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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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SECTION 1. INTRODUCTION

Intersection navigation is a particularly hazardous component of driving. For example, in 2003, more than 9,213 Americans lost their lives as a result of intersection-related crashes.(1) In total, intersection-related crashes account for more than 2.7 million crashes each year, which amounts to more than 45 percent of all reported crashes.(1) Even though intersections comprise just a small amount of the total roadway surface area, they contribute to a relatively high proportion of crashes because they are the critical points in the roadway system where traffic movements are most frequently in conflict with each other.

In addition to a greater frequency of conflicts, intersections generally are more complex and difficult to navigate than most other stretches of roadway. More specifically, intersections can be visually complex, requiring that drivers scan several different areas and keep track of several different elements to get the information they need to safely pass. Also, there are more hazards to deal with in terms of pedestrians and other traffic, such as turning and crossing vehicles that can encroach into a driver's path. Intersections also represent action points in which drivers may frequently have to make a response based on emerging traffic conditions under time pressure (e.g., change lanes to continue past stopping vehicles or decide to stop on a yellow light). Thus, intersection driving involves a multitude of different elements and hazards that can combine to increase the difficulty and workload that drivers face. When drivers are unable to meet these higher demands, their risk of making critical driving errors that can lead to conflicts with other road users also increases.(2)

The purpose of this report is to identify the information processing bottlenecks that drivers face in specific intersection driving scenarios. These bottlenecks represent situations in which drivers may become overloaded by driving demands, which could result in drivers conducting important driving tasks in improper fashion (e.g., taking too quick a look at oncoming traffic while turning left and failing to see an oncoming vehicle) or skipping certain tasks altogether (e.g., failing to check the blind spot while making a lane change under time pressure). Information processing bottlenecks can arise as the complexity/difficulty of normal driving tasks is increased or as additional tasks are added, or both, as drivers deal with emerging situations that require analysis and appropriate responses. Identifying and characterizing these bottlenecks can provide useful information for future safety efforts by identifying situations in which drivers could benefit from countermeasures that reduce driving demands, and also by identifying the types of measures that would be most relevant to the underlying problems.

To identify and characterize information processing bottlenecks, this report presents a series of task analyses to determine key functions performed by drivers as they approach and navigate through different intersection scenarios. A task analysis is the systematic analysis or breakdown of how specific tasks are accomplished in a situation, such as what subtasks are required and in what sequence they occur. The focus of the present task analysis is on identifying the underlying information processing elements, including the perceptual, cognitive, and psychomotor subtasks associated with each individual driving task. A key benefit of using a task analysis approach is that it provides specific information about driver activities at various points during intersection navigation that is not available in other approaches, such as crash data analyses, performance studies, and focus group research. This information about driver activities can also be quantified in terms of overall workload to identify when and under what conditions that information processing bottlenecks occur.

Seven distinct driving scenarios were investigated in the task analysis. Each scenario was then separated into segments, tasks, and subtasks/information processing elements. Where possible, existing task analyses were used as a starting point for the task analyses provided in this report.

The body of this report contains three technical sections. Section 2 describes the methods used to conduct the task analyses. It includes a description of the basis for the task analyses, the process of selecting scenarios to include in the task analyses, and procedures used to develop and present the task analyses. Section 3 provides the results from the individual task analyses. The results are presented in seven subsections, corresponding to the seven unique intersection driving scenarios captured in the task analyses. Each scenario includes the following components:

  • Basic description.
  • Details and assumptions.
  • Diagram of individual segments and vehicle locations within the scenario.
  • Timeline of key activities, milestones, and constraints within the scenario.
  • List of segments, tasks, and subtasks.
  • Detailed analysis of individual segments, including information processing elements, workload estimates, a scenario diagram, and an analysis of task timing and duration characteristics.
  • Profiles of total and average workload estimates.
  • Summaries of key bottlenecks, constraints, and driving challenges.

Section 4 provides a summary of key findings and conclusions from the task analyses. Appendix A provides a detailed discussion of the equations and assumptions associated with the development of the vehicle timing and dynamics calculations performed for each of the seven driving scenarios included in the report.

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