceptual and cognitive subtasks in this segment do have relatively high average workload estimates. Otherwise, the average workload levels peak during the Approach segment for perceptual subtasks and peak during the initial part of the Execute Turn segment cognitive subtasks.

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 64. 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 lists key information processing bottlenecks identified in each of the segments.

Approach nature of bottleneck: Visual demands:

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

Deceleration nature of bottleneck: Several concurrent tasks:

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

Intersection Entry nature of bottleneck: High time pressure for several tasks:

• A high number of demanding tasks are concurrent and draw on information distributed throughout the visual scene. In addition, a relatively high number of those tasks must also be performed under high time pressure.

Execute Turn nature of bottleneck: Concurrent tasks:

• Concurrent demanding cognitive subtasks take place during the initial part of the segment. These effects are mitigated somewhat because most of these tasks are routine, automatic activities.

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