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Publication Number: FHWA-HRT-06-033
Date: August 2006

Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks

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Scenario 4–Straight on Green Light

Description

This scenario involves the subject vehicle going straight through the intersection on a green light after changing lanes to avoid a vehicle that is stopping to turn in the left lane. Figure 29 shows the scenario diagram and provides additional details regarding the scenario. Briefly described, this scenario involves the driver approaching the intersection and deciding that the light will remain green long enough to get across the intersection. After making the decision to proceed through the intersection, the subject driver observes a left-turning vehicle decelerating in the lane of travel, which requires the driver to change lanes before crossing through the intersection.

This scenario is divided into four segments (Approach, Prepare for Lane Change, Execute Lane Change, and Intersection Entry). The primary reason for parsing this scenario into these segments is that each segment has a different overall driving goal (table 41). Unlike other scenarios, the speed does not vary greatly between segments.

Table 41. Scenario 4–Straight on Green Light driving objectives and speed characteristics for each scenario segment as a basis for the scenario partitioning.

Segment

Driving Objectives

Speed Characteristics

Approach

Gather information about the situation.

Traveling at full speed with some deceleration.

Prepare for Lane Change

Determine if a lane change is feasible/safe.

Traveling near full speed.

Execute Lane Change

Maneuver into right lane.

Traveling near full speed with speed adjustments.

Intersection Entry

Enter and proceed through the intersection.

Traveling near full speed with speed adjustments.

Because the light remains green throughout this scenario, the dilemma-zone or red-light-running crash issues described in Scenario 3 have minimal relevance for identifying potential areas of difficulty. On the other hand, the lane-change element of this scenario provides a potential source of insight into the factors that might lead to crashes in this scenario. Lane change/merge-related crashes accounted for 3.7 percent of all crashes according to 1991 General Estimates System (GES) data, and 22.6 percent of those occurred at intersections or were intersection related.(21) Most of these crashes involved little or no longitudinal gap and a small speed differential between the subject and the POV. Also, crashes were roughly equally distributed among collisions with the front (31.7 percent), middle (26 percent), and rear of the POV (35 percent). The primary causal factors were “looked but did not see” (61.2 percent) and “misjudged” gap/velocity (29.9 percent). This finding suggests that the tasks associated with determining whether it is safe to change lanes in the Prepare for Lane Change segment would be key performance bottlenecks in this scenario, especially with the inherent time limitations created by the approaching intersection. Figure 29 lists Scenario 4 details and shows the scenario diagram.

Scenario 4 Diagram

View Alternative Text

  • Segments are demarked by dotted lines.
  • Approximate speed in km/h is indicated on the right.

Note: Illustration dimensions and vehicle positions are not to scale.

Figure 29. Scenario 4–Straight on Green Light diagram, details, and assumptions.


Scenario Details

Activity:

Straight on Green Light

Scenario Segments:

  • Approach
  • Lane Change
  • Decision to Proceed
  • Intersection Entry

Intersection Configuration:

Four-lane urban signalized intersection without a dedicated left-turn lane.

Traffic Volume:

Moderate (driving speeds are at speed limit and other lead/following/adjacent vehicles are present).

Assumptions/Complicating Factors:

  • There is no dedicated left-turn lane or turn signal.
  • The signal will remain green until the subject vehicle crosses the intersection, and the visual cues (e.g., pedestrian signal) are consistent with this situation.
  • A left-turning vehicle is stopped ahead in the subject vehicle's lane.
  • The decision to go through the intersection can be made far enough away from the intersection that there is enough room to execute the lane change.
  • There is traffic in the right lane that is in front and behind the subject vehicle traveling at speed, but otherwise there is a sufficient gap to change lanes.
  • The subject is being followed by another vehicle in the originating lane.

Several assumptions were made about the situational aspects of the scenario. The justifications for these are summarized in figure 29 and more fully described in table 42.

Table 42. Scenario 4–Straight on Green Light assumptions and corresponding justifications.

Assumption

Justification

The signal will remain green until the subject vehicle crosses the intersection and the visual cues (e.g., pedestrian signal) are consistent with this green light.

This situation provides some motivation for changing lanes, rather than stopping and waiting for a new cycle.

A vehicle that is far ahead signals for a left turn during the Approach Segment.

This situation necessitates a lane change, but because the vehicle is far ahead, it will not interfere with the subject vehicle's ability to change lanes.

The decision to go through the intersection can be made far enough away from the intersection to ensure enough room to execute the lane change.

This situation makes the lane change possible.

There is traffic in the right lane that is in front and behind the subject vehicle traveling at speed, but otherwise there is a sufficient gap to change lanes.

This situation makes the lane change more difficult and introduces the risk of a traffic conflict while still making it possible to change lanes.

Scenario 4 Timeline

An approximate timeline showing the key temporal milestones for Scenario 4 was calculated based on vehicle kinematics (figure 30). These milestones were used to make judgments about the pacing of tasks within segments in addition to providing a basis for the overall sequencing of certain tasks.

View Alternative Text

Figure 30. Scenario 4–Straight on Green Light

timeline of key segment phases duration and event/task milestones.

One aspect of the timing that deserves discussion is that the times provided for planning and executing lane changes (2.5 and 3.0 sec, respectively), are shorter than what is typically observed in other research (see also appendix A). Shorter times were used to reflect the fact that the lane changes must be completed quickly because the driver is approaching the intersection.

Task Analysis Table

The results of the task analysis organized by scenario segment are shown in the task analysis table (table 43). The task analysis results are duplicated for individual segments in the segment analyses tables in the next sections, which more fully discuss the organization and content of the tasks and information processing subtasks.

Table 43. Scenario 4–Straight on Green Light task analysis table.

Task

Perceptual Subtasks

Cognitive Subtasks

Psychomotor Subtasks

4.1. Approach

4.1.1 Maintain safe lane position.

Visually observe roadway ahead.

Verify correct lane position.

Make necessary adjustments to steering.

4.1.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

Determine whether perceptual input indicates if current situation is safe/unsafe.

Head and eye movements for scanning.

4.1.4 Decelerate.

Visually assess distance to intersection.

Determine when vehicle is close enough to intersection to begin deceleration.

Coast (foot off accelerator) and/or gentle braking.

4.1.5 Identify intersection characteristics.

Visually identify lane configurations, pavement marking and signs, signal location, etc.

Determine if any nonroutine actions are required.

Head and eye movements for scanning.

4.1.6 Observe status of light.

Visually observe traffic signal.

Identify color/status of traffic light.

Head and eye movements to view traffic signal.

4.1.7 Determine if the light is about to change.

Visually observe the scene for key cues (check pedestrian signal).

Identify that walk pedestrian signal means that the light will stay green.

Head and eye movements for scanning.

4.1.8 Observe that lead vehicle is signaling a left turn.

Visually observe lead vehicle left-turn signal.

Determine that lead vehicle intends to turn left.

Head and eye movements for viewing lead vehicle.

4.2. Prepare for Lane Change

4.2.1 Maintain safe lane position.

Visually observe roadway ahead.

Verify correct lane position.

Make necessary adjustments to steering.

4.2.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

Determine whether perceptual input indicates if current situation is safe/unsafe.

Head and eye movements for scanning.

4.2.4 Decide whether lane change is legally permissible (e.g., lane markings are not solid).

Scan for signs prohibiting lane change.

Visually observe pavement markings.

Make yes/no decision regarding legality based on observed information.

Head and eye movements for looking for indicators.

4.2.5 Determine whether there is a sufficient gap in right lane.

Visually observe leading or adjacent traffic in right lane.

Determine if there is sufficient space in the right lane or if deceleration is required.

Head and eye movements for viewing right lane.

4.2.6 Check rearview mirror for rear-approaching traffic in right lane.

Observe presence and relative speed of rear-approaching traffic.

Determine if there will be a potential conflict with rear-approaching traffic.

Head and eye movements to observe rearview mirror.

4.2.7 Check blind spot for rear-approaching traffic in right lane.

Visually observe right lane over shoulder.

Identify if any vehicles are present.

Head and eye movements to perform shoulder check.

4.3. Execute Lane Change

4.3.1 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

Determine whether perceptual input indicates if current situation is safe/unsafe.

Head and eye movements for scanning.

4.3.3 Activate turn signal.

Locate the control (automatic behavior).

Identify which direction to activate the control (automatic behavior).

Activate turn signal control.

4.3.4 Adjust vehicle speed to avoid conflicts with right-lane lead vehicle traveling at constant speed.

Visually assess distance and relative speed of lead vehicle in right lane.

Determine if distance and relative speed are safe.

Decelerate if necessary.
Head and eye movements to view vehicle.

4.3.5 Adjust vehicle speed to avoid conflicts with right-lane following vehicle traveling at constant speed.

Visually assess distance and relative speed of following vehicle in right lane.

Determine if distance and relative speed are safe.

Accelerate if necessary
Head and eye movements to view vehicle.

4.3.6 Change lane.

Visually observe vehicle lateral position.

Identify when vehicle is completely in new lane.

Steer into new lane and adjust vehicle position.

4.4. Intersection Entry

4.4.1 Maintain safe lane position.

Visually observe roadway ahead.

Verify correct lane position.

Make necessary adjustments to steering.

4.4.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

Determine whether perceptual input indicates if current situation is safe/unsafe.

Head and eye movements for scanning.

4.4.3 Maintain safe distance from lead vehicle traveling at constant speed.

Visually assess distance and relative speed of leading vehicle.

Determine if lead-vehicle trajectory is safe.

Reduce speed if necessary.
Head and eye movements to view vehicle.

4.4.4 Check that there will be no conflict with following vehicle.

Visually assess distance and relative speed of following vehicle.

Determine if following-vehicle closing trajectory is safe.

Head and eye movements to observe rearview mirror.

4.4.5 Check for red-light- running cross traffic.

Visually observe vehicles in left and right cross lanes.

Determine if crossing vehicles are stopped or will stop in time.

Head and eye movements for observing cross traffic.

4.4.6 Check for oncoming vehicle trying to turn left across path.

Look for signs of vehicle turning (turn signal, stopped in intersection).

Decide if oncoming vehicle is turning.

Head and eye movements to view oncoming vehicle.

Segment Analysis

Scenario 4, Segment 1, Approach

The Approach segment involves the subject vehicle traveling at full speed until the subject vehicle driver observes that the lead vehicle is signaling to turn (which prompts the decision to consider changing lanes-next segment). The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 44. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 31 and table 45.

Table 44. Scenario 4–Straight on Green Light Approach segment

tasks and information processing subtasks.

Task

Perceptual Subtasks

Cognitive Subtasks

Psychomotor Subtasks

4.1.1 Maintain safe lane position.

Visually observe roadway ahead.

1

Verify correct lane position.

1

Make necessary adjustments to steering.

1

4.1.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.

Listen for indications of unsafe situations.

7

Determine whether perceptual input indicates if current situation is safe/unsafe.

4

Head and eye movements for scanning.

1

4.1.3 Decelerate.

Visually assess distance to intersection.

4

Determine when vehicle is close enough to intersection to begin deceleration.

6

Coast (foot off accelerator) and/or gentle braking.

3

4.1.4 Identify intersection characteristics.

Visually identify lane configurations, pavement marking and signs, signal location, etc.

6

Determine if any nonroutine actions are required.

6

Head and eye movements for scanning.

1

4.1.5 Observe status of light.

Visually observe traffic signal.

2

Identify color/status of traffic light.

 

2

Head and eye movements to view traffic signal.

1

4.1.6 Determine if the light is about to change.

Visually observe the scene for key cues (check pedestrian signal).

3

Identify that walk pedestrian signal means that the light will stay green.

2

Head and eye movements for scanning.

1

4.1.7 Observe that lead vehicle is signaling a left turn.

Visually observe lead vehicle left-turn signal.

1

Determine that lead vehicle intends to turn left.

1

Head and eye movements for viewing lead vehicle.

1

Some assumptions about the task analysis and workload estimation warrant discussion. One is the assumption that the pedestrian signal is visible to the driver during the approach, and it displays the walk signal indicating that the light will remain green in the near future. Observing the pedestrian signal was mentioned as a common strategy in intersection approaches by drivers of all ages in the focus group component of this project.(22) This assumption consequently simplifies task 4.1.7 (determine if the light is about to change) and provides some variety from the other scenarios. It is also the most likely situation because the light is assumed to be early in the green phase. Another assumption is that the lead vehicle does not start signaling the turn until after the subject vehicle has completed task 4.1.6, which occurs because this event is the boundary between this segment and the next.

Scenario 4, Segment 1 Diagram

View Alternative Text

Blue dotted outlines indicate general distribution of primary information in key perceptual tasks.

Note: Illustration dimensions and vehicle positions are not to scale.

Figure 31. Scenario 4–Straight on Green Light Approach segment diagram.


Table 45. Relative timing and duration of segment tasks and summary of key findings for the
Approach segment of Scenario 4–Straight on Green Light.

View Alternative Text

Potential contributions to high workload and information processing bottlenecks:

  • Demanding perceptual tasks in 4.1.2 and 4.1.4.
  • Cognitive tasks involving evaluation or judgment of several factors in 4.1.3 and 4.1.4.

Mitigating factors:

  • Most of the difficult tasks are self-paced.
  • Many tasks are routine, automatic activities.

Task Pacing and Timing - Task 4.1.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Task 4.1.7 (observe that lead vehicle is signaling a left turn) is forced-paced because the driver has a limited amount of time to notice the turning vehicle and decide to change lanes because the intersection is rapidly approaching.

Regarding the task ordering, tasks 4.1.3 through 4.1.6 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 other scenario Approach segments. Task 4.1.7 was simply assumed to occur last because it was the transition point to the next segment.

Scenario 4, Segment 2, Prepare for Lane Change

The Prepare for Lane Change segment spans the time from when the subject driver observes the lead vehicle signaling to turn to when the driver decides that it is legal and safe to make a lane change. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 46. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 32 and table 47.

Table 46. Scenario 4–Straight on Green Light Prepare for Lane Change segment of tasks and information processing subtasks.

Task

Perceptual Subtasks

Cognitive Subtasks

Psychomotor Subtasks

4.2.1 Maintain safe lane position.

Visually observe roadway ahead.

1

Verify correct lane position.

1

Make necessary adjustments to steering.

1

4.2.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

7

Determine whether perceptual input indicates if current situation is safe/unsafe.

4

Head and eye movements for scanning.

1

4.2.3 Decide whether lane change is legally permissible (e.g., lane markings are not solid).

Scan for signs prohibiting lane change.
Visually observe pavement markings.

7

Interpret information regarding legality based on observed information.

5

Head and eye movements for looking for indicators.

1

4.2.4 Determine whether there is a sufficient gap in right lane.

Visually observe leading or adjacent traffic in right lane.

4

Determine if there is sufficient space in the right lane or if deceleration is required.

4

Head and eye movements for viewing right lane.

1

4.2.5 Check rearview mirror for rear-approaching traffic.

Identify presence and relative speed of rear-approaching traffic.

4

Determine if there will be a potential conflict with rear-approaching traffic.*

5

Head and eye movements to observe rearview mirror.

1

4.2.6 Check blind spot for rear-approaching traffic in right lane.

Visually observe right lane over shoulder.

1

Identify if any vehicles are present.

2

Head and eye movements to perform shoulder check.

1

* Difficulty 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 task 4.2.4 (determine whether there is a sufficient gap in right lane) was added to this segment because it is first necessary to determine whether there is room to change lanes and if a speed adjustment is necessary before checking for rear-approaching traffic.(24) It is also noteworthy that no separate task is defined for the actual decision about whether to change lanes. This decision process is the culmination of tasks 4.2.3 through 4.2.6, and the assumption is that the decision is an implicit result of resolving these tasks. Finally, the workload value of the task 4.2.5 (check rearview mirror for rear-approaching traffic) cognitive subtask was incremented because the driver must deal with degraded information from the rearview mirror.

Scenario 4, Segment 2 Diagram

View Alternative Text

Blue dotted outlines indicate general distribution of primary information in key perceptual tasks. 

Note: Illustration dimensions and vehicle positions are not to scale.

Figure 32. Scenario 4–Straight on Green Light Prepare for Lane Change segment diagram.


Table 47. Scenario 4–Straight on Green Light Prepare for Lane Change segment relative timing and duration of segment tasks and summary of key findings.

View Alternative Text

Potential contributions to high workload and information processing bottlenecks:

  • Perceptual tasks involving scanning in 4.2.2, & 4.2.3.
  • Cognitive tasks involving interpretation of information in 4.2.3 and evaluation a single factor under difficult conditions in 4.2.5.
  • Tasks 4.2.3 through 4.2.6 are forced-paced and must be completed in a relatively short period of time.
  • Visual information sources are distributed throughout the scene.

Mitigating factors:

  • None.

Task Pacing and Timing - Task 4.2.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Tasks 4.2.3 through 4.2.6 are forced-paced because they must be completed before initiating the lane change, which is itself constrained by the approaching intersection.

In the task ordering, tasks 4.2.3 through 4.2.6 are shown as overlapping sequential tasks that follow the sequence described in the McKnight and Adams task analysis.(9) The reason for presenting them in this way is that these tasks all involve visual information acquisition (which can only be done in sequence) that requires precise yet rapid deployment of visual gaze and attention to distributed locations throughout the visual scene. Consequently, there is likely to be some interference between sequential tasks; however, because more time is available than in other time-limited segments with task interference (e.g., Decision to Proceed in Scenario 3), the overlap was limited to adjacent tasks, rather than making them all simultaneous.

Scenario 4, Segment 3, Execute Lane Change

The Execute Lane Change segment spans the time from when the subject vehicle initiates the lane change until the driver is established in the new lane. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 48. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 33 and table 49.

Table 48. Scenario 4–Straight on Green Light Execute Lane Change segment

tasks and information processing subtasks.

Task

Perceptual Subtasks

Cognitive Subtasks

Psychomotor Subtasks

4.3.1 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

7

Determine whether perceptual input indicates if current situation is safe/unsafe.

4

Head and eye movements for scanning.

1

4.3.2 Activate turn signal.

Locate the control (automatic behavior).

1

Identify which direction to activate the control (automatic behavior).

1

Activate turn signal control.

1

4.3.3 Adjust vehicle speed to avoid conflicts with right-lane lead vehicle traveling at constant speed.

Visually assess distance and relative speed of lead vehicle in right lane.

4

Determine if distance and relative speed are safe.

6

Decelerate if necessary.
Head and eye movements to view vehicle.

3

4.3.4 Adjust vehicle speed to avoid conflicts with right-lane following vehicle traveling at constant speed.

Visually assess distance and relative speed of following vehicle in right lane.

4

Determine if distance and relative speed are safe.*

7

Accelerate if necessary.
Head and eye movements to view vehicle.

3

4.3.5 Change lane.

Visually observe vehicle lateral position.

1

Identify when vehicle is completely in new lane.

2

Steer into new lane and adjust vehicle position.

3

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

Several aspects of the task analysis and workload estimation warrant discussion. The first is that task 4.3.3 (activate turn signal) was included in this segment because this task should be performed after determining that it is legal and safe to change lanes. Although this task could have been included in the previous segment, it was included here largely because research indicates that in practice, many drivers do not activate the turn signal until the lane change is underway.(25) Also, the turn signal activation workload demands were assigned the minimum value of 1 because this is a learned automatic activity. Another point is that the maintain safe lane position task was not included in this segment because the subject vehicle is changing lanes during this segment.

One assumption in this segment is that tasks 4.3.5 and 4.3.6 (avoiding conflicts with lead and following vehicles in the right lane) do not conflict (e.g., that a speed change required to avoid the lead vehicle will not cause a conflict with the following vehicle) because it is likely that the driver would have decided earlier that there was an insufficient gap to change lanes. Finally, the estimated workload value for the task 4.3.5 cognitive subtask was incremented by a value of 1, because determining whether the relative speed and distance of the following vehicle are safe is more difficult to do using the degraded indirect visual information from the rearview mirror.

Segment 3 Diagram

View Alternative Text

Blue dotted outlines indicate general distribution of primary information in key perceptual tasks.

Note: Illustration dimensions and vehicle positions are not to scale.

Figure 33. Scenario 4–Straight on Green Light Execute Lane Change segment of scenario diagram.


Table 49. Scenario 4–Straight on Green Light Execute Lane Change segment of relative timing and duration of segment tasks and summary of key findings.

View Alternative Text

Potential contributions to high workload and information processing bottlenecks:

  • Demanding cognitive tasks involving evaluation of multiple factors in 4.3.3 & 4.3.4.
  • Tasks 4.3.2 to 4.3.5 are forced-paced and must be completed in a relatively short period of time.

Mitigating Factors:

  • Most of the tasks are routine.

Task Pacing and Timing - Tasks 4.3.2 through 4.3.5 are forced-paced because they must be completed before the subject vehicle gets too close to the intersection.

In the task ordering, task 4.3.2 (activate the turn signal) initiates the segment, whereas tasks 4.3.3 and 4.3.4 are ongoing both before and during the actual lane-change maneuver (task 4.3.5).

Scenario 4, Segment 4, Intersection Entry

The Intersection Entry segment spans the time from when the driver has completed the lane change and is established in the new lane to when the subject vehicle crosses to the other side of the intersection. The tasks, information processing subtasks, and workload estimates associated with this segment are shown in table 50. The scenario diagram, relative timing of tasks, and potential contributions to information processing bottlenecks and mitigating factors are shown in figure 34 and table 51.

Table 50. Scenario 4–Straight on Green Light Intersection Entry segment

tasks and information processing subtasks.

Task

Perceptual Subtasks

Cognitive Subtasks

Psychomotor Subtasks

4.4.1 Maintain safe lane position.

Visually observe roadway ahead.

1

Verify correct lane position.

1

Make necessary adjustments to steering.

1

4.4.2 Check surround for unsafe situations.

Scan for potential obstacles/hazards.
Listen for indications of unsafe situations.

7

Determine whether perceptual input indicates if current situation is safe/unsafe.

4

Head and eye movements for scanning.

1

4.4.3 Maintain safe distance from lead vehicle traveling at constant speed.

Visually assess distance and relative speed of leading vehicle.

4

Determine if distance and relative speed are safe.

4

Reduce speed if necessary.

Head and eye movements to view vehicle.

3

4.4.4 Check that there will be no conflict with following vehicle.

Visually assess distance and relative speed of following vehicle.

4

Determine if following- vehicle closing trajectory is safe.*

5

Head and eye movements to observe rearview mirror.

3

4.4.5 Check for red-light- running cross traffic.

Visually observe vehicles in left and right cross lanes.

5

Determine if crossing vehicles are stopped or will stop in time.

4

Head and eye movements for observing cross traffic.

1

4.4.6 Check for oncoming vehicle trying to turn left across path.

Look for signs of vehicle turning (turn signal, stopped in intersection).

4

Decide if oncoming vehicle is turning.

6

Head and eye movements to view oncoming vehicle.

1

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

Task 4.4.4 (check that there will be no conflict with following vehicle) is included in this segment as a quick doublecheck for potential conflict with the following vehicle. This action is a check for the possibility that the subject vehicle has misjudged the available space, or that the following vehicle was distracted and adopted an unsafe trajectory. In the McKnight and Adams task analysis,(9) the corresponding task is designed to check to see if the following vehicle is trying to overtake or pass. This situation is unlikely to apply this close to the intersection; thus, the purpose of task 4.4.4 was just to doublecheck for conflict.

Also, tasks 4.4.3 (maintain safe distance from lead vehicle traveling at constant speed) and 4.4.4 cognitive subtasks were 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 for this type of treatment 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) In addition, the estimated workload value for the task 4.4.4 cognitive subtask 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.

Scenario 4, Segment 4 Diagram

View Alternative Text

Blue dotted outlines indicate general distribution of primary information in key perceptual tasks.

Note: Illustration dimensions and vehicle positions are not to scale.

Figure 34. Scenario 4–Straight on Green Light Intersection Entry segment diagram.


Table 51. Scenario 4–Straight on Green Light Intersection Entry segment of relative timing and duration of segment tasks and summary of key findings.

View Alternative Text

Potential contributions to high workload and information processing bottlenecks:

  • Concurrent and successive visual scanning (4.4.2) and visual monitoring/localization tasks (4.4.5 & 4.4.6) that must be completed quickly.
  • Some of the critical safety-related tasks are forced-paced.

Mitigating Factors:

  • Several of the tasks are routine, automatic activities.

Task Pacing and Timing - Task 4.4.1 (lane maintenance) is forced-paced because it is part of the ongoing task of driving. Tasks 4.4.5 (check for red-light-running traffic) and 4.4.6 (check for oncoming turning vehicles) are forced-paced because they have to be completed as the subject vehicle is rapidly approaching the intersection.

In the task ordering, task 4.4.3 (maintain safe distance from following vehicle) occurs first because this task should be performed soon after arriving in the new lane because its purpose is to doublecheck for potential conflicts. Also, tasks 4.4.5 and 4.4.6 are ordered based on which one is encountered first, and the other tasks are concurrent.

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 35 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 35, the workload levels peak at moderate to high levels for the perceptual elements during the Prepare for Lane Change segment and remain at moderate levels beyond that. The cognitive elements peak at moderate to high levels at the Execute Lane Change segment and remain at moderate levels during the Prepare for Lane Change and Intersection Entry segments.

Figure 36, which displays the average workload estimate of all the tasks in play during a particular segment interval, shows that there is a peak at high levels for cognitive elements during the Execute Lane Change segment. This high average workload level associated with the underlying tasks is likely responsible for the corresponding peak in the combined workload profile for cognitive tasks (figure 35). Similarly, average workload levels are relatively high for the perceptual elements in all segments, but peak in the Prepare for Lane Change segment, which suggests that the corresponding peak in the combined workload profile (figure 35) arises from high workload levels of the tasks, rather than solely from a high number of overlapping tasks.

View Alternative Text

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

Figure 35. Scenario 4–Straight on Green Light total estimated workload ratings for all tasks in each scenario segment.

View Alternative Text

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.

Figure 36. Scenario 4–Straight on Green Light average estimated workload ratings per task for each scenario 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 52. 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.

Table 52. Combined and average workload ratings, pacing of key tasks, and nature

of bottlenecks that indicate potential problems for each scenario segment.

Segment

Combined Workload

Average
Workload

Pacing of Key Tasks

Nature of Bottlenecks

Approach

 

High perceptual workload.

Some forced-paced tasks.

Perceptual tasks involving scanning or reading and cognitive tasks involving judging multiple factors.

Prepare for Lane Change

Moderate perceptual workload.

High perceptual workload.

Forced-pacing of key tasks.

Rapid sequence of perceptual and cognitive tasks with moderate to high workload that involve distributed information and high time pressure.

Execute Lane Change

Moderate cognitive workload.

High perceptual and cognitive workload.

Forced-pacing of key tasks.

Concurrent and continuous perceptual and cognitive tasks with moderate to high workload conducted under time pressure.

Intersection Entry

 

High perceptual workload.

Forced-pacing of some key tasks.

Perceptual and cognitive tasks with moderate to high workload that involve distributed information with some time pressure.

Prepare for Lane Change nature of bottleneck: High time pressure:

  • Several tasks involving moderate to high perceptual and cognitive workload focused on obtaining information from the environment are needed to decide whether or not a lane change can be made. These tasks must be performed in rapid succession under high time pressure because the time available to make the decision is limited by the approaching intersection.

Execute Lane Change nature of bottleneck: High time pressure:

  • There are concurrent and ongoing perceptual and cognitive tasks with moderate to high workload conducted under time pressure because the subject vehicle must safely get into the new lane with sufficient time to perform the Intersection Entry tasks. These effects are mitigated somewhat because most of these tasks are routine, automatic activities.

Intersection Entry nature of bottleneck: Distributed information:

  • There are perceptual and cognitive tasks with moderate to high workload that involve information distributed throughout the visual scene with time pressure for some of the safety-related tasks. These effects are partially diminished because some of these tasks are routine, automatic activities.

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ResearchFHWA
FHWA
United States Department of Transportation - Federal Highway Administration