Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
2.1 Organization of the HFG This document is divided into five parts. Part I, Introduction, is a short introduction to the document. The first chapter explains why having human factors guidance is useful. This second chapter explains how to use the document and take advantage of its features. Part II, Bringing Road User Capabilities into Highway Design and Traffic Engineering Practice, describes a human factors approach to roadway design, presents basic principles and methods, and provides key information about basic road user capabilities. Part II is about road users and how to take their needs into account. It is the basis from which the guidance in Parts III and IV is derived. Parts III and IV present the actual guidance statements within this document. Part III, Human Factors Guidance for Roadway Location Elements, is organized around specific roadway location elements, such as signalized intersections and work zones. Part IV, Human Factors Guidance for Traffic Engineering Elements, deals with traffic engineering elements such as fixed signage, vari- able message signs, markings, and lighting. The guidance among many of these chapters is inter- related and the chapter sections link to one another. Part V, Additional Information, presents tutorials (see Section 2.4) and collects other informa- tion that may be useful when using the HFG. 2.2 Scope and Limitations of the HFG The HFG is intended to serve a number of important purposes. Specifically, the HFG provides the following: â¢ An introduction to the field of human factors as it is applied to highway design and traffic engineering â¢ Guidance for more optimal design of highways and traffic control devices â¢ Information linking human factors data and analysis with related guidance in other key highway design and traffic engineering reference documents â¢ Help in solving problems related to road user considerations, including identifying probable human factors causes or countermeasures â¢ Objective, defensible information that can be used to support and justify design decisions In addition, the HFG has some limitations. Specifically, the HFG is not the following: â¢ An alternative to primary design references in highway design and traffic engineering. It is intended to complement and amplify aspects of these other references, such as the 2-1 C H A P T E R 2 How to Use This Document
MUTCD (FHWA, 2009), A Policy on Geometric Design of Highways and Streets (AASHTO, 2011), the Traffic Control Devices Handbook (Pline, 2001), the Highway Safety Manual (2010), and other guidance. â¢ A source for comprehensive design specifications nor a redundant treatment of other documents. The HFG is meant to add to, and refine, existing guidance. â¢ A textbook or tutorial on human factors or a comprehensive source of human factors literature. â¢ A guide to crash investigation or a comprehensive reference for safety diagnosis. 2.3 The Two-Page Format In the HFG, a consistent two-page format is used to present the individual human factors guidelines provided in Chapters 5 through 21. On each page, the main issue being addressed by the guideline (e.g., When and How to Use Sight Distance Information, How to Diagnose Sight Distance Problems, etc.) is indicated by centered, bold type within the header. As described in more detail below, the left-hand page presents the title of the guideline; an introduction and overview of the guideline; the guideline itself; the rating associated with the guideline; and a graphic, table, or figure that augments the text information. The right-hand page provides the more detailed supporting rationale for the guideline that a highway designer or traffic engineer may need in order to perform his or her day-to-day design tasks, as well as special design consid- erations, cross-references to related guidelines, and a list of key references. A sample guideline, with key features highlighted, is shown in Figure 2-1; a detailed description of the presentation format of the guidelines follows. HFG HOW TO USE THIS DOCUMENT Version 2.0 2-2 HFG SIGHT DISTANCE Version 0.01 KEY COMPONENTS OF SIGHT DISTANCE Introduction Sight Distance (SD) is the distance that a vehicle travels before completing a maneuver in response to some roadway element or condition that necessitates a change of speed and/or path. Sight Distance is based on two key components: 1) A Reaction Time (RT) required to initiate a maneuver (pre-maneuver phase), and 2) The time required to safely complete a maneuver (Maneuver Time; MT). The reaction time includes the time needed to see/perceive the roadway element, time needed to complete relevant cognitive operations (e.g., recognize hazard, read sign, decide how to respond etc.), and time needed to initiate a maneuver (e.g., take foot off accelerator and step on brake pedal). Maneuver Time includes actions and time required to safely coordinate and complete a required driving maneuver (e.g., stop at intersection, pass a vehicle, etc). Typically, a vehicle maintains its current speed and trajectory during the reaction time phase, while changing its speed and/or path during the maneuver time phase. Design Guidelines Sight Distance = Distance traveled while driver perceives, makes decisions about, and initiates action in response to roadway element (RT) + Distance traveled while the driver completes an appropriate maneuver (MT) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data SCHEMATIC SHOWING THE REACTION TIME AND MANEUVER TIME COMPONENTS OF SIGHT DISTANCE Sufficient Sight Distance Insufficient Sight Distance A B Reaction Time Maneuver Time Reaction Time Maneuver Time Hazard (2ft high) Driverâs Eye (3.5 ft High) Line of Sight Diagram A: The hazard is visible to the driver far enough away that there is sufficient distance for the driver to recognize and react to the hazard and to complete the maneuver necessary to avoid it. Diagram B: Because of the steeper vertical crest, the driverâs sight distance is shorter than in Diagram A making it possible for a hazard to be hidden from sight until there is insufficient distance to avoid it. *Note: distances not to scale 5-1 HFG SIGHT DISTANCE Version 0.01 Discussion Before drivers can execute a maneuver, they must first recognize there is a need for some action and decide what that action should be. Therefore, this mental activityâperception, cognition, and action planningâprecedes an overt vehicle control action and takes some amount of time. The reaction time is typically defined as the period from the time the object or condition requiring a response becomes visible in the driverâs field to view to the moment of initiation of the vehicle maneuver (e.g., first contact with the brake pedal). Although a particular reaction time value (e.g., 2.5 s from AASHTO 2004) is used in deriving sight distance requirements for a given design situation, this âreaction timeâ value should not be viewed as a fixed human attribute, since it is influenced by many factors. Some the of the key factors that influence reaction time are shown in the table below. FACTORS THAT AFFECT THE DIFFERENT COMPONENTS OF REACTION TIME Factor Explanation Low contrast (e.g., night) It takes longer to perceive low-contrast objects Visual glare Objects are perceived less quickly in the presence of glare Older Age Older drivers less sensitive to visual contrast and are more impaired by visual glare (e.g., oncoming headlights) Object size /height Smaller objects/text require drivers to be closer to see them Driver expectations It takes substantially longer to perceive unexpected objects Seeing/ Perceiving Visual complexity It takes longer to perceive objects âburiedâ in visual clutter Older age Older drivers require more time to make decisions Cognitive elements Complexity Drivers require more time to comprehend complex information or situations and to initiate more complex or calibrated maneuvers Initiating Actions Older age Older drivers require more time to make vehicle control movements and they may be limited their range of motion In contrast to the reaction time, the maneuver time is primarily affected by the physics of the situation, including vehicle performance capabilities. In particular, tire-pavement friction, road-surface conditions (e.g. ice), downgrades, etc. can increase maneuver time or make some maneuvers unsafe at higher speeds. Maneuver time is also affected to a lesser extent by driver-related factors (e.g., deceleration profile), but these factors are highly situation specific since the maneuvers are very different (e.g., emergency stop, passing, left turn through traffic etc.). These factors are covered in more detail in the relevant guideline sections (see GLâ¦). Design Issues It is important to note that although most design requirements are expressed as a design distance, from the driverâs perspective the critical aspect is time. It takes time to recognize a situation, understand its implications, decide on a reaction, and initiate the maneuver. While this process may seem almost instantaneous to us when driving, it can translate into hundreds of feet at highway speeds before a maneuver is even initiated. Speed selection is also critical, since the relative speed between the driver and the hazard determines how much distance is traversed in the time it takes the driver to initiate and complete the maneuver (see Speed GL). Cross References Specific types of sight distance (pg. 5-X, 5-Xâ¦); Greenbook section on calculating sight distance Curves, Traffic engineering elements (signs), decision sight distance? (these are not currently included as HFG topics) Key References None 5-2 Right-hand page References Cross References Design Issues Discussion Abbreviated Handbook Title (Both Pages) Abbreviated Chapter Title (Both Pages) Revision Version (Both Pages)Guideline Title Bar Scale Rating Introduction Design Guideline Left-hand page Page Numbers Figure, Table, or Graphic Figure 2-1. Guideline format used in the HFG.
2.3.1 The Left-Hand Page The guideline title is indicated by centered, bold type at the top of the left-hand page. 18.104.22.168 Introduction This subsection briefly defines the guideline and provides basic information about the road- way design parameter and the guideline. For example, this subsection might be used to provide the unit of measurement (e.g., visual angle, meters, foot-lamberts) for the guideline or to pro- vide equations for the derivation of certain parameters. 22.214.171.124 Design Guideline This subsection presents a quantitative guideline (when possible), either as a point value, a range, or an explicit recommendation. The guideline is always presented prominently and is enclosed in a blue box that is centered on the page. In some cases, the guideline is presented qualitatively in general terms (e.g., âIf the operating speed of a roadway is substantially higher than the design speed, then it may be appropriate to increase sight distance for higher traveling speeds.â). However, in most cases, the design guide- line is presented quantitatively (e.g., âThe reaction time component of stopping sight distance can be expected to be 1.6 s under good-visibility, good-traction conditions.â). 126.96.36.199 The Bar-Scale Rating System For some design topics, enough empirical data exist to provide well-supported guidelines, and the use of expert judgment is minimal. For others, empirical data have provided only the foun- dation for a decision about what the guideline should be, but experience and judgment have been used to determine the final guideline. For yet other topics, little or no empirical data were avail- able, and the guideline was based primarily on expert judgment. To aid highway designers and traffic engineers in making design trade-offs, individual guidelines have been rated according to the relative contribution that empirical data and expert judgment have each made to the guideline. Specifically, each guideline has been rated along a continuum, with each guideline falling somewhere between âBased Primarily on Expert Judgmentâ and âBased Primarily on Experimental Data.â These terms are defined below. Based Primarily on Expert Judgment. Little or no empirical data were used to develop this guideline. Expert judgment and design convention were used to develop this guideline. Based Equally on Expert Judgment and Experimental Data. Equal amounts of expert judg- ment and experimental data were used to develop this guideline. There may have been a lack of consistency in the research finding, requiring greater amounts of expert judgment. Or, research may have been lacking in this area, requiring the results of research from related content domains to be interpreted for use in this context. Based Primarily on Experimental Data. The guideline is based on high quality and consis- tent data sources that apply directly to the guideline. Empirical data from highly relevant con- tent domains (e.g., transportation human factors, driver performance data) were primarily used to develop this guideline. Little expert judgment was required to develop this guideline. 188.8.131.52 Figure, Table, or Graphic This subsection provides a figure, table, or graphic to augment the guideline. This figure, table, or graphic provides âat-a-glanceâ information considered to be particularly important to the conceptualization and use of the guideline. It provides a visual representation of the guideline 2-3 HFG HOW TO USE THIS DOCUMENT Version 2.0
(or some aspect of the guideline) that may be difficult to grasp from the design guideline itself, which is quantitative and text based. This figure, table, or graphic may take many forms, including a drawing depicting a generic application of a guideline or a particular design issue, a flowchart of measurement procedures for the guideline, a table that summarizes the guideline, or schematic examples of specific design solutions. 2.3.2 The Right-Hand Page 184.108.40.206 Discussion This subsection briefly summarizes the rationale behind the choice of the guideline. In par- ticular, the discussion explains the logic, premises, assumptions, and train-of-thought associated with development of the guideline. The focus is on a presentation of driver limitations and capa- bilities deemed relevant to the particular guideline topic. The discussion can take many forms, including a brief review of applicable empirical studies, references to traditional design practice, or an analysis of relevant information. The discussion is presented primarily to help HFG users understand the guideline and to help them explain or justify the guideline to other members of their respective development teams. Also, because these human factors guidelines are expected to be revised as additional empirical data become available, this subsection will be useful to future developers of the guidelines. In particular, the discussion will enable future guideline developers to determine how new infor- mation on road usersâ capabilities and limitations can (or should) be integrated into the exist- ing guidelines. For example, the design guideline âDetermining Stopping Sight Distanceâ in Chapter 5 has been developed through consideration of experimental data gathered under a range of visibility (good and poor) and vehicle traction (good and poor) conditions. Thus, this guideline is presented as being the sum of driver reaction time plus vehicle deceleration, under a range of visibility/ traction conditions. If new driver performance analyses or data for these conditions are obtained (or if new assumptions are made), future design guideline developers will be able to evaluate the quality and applicability of this new information relative to the discussion in the current design guideline âDetermining Stopping Sight Distanceâ and determine what (if any) changes should be made to the design guideline. 220.127.116.11 Design Issues This subsection presents special design considerations associated with a particular guideline. These special considerations may include design goals from the perspective of other disciplines (e.g., highway engineering, urban planning, physiology), interactions with other guidelines, spe- cial difficulties associated with the guidelineâs conceptualization or measurement, or special per- formance implications associated with the guideline. 18.104.22.168 Cross References This subsection lists the titles and page numbers of other guidelines within the handbook that are relevant to the current guideline. 22.214.171.124 References This subsection lists the references associated with the formulation of the guideline. Each of these references will have been assigned a reference number that was used to note it within the text of the design guideline (e.g., as part of the introduction, discussion, or design issues sections). A complete reference section is provided in Chapter 23 of this document. HFG HOW TO USE THIS DOCUMENT Version 2.0 2-4
2.4 Tutorials Tutorials are provided in the HFG for important topics, special issues, and detailed procedures that cannot be addressed within the two-page constraints of individual guidelines. 2.5 Other Features A Glossary is provided in Chapter 24. Technical words and phrases are defined in the Glos- sary and listed in the Index (Chapter 25). Abbreviations are provided in Chapter 26. Also, equa- tions are numbered sequentially and listed separately in Chapter 27. 2-5 HFG HOW TO USE THIS DOCUMENT Version 2.0