Human Factors Guidelines for Road Systems: Second Edition (2012) / Chapter Skim
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Tutorial 1: Real-World Driver Behavior Versus Design Models .
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Tutorial 1: Real-World Driver Behavior Versus Design Models Much of the information on sight distance presented in Chapter 5 reflects the application of empirically derived models to determine sight distance requirements. Such models, while valuable for estimating driver behavior across a broad range of drivers, conditions, and situations, have limitations.
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This model shows that the sight distance requirement is composed of (at least) two distances: there is a distance traveled while the driver perceives and evaluates a situation (determined by PRT and vehicle speed)
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Conditions or Events that Occur Prior to a Hazardous Event/Object Becoming Visible to the Driver The model shown in Figure 22-1 is not sensitive to events that happen prior to the moment that the hazardous object or event becomes visible to the driver. In reality, the driver's ability to react to a hazardous object or event may be strongly influenced by previously occurring conditions or events.
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The basic sight distance behavioral model (Figure 22-1) makes assumptions about driver cognitive state and speed choice as the hazardous event is encountered.
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is generally reasonable from a design perspective, however, because it is somewhat conservative. Specifically, those drivers who encounter a situation without planning or anticipation are those most likely to be in need of the full sight distance requirement.
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distance, driver/vehicle capabilities) determines options and shapes the way drivers respond, and often multiple options are available to the driver.
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ical observations made at the site may be at variance with the predicted behaviors. Even when design equations are based on "good" data, the generality of the models suggests that credence should be given to any empirical data that can be collected at the site itself.
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Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies Introduction The previous sections of this document -- especially Chapter 5 -- have provided design guidelines for human factors aspects of various sight distance concepts. However, for users to implement these guidelines in a practical sense, it is desirable to provide a procedure for their operational application.
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H FG T U TO RIA LS Version 2.0 22-10 Figure 22-3. Flow diagram of six-step diagnostic process.
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22-11 HFG TUTORIALS Version 2.0 Step 1A: Identify Hazard and Prepare Site Diagram The specific hazard location under investigation is identified and the approach roadway is diagrammed. Example of hazards requiring sight distance consideration and the associated sight distance concepts are as follows.
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HFG TUTORIALS Version 2.0 22-12 Step 1C: Observe Erratic Vehicle Maneuvers on Approach Observations of vehicle movements should be considered in situations of sufficiently high traffic volumes to justify this type of study, e.g., 100 vehicles per hour (vph) and above.
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22-13 HFG TUTORIALS Version 2.0 Step 1F: Note Factors Affecting Flow Speeds Certain roadway environmental features are known to affect drivers' selection of speed. Examples are pavement defects, narrow shoulder widths and protruding bridge piers, abutments, guardrails, median barriers, etc.
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HFG TUTORIALS Version 2.0 22-14 Step 1I: Label the Diagram with Specified Symbols SDHAZ -- Sight distance to a potential hazard. The point at which a location or object is first detectable to an approaching motorist.
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22-15 HFG TUTORIALS Version 2.0 Step 2: Conduct Preliminary Engineering Analyses This step involves the application of traditional traffic engineering techniques (e.g., AASHTO Design Policy geometric design criteria and DSD warrant) as a preliminary determinant of site deficiencies.
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HFG TUTORIALS Version 2.0 22-16 Step 2D: Examine Approach with respect to DSD Warrants The approach to the hazard location also must be examined for conditions of visual clutter meeting requirements for DSD application. In particular, these conditions could take the form of roadside distractions and/or complex TCDs at intersections along the approach.
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22-17 HFG TUTORIALS Version 2.0 Step 3: Apply Crash Data This step involves the integration of traffic crash data into the analysis. The objective is to locate specific crash-prone locations within the roadway segment, which may be indicative of sight distance problems.
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HFG TUTORIALS Version 2.0 22-18 Step 3C: Examine Potential Sight Distance Causation Effect Certain patterns of crash behaviors (i.e., pre-collision maneuvers) are suggestive of sight distance problems: for example, single-vehicle or run-off-road crashes with a fixed object that may appear visible under some conditions but may not be easily detectable to drivers during conditions of more limited visibility (e.g., darkness)
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22-19 HFG TUTORIALS Version 2.0 Step 4A: Establish and Plot Action Points Along Approach Segment Specific locations within the study roadway section requiring a driver action (e.g., maneuver) will be identified and plotted.
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HFG TUTORIALS Version 2.0 22-20 Step 4B: Establish and Plot Information Sources and Associated Sight Distances Along Approach Segment Any driver action (e.g., hazard avoidance) must be based on information available to the driver.
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22-21 HFG TUTORIALS Version 2.0 Step 4C: Define Component Driver Response Sections Within Approach Segment Distinctly different driver informationprocessing tasks are associated with each detection and maneuver activity. In this step, roadway sections will be designated and plotted to illustrate the required travel distances over which the driver would perform these varied information-processing and maneuver tasks.
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HFG TUTORIALS Version 2.0 22-22 Step 5A: Determine the Relevant Geometric Design Sight Distance Application The analysis of driving task requirements involves application of the appropriate sight distance value for the given task. Sight distance requirements (to accommodate both the information-processing and maneuver tasks)
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Step 5B: Determine Driving Task Requirements Within Each Component Roadway Segment Case 1: Direct line of sight to hazard; no traffic control SDHAZ ➞ A In this case, PRT and MT are determined from Section 5.2. Case 2: Intervening traffic control device (i.e., warning of hazard)
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HFG TUTORIALS Version 2.0 22-24 Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task Component No TCDs present: SDHAZ ➞ A Apply applicable PRT and MT requirement corresponding to predetermined condition (i.e., SSD, ISD, DSD, or PSD as determined in Step 5A)
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22-25 HFG TUTORIALS Version 2.0 Step 5D: Assess the Adequacy of the Available Sight Distance Components Case 1: Direct line of sight to hazard; no traffic control SDHAZ ➞ A Does the subsection length SDHAZ ➞ A allow sufficient time for the driver to perform any required hazard avoidance maneuver? Case 2: Intervening traffic control device (i.e., warning of hazard)
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Step 6: Develop Engineering Strategies for Improvement of Sight Distance Deficiencies In this final step, the practitioner recommends improvement (e.g., traffic control device applications or minor design modifications) to correct deficiencies.
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22-27 HFG TUTORIALS Version 2.0 Example Application: Sight Distance Diagnostic Procedure The example driving situation consists of a 55-mi/h, two-lane rural roadway that approaches a 35-mi/h curve followed by a stop-controlled intersection. The intersection approach is to a main highway, which requires application of destination guide signing.
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Signed Intersection Approach Segment Steps 2A through 2D: Examine Site with Respect to AASHTO Design and DSD Criteria. For the purpose of this example, it is assumed that geometrics conform to AASHTO and that DSD criteria (e.g., visually cluttered environmental conditions)
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• Case 1, direct line of sight to hazard (i.e., 35-mi/h speed zone to intersection) : SDHAZ ➞ A • Case 2: Three intervening traffic control devices – A route shield assembly: SDTCD1 ➞ LDTCD1 ➞ TCD1 ➞ A – A destination name sign: SDTCD2 ➞ LDTCD2 ➞ TCD2 ➞ A – A stop sign: SDTCD3 ➞ LDTCD3 ➞ TCD3 ➞ A This roadway segment is diagrammed in Figure 22-7.
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Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve) : SDHAZ ➞ A Because DSD does not apply (determined previously)
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Step 5B: Determine the Driving Task Requirements. Considering the two possibilities (i.e., Case 1 in which the driver proceeds to the intersection ahead while ignoring the signs, and Case 2 whereby the driver observes and comprehends the intermediate signs)
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The legibility distance of symbol signs has been researched in a laboratory study (Dewar, Kline, Schieber & Swanson, 1994) and found to significantly exceed that of legend signs (despite the high degree of variability in the study data)
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The first guide sign assembly contains two numbers and two symbols, requiring 3.0 s of reading time; the second contains two designation names and two symbols, also requiring 3.0 s; and the third is a simple and familiar one-word regulatory sign, requiring 1 s. Thus the total sign reading time is 7.0 s.
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Step 6: Develop Engineering Strategies for Improvement of Sight Distance Deficiencies Not conducted as part of this example. HFG TUTORIALS Version 2.0 22-34 Sign Legibility Distance (ft)
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22-35 HFG TUTORIALS Version 2.0 Tutorial 3: Detailed Task Analysis of Curve Driving A task analysis of the different activities that drivers must conduct while approaching and driving through a single curve (with no other traffic present) was conducted to provide qualitative information about the various perceptual, cognitive, and psychomotor elements of curve driving.
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HFG TUTORIALS Version 2.0 22-36 Table 22-6. Driving tasks and information-processing subtasks associated with a typical curve.
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• Groeger, J
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HFG TUTORIALS Version 2.0 22-38 Tutorial 4: Determining Appropriate Clearance Intervals Methods for determining appropriate clearance interval length vary from agency to agency, and there is no consensus on which is the best method. The Institute for Transportation Engineers recommends several procedures for determining clearance interval duration in a 1994 informational report (see ITE, 1994)
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22-39 HFG TUTORIALS Version 2.0 Tutorial 5: Determining Appropriate Sign Placement and Letter Height Requirements When determining the appropriate sign placement, it is important to consider a number of driver-related factors. The Traffic Control Devices Handbook (Pline, 2001)
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HFG TUTORIALS Version 2.0 22-40 Another method for calculating reading time, cited in previous studies, applies to complex signs in high-speed conditions. The formula provided is: After finding the reading time, convert it into a reading distance by multiplying by the travel speed.
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22-41 HFG TUTORIALS Version 2.0 Step 4. Calculate the Information Presentation Distance The information presentation distance is the total distance from the choice point (e.g., intersection)
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HFG TUTORIALS Version 2.0 22-42 one symbolic arrow. The sign is placed 200 ft in advance of the intersection.
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22-43 HFG TUTORIALS Version 2.0 Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions The amount of information that can be displayed on a CMS is limited by the amount of time that the driver has to read the message. This amount of time in turn is determined by the legibility distance of the sign and the traveling speed of the passing vehicle.
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HFG TUTORIALS Version 2.0 22-44 Step 2. Use the Driver Speed to Find the Base Maximum Number of Information Units Allowed in a Message The maximum number of information units is derived from the legibility distance of the CMS (which depends on the technology used)
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22-45 HFG TUTORIALS Version 2.0 In general, permanent CMSs that are mounted over the roadway are not affected by crest vertical curves (Dudek, 2004)
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HFG TUTORIALS Version 2.0 22-46 for Displaying Messages with Dynamic Characteristics. Although the blanking time was only tested between phases 1 and 2 (not between 2 and 1)
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22-47 HFG TUTORIALS Version 2.0 • Omit redundant information Example: Original Message: Shortened Message: MAJOR ACCIDENT MAJOR ACCIDENT ON I-276 NORTH PAST I-80 PAST I-80 2 LEFT LANES CLOSED 2 LEFT LANES CLOSED KEEP RIGHT If the CMS is on I-276, the same freeway as the accident, the information is evident to the drivers and may be omitted. The information units "2 Left Lanes Closed" and "Keep Right" are redundant because drivers can assume that if the two left lanes are closed, they will need to move to the right.
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HFG TUTORIALS Version 2.0 22-48 Step 5C. Use Priority Reduction Principles If the message still contains more information units than should be displayed, the information units should be reduced in order of priority.
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