Human Factors Guidelines for Road Systems: Second Edition (2012)

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Tutorial 1: Real-World Driver Behavior Versus Design Models . . . . . . . . . . . . . . . . . . . . . . .22-2 Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies . . . . . . . . .22-9 Tutorial 3: Detailed Task Analysis of Curve Driving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22-35 Tutorial 4: Determining Appropriate Clearance Intervals . . . . . . . . . . . . . . . . . . . . . . . . . . .22-38 Tutorial 5: Determining Appropriate Sign Placement and Letter Height Requirements . .22-39 Tutorial 6: Calculating Appropriate CMS Message Length under Varying Conditions . . .22-43 22-1 C H A P T E R 22 Tutorials

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 valu- able for estimating driver behavior across a broad range of drivers, conditions, and situations, have limitations. This tutorial discusses how driver behavior as represented in sight distance models may dif- fer from actual driver behavior. The design models presented in Chapter 5 use simplified con- cepts of how the driver thinks and acts. This simplification should not be viewed as a flaw or error in the sight distance equations. These models are a very effective way of bringing human factors data into design equations in a manner that makes them accessible and usable. After all, the intent of a sight distance equation is not to reflect the complexities of human behavior but to bring what we know about it into highway design in a concise, practical way. However, like any behavioral model, models for deriving sight distance requirements are not precise predictors of every case and there may be some limitations to their generality. Therefore, having an under- standing of certain basic principles of human behavior in driving situations is useful to better interpret these models and to understand how they may differ from the range of real-world driv- ing situations. Sight distance formulas for various maneuvers (presented in Chapter 5) differ from one another, but they share a common simple behavioral model as part of the process. The model assumes that some time is required for drivers to perceive and react to a situation or condition requiring a particular driving maneuver (i.e., PRT), which is followed by some time (i.e., MT) and/or distance required to execute the maneuver. Sight distance equations for some maneuvers may contain additional elements or assumptions; however, all have this basic two-stage model somewhere at their core. The two equations that follow show two versions of the general, two-component model. In both versions, the first term shows the distance traveled during the PRT component and the sec- ond term shows the distance traveled during the MT component. The difference is that the first equation shows a case where the distance traveled while executing the maneuver is based on the time required to make that maneuver (for example, the time to cross an intersection from a Stop), while the second equation shows a case where the distance traveled while executing the maneuver is based directly on the distance required to complete the maneuver (for example, braking distance for an emergency stop). For both forms of this general equation, vehicle speed (V) influences the second (MT) component. The general form of the sight distance equation is: Where: d = required sight distance V = velocity of the vehicle(s) tprt = PRT tman = MT dmanV = distance required to execute a maneuver at velocity V k = a constant to convert the solution to the desired units (feet, meters) d kVt kVt where maneuver time is iSD prt man= + , nput or d kVt d where maneuver tiSD prt manV= + , me is input HFG TUTORIALS Version 2.0 22-2

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) and a distance traveled while executing the maneuver (determined by maneu- ver time/distance and vehicle speed). Figure 22-1 depicts the activities and sequence of activities associated with this simple model. As the figure shows, the PRT component is itself viewed as a series of steps. These individual steps are not explicit in the design equation but are included in the assumptions that underlie the PRT value. Design equations and their assumptions for specific maneuvers were discussed in Chapter 5. The sequential model of driver behavior shown in Figure 22-1 is a shared common conceptual underpinning of various sight distance equations. However, in some respects, we can consider this model to be a “convenient fiction,” in part because it depicts a simple, fixed, linear, and mechanistic process. While the model provides a useful basis for deriving approximate quantitative values for design requirements that work for many situations, real-world driving behavior is far more complex than the model suggests. While highway designers and traffic engineers are often required to work with less complex (i.e., imper- fect) models of human visual perception, attention, information processing, and motivation, it is important that they understand those factors that may affect the application of design sight distance models for specific situations. Such an understanding will help them to prevent, recog- nize, or deal with sight distance issues that may arise. For a particular situation, the standard sight distance design equation might either underestimate or overestimate the actual needs of a driver. Subsequent sections of this tutorial deal with specific factors that affect the driver response and provide guidance for working with them. Before these specific factors are considered, it will be useful to have an appreciation of how the simple driver models that underlie sight distance requirements contrast with the real complexities of driver behavior. There are a number of factors or conditions associated with driver responses to a hazardous event or object that are not reflected in the basic sight distance model, but nonetheless can have a profound effect on driver behavior and overall roadway safety: • Conditions or events that occur prior to a hazardous event/object becoming visible to the driver • How and when the driver processes relevant information • Driving as an “episodic” activity versus driving as a “smooth and continuous” activity • The nature of the hazardous object or event • The nature of the driver’s response • Individual differences across drivers • The quality and applicability of the empirical research used to develop the driver models Each of these is discussed in more detail below. 22-3 HFG TUTORIALS Version 2.0 Figure 22-1. Diagrammatic version of the basic sight distance model.

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 condi- tions or events. For example, drivers traveling on a roadway with few access points and little traf- fic may be unprepared to stop for a slow-moving vehicle ahead. In contrast, if drivers had been encountering numerous commercial driveways and intersections, with entering truck traffic, they might more readily react. Roadway design and operational features in advance of a haz- ardous event/object becoming visible are potentially important influences on behavior that are not explicit in the basic sight distance model. Figure 22-2 shows an expansion of the basic model, with added “driver state” factors (e.g., anticipation, situational awareness, caution, and locus of attention) that increase or decrease the driver’s cognition preparation for a hazardous condition or event. In Figure 22-2, an addition component to the model is shown prior to the event becoming visible. One element of the additional component is cognitive preparation. This general term encompasses the various active mental activities that can influence response times and deci- sions, such as driver expectancies, situational awareness, a general sense of caution, and where attention is being directed by the driver. Part II: Bringing Road User Capabilities into High- way Design and Traffic Engineering Practice provides some further explanation of these factors. As the arrows in the figure show, the driver’s cognitive preparation as he or she encounters a hazardous object or event can influence the speed of detection, the speed and accuracy of recognizing the situation, and the speed and type of decision made about how to respond. The critical point is that the PRT associated with a particular hazardous object or event is influenced by the conditions or events preceding the driver’s perception of the haz- ardous object or event. The second element in the additional component in Figure 22-2 that occurs prior to the driver’s perception of the hazardous object or event is speed selection. As discussed earlier, speed can have perceptual effects, influencing how easily a target object is detected or how accurately gaps are judged. Speed may affect the driver’s sense of urgency, which can influence what maneu- ver options are considered and their relative appeal. Speed also may directly affect the difficulty, as well as the required time or distance, of the maneuver. Therefore, the driver’s speed choice prior to the event may influence the driver’s decision process; it may also influence the time avail- able for the driver’s response. HFG TUTORIALS Version 2.0 22-4 Figure 22-2. Added elements to basic sight distance behavioral model.

The basic sight distance behavioral model (Figure 22-1) makes assumptions about driver cog- nitive state and speed choice as the hazardous event is encountered. In reality, the driver does not arrive at the situation as a “blank slate.” The locus of a sight distance problem, or its solu- tion, therefore may turn out to be in advance of the problem site itself. How and When the Driver Processes Relevant Information The basic sight distance model shows a chain of mental and physical events taking place in the following sequential fashion: 1. A hazardous object or event becomes visible. 2. The presence of this object or event is detected by the driver. 3. The object or event is recognized and understood by the driver. 4. The driver makes a decision about what maneuver is needed to avoid or respond to the object or event. 5. The maneuver is initiated. 6. Once initiated, the maneuver is fully executed. Each event in this chain takes some amount of time to occur, and—according to the basic model—one step does not begin until the previous step is complete. This assumed “serial pro- cessing” model is indeed one way a driver might respond, but it may not be typical. For exam- ple, if a driver sees some vague object ahead of the vehicle that might or might not be in the roadway, he or she may begin to brake even before the object is fully recognized. Also, once the object is fully recognized, the maneuver may be reconsidered (e.g., stopped, slowed, accelerated, or otherwise revised). Contrary to the serial processing assumed by the basic model, the mental processes shown by the various boxes in Figure 22-1 may actually occur in parallel, in a differ- ent sequence, or with modifications (feedback loops) as the process progresses. The assumed lin- ear response sequence is therefore really a simplified case used for design purposes. It should not be viewed as a universal or invariant representation of the more complex perceptual and cogni- tive activity in complex driving situations. Importantly, consistency in geometric design is required in order to meet driver expectations and to avoid surprising the driver. Driving as an “Episodic” Activity versus Driving as a “Smooth and Continuous” Activity Related to the previous point, the basic sight distance model reflects an “episodic” perspective of real-world driving. That is, some object or event becomes visible, and some driver maneuver(s) in response to the object or event are initiated and executed. Then, another object or event becomes visible, and another maneuver takes place. Real-world driving however, is normally smooth and continuous; it is not a jerky sequence of separate, individual episodes. Yet for ease of analysis, we often break driver behavior into individual events each requiring their own separate response, or we treat the roadway as a succession of discrete segments or zones. To the driver, though, the road- way and the driving task are generally smooth and continuous. Real drivers do not just react to events that randomly occur; they plan and predict and manage and adapt to events as they go along. Adopting an “episodic” perspective is useful for developing models of driver behavior that are both simple and reasonably predictive. A “smooth and continuous” perspective of real-world driving is much more difficult to model and quantify, especially in a manner that will easily generate a sim- ple design parameter. From a human factors perspective, sight distance models are based on a lit- tle bit of driver performance data that describe how a driver might react, but may not reflect how drivers always or even typically behave. The use and application of the simpler sight distance model 22-5 HFG TUTORIALS Version 2.0

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. The Nature of the Hazardous Object or Event For each sight distance design application, the analysis is based around some object, event, or roadway feature to which the driver must respond with a driving maneuver. That object, event, or roadway feature might be debris in the roadway, braking by a vehicle ahead, an approaching vehicle on a conflicting path, a freeway lane drop, a change in signal phase, a pedestrian entering the road, a railroad gate, an animal, a vehicle entering from a driveway, or many other things. The PRT process begins with the potentially hazardous object or event (the “visual target”) becoming visible to the driver followed by some time to visually detect and recognize that target. Design equations have to include some estimate of when a target becomes visible and how long driver reaction will take. The many examples of potential hazards suggest just how different these may be as visual targets; therefore, making a single assumption is an obvious simplification. A target object may be large or small, bright or dull, familiar or unfamiliar, moving or stationary, or have other attributes that affect the driver’s ability to accurately and quickly detect and recognize it. Explicitly or implicitly, design equations have to make some assumption about the characteris- tics of the visual target. Furthermore, visibility conditions may vary with weather, glare, light condition, roadway lighting, and intervening traffic (especially truck traffic). Again, design equa- tions must be based on some assumption about visibility conditions. A PRT model requires the user to be able to specify the point in time or space that the hazard becomes visible to the driver. However, this too may be an oversimplification. For example, there is usually no sharp threshold where an object in the road suddenly goes from being invisible to visible. Most hazards do not occur all at once, but evolve over some time, such as a vehicle mov- ing into a lane in front of a driver. Some events might have a preview, such as a vehicle positioned in a driveway prior to its pulling out or children playing near the road prior to entering the road. Some events might have multiple cues; for example, a freeway lane drop has an initial taper, lane markings, and the point where the lane finally disappears. Sometimes the important visual target is not the hazard object or event itself but a cue about the hazard; for example, brake lights on a vehicle ahead may be a warning cue about a sudden severe deceleration, but they may also reflect a minor tap on the brake. Drivers cannot respond to the brake light in the same way they respond to recognition of the actual deceleration. Overall then, the driver’s response to a hazardous event or object will reflect specific physical characteristics, visibility conditions, and the evolving nature of the hazard itself. The Nature of the Driver’s Response The behavioral components of sight distance models are based around some very specific maneuver in response to the object/event, with fixed assumptions about response parameters. For example, when responding to an unexpected need to stop, AASHTO (2004) assumes a braking maneuver with a deceleration of 3.4 m/s2 (11.2 ft/s2). Braking may be a reasonable response to assume, and 3.4 m/s2 may be a reasonable deceleration to assume, but this certainly does not mean that braking at this level is always the driver’s response to an unexpected hazard. The maneuver time and maneuver distance components of sight distance models are in many cases based on good empirical research and human factors considerations and work well for most applications. Still, the use of a single standard value is a convenient simplification. Actual maneuvers can be influenced by various factors. The perceived urgency of the situation (based on available time/ HFG TUTORIALS Version 2.0 22-6

distance, driver/vehicle capabilities) determines options and shapes the way drivers respond, and often multiple options are available to the driver. For example, for an unanticipated stop, a driver may brake severely, or brake gradually and steer around, or swerve sharply. The surrounding physical, traffic, and social environment will affect these options: is there a lane or shoulder to steer around, are there adjacent or following vehicles, is the obstacle a piece of debris or a child, is there a passenger in the vehicle? Drivers also make trade-offs between speed versus control when executing maneuvers. The AASHTO deceleration value of 3.4 m/s2 represents an estimate of a “comfortable deceleration” with which almost all drivers can maintain good vehicle control. In this sense it is appropriate for general design, but does not necessarily describe what drivers can do or actually do under all conditions or circumstances. Furthermore, once a driver initially selects and begins to execute a particular maneuver, the maneuver is not simply executed in a fixed man- ner. As Figure 22-2 illustrates, the situation is monitored and the maneuver is re-evaluated as it is being executed. The response may be refined or modified as it progresses. Drivers may not respond to a situation with a maximum response (e.g., maximum braking or steering), but may initiate a more controlled action and monitor the situation before committing to a more extreme action. For instance, they may begin gradual braking and check their mirrors for following traf- fic before decelerating more sharply or swerving. Individual Differences Across Drivers The diverse driving population ranges widely in capabilities and behaviors. Drivers vary in experience, visual acuity, contrast sensitivity, useful field of view, eye height, information process- ing rate, tolerance for deceleration, physical strength, and other factors related to PRT and MT. A design equation will typically be based around a design driver with some assumed set of attri- butes. To be conservative, the assumptions do not usually represent a typical driver, but rather reflect less capable drivers (e.g., 15th percentile in terms of some attribute). Assumptions are made about the state of the driver as well. For example, data are generally based on drivers who are sober and alert. Yet impaired or fatigued drivers may represent a large part of the crash risk. Alcohol, drugs, medication, and fatigue can have dramatic effects on the psychological processes that underlie PRT and MT. Driver distraction by activity within the vehicle is also a common occur- rence that is not reflected in the design model. In-vehicle technologies, such as cell phones, nav- igation systems, and infotainment systems, are increasingly common. The multitasking driver is an increasing concern, but PRT models do not reflect this possibility. The Quality and Applicability of the Empirical Research Used to Develop the Driver Models The values used in design equations may or may not be derived from good empirical sources. In some cases (e.g., brake reaction time), there are numerous empirical studies and reasonably good agreement among them. In other cases, empirical data are very limited, are of lesser qual- ity, or are only weakly applicable to the design issue in question. The quality and applicability of the numbers that come from empirical studies are sometimes questionable on a number of grounds: the sample of drivers may be small or unrepresentative; the situations evaluated may be limited and may not generalize well; the research may be out of date (given changes in road- ways, traffic, vehicles, traffic control devices, and driver norms); the research setting (test track, simulator, laboratory) may lack validity; and results may conflict with results from other stud- ies. It would be wrong to assume that sight distance design equations are necessarily based on a strong, high-quality empirical foundation that readily generalizes to all cases. Another concern related to data quality and applicability is the inability of general design equations based on simple behavioral models to incorporate site-specific considerations. Empir- 22-7 HFG TUTORIALS Version 2.0

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. In summary, sight distance requirements are based on a highly simplified and mechanistic model of driver behavior and capabilities. This approach is reasonable and generally success- ful. The general assumptions often work well enough to approximate the needs of most driv- ers; however, it is important to recognize that this simple model has a number of limitations as a description of actual driver performance. When difficult sight distance problems are being diagnosed or addressed, it may be useful for the highway designer or traffic engineer to recog- nize how design models simplify driver actions and to acknowledge the realities of more com- plex driver perception and behavior. HFG TUTORIALS Version 2.0 22-8

Tutorial 2: Diagnosing Sight Distance Problems and Other Design Deficiencies Introduction The previous sections of this document—especially Chapter 5—have provided design guide- lines for human factors aspects of various sight distance concepts. However, for users to imple- ment these guidelines in a practical sense, it is desirable to provide a procedure for their operational application. Therefore, this section comprises a hands-on tool whereby practition- ers can apply human factors techniques to analyze sight distance problems and other design defi- ciencies at a selected highway location. A starting point for development of the current procedure was a review of previously docu- mented procedures for conducting on-site driving task analyses (Alexander & Lunenfeld, 2001) that applied techniques such as commentary drive-thru procedures to generate checklist subjective- scaled ratings of hazard severity and information load. The current in-situ sight distance diagnos- tic procedure includes application of previously available engineering tools, e.g., AASHTO (2004) analyses of geometric requirements and MUTCD (FHWA, 2003) traffic control device require- ments, and augments these techniques with those sight distance concepts presented in Chapter 5 of this HFG. This sight distance diagnostic procedure consists of a systematic on-site investigation tech- nique to evaluate the highway environment to support the concepts of interest, i.e., SSD, PSD, ISD, and DSD. The highway location is surveyed, diagrammed, and divided into component sections based on specific driving demands (e.g., requirement to perform a maneuver). Then each section is analyzed in terms of its suitability to support the required task (e.g., informa- tion provided to driver and allotted time to the complete required task). This procedure enables the practitioner to compare the available sight distance with the required sight distance to safely perform the driving task. The Six-Step Procedure The procedure consists of the following six steps: 1. Collect field data to describe roadway characteristics and other environmental factors affect- ing sight distance requirements and driver perception of a potential hazard. 2. Conduct engineering analyses applying traditional techniques, e.g., AASHTO design criteria and MUTCD compliance, to initially assess site characteristics or deficiencies. 3. Examine crash data and prepare collision diagram to seek possible association between safety and a sight distance problem. 4. Establish component roadway sections in which drivers respond to specific visual cues in order to initiate a maneuver to avoid a hazard. 5. Analyze driving task requirements (PRT and MT) and determine the adequacy of each com- ponent roadway section to support these requirements. 6. Develop engineering strategies for improvement of sight distance deficiencies. A flow diagram overview of the process is shown in Figure 22-3. Following the description of the six-step procedure, an example application is provided. 22-9 HFG TUTORIALS Version 2.0

H FG T U TO RIA LS Version 2.0 22-10 Figure 22-3. Flow diagram of six-step diagnostic process.

22-11 HFG TUTORIALS Version 2.0 Step 1A: Identify Hazard and Prepare Site Diagram The specific hazard location under investiga- tion is identified and the approach roadway is diagrammed. Example of hazards requir- ing sight distance consideration and the associated sight distance concepts are as follows. • A hidden intersection (SSD) • An exit from a shopping mall in a heavily lit (e.g., visually cluttered) setting (DSD) • A vehicle approaching an intersection (ISD) • An oncoming vehicle in a passing zone (PSD) Note distances from hazard to the following features: (1) traffic control devices, (2) intersecting driveway or roadways, and (3) sight distance obstructions. References: Lunenfeld, H., and Alexander, G. J. (1990). A User’s Guide to Positive Guidance (FHWA- SA-90-017). Washington, DC: FHWA. Procedure Product/Application Step 1B: Collect Operating Speed on Approach Spot speeds for randomly selected vehicles are to be observed at a sufficient advance distance upstream from the hazard beyond which slowing in response to the hazard is expected. Candidate speed collection tech- niques are radar/laser detection, automated speed recorders, and manual timing. Refer- ences noted in the column to the right describe appropriate procedures to ensure random vehicle selection and suitable sam- ple sizes. In the event that the approach roadway section is characterized by horizontal or vertical curvature, speed collection points should be selected so as to represent opera- tional speeds at these locations. The product of this step will be a statistical distribution of speeds from which means and/or percentile values will be applied to estimate vehicle speed for the approach roadway under study. References: Hanscom, F. R. (1987). Validation of a non-automated speed data collection methodology. Transportation Research Record, 1111, 54–61. Robertson, H. D. (Ed.). (2000) Manual of Transportation Engineering Studies, Wash- ington, DC: Institute of Transportation Engineers. Procedure Product/Application Step 1: Collect Field Data This step involves making specific field measurements and observations. Data are to be gath- ered both at the location of the designated hazard as well as the approach roadway section imme- diately in advance of the hazard. Approach distances over which field measurements should be gathered are determined from Table 22-1 at the end of this step. Approach distances were derived from approximated perception-reaction and sign reading times applied to the desig- nated operating speeds.

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. Typical target vehicle behaviors indicative of a sight distance problem are sudden slowing (e.g., observable break light activation) and abrupt lane changes when these maneuvers are not induced by other vehicles in the traffic stream. A considerable literature base is available regarding the conduct and interpretation of “traffic conflicts” studies; however, the reader is cautioned that traffic conflicts studies are limited to interactions between vehicles. A sight distance–induced erratic maneuver, on the other hand, can involve a single vehicle. Methodological literature addressing conflicts study is helpful with respect to observational techniques. The outcome of this step should be insight- ful with respect to possible sight distance– induced vehicle behaviors. References: Parker, M. R., and Zegeer, C. V. (1989). Traffic Conflict Techniques for Safety and Operations (FHWA-IP-88-026 (Engineer’s Guide) and FHWA-IP-88-027 (Observer’s Guide)). Washington, DC: FHWA. Taylor, J. I., and Thompson, H. T. (1977). Identification of Hazardous Locations: A Users Manual (FHWA-RD-77-82). Washington, DC: FHWA. Procedure Product/Application Step 1D: Inventory Existing Traffic Control Devices Document existing signs, signals, and pave- ment markings along with their respective distances from the hazard under study. Document the age of these signs, signals, and markings, as well. The letter heights and mounting heights of signs need to be re- corded. Document any visual obstructions. The resulting device inventory will be sub- sequently applied in this diagnostic analy- sis to evaluate the suitability of provided information, as well as visual distractions and information processing demands on motorists as they approach the hazard under study. Procedure Product/Application Step 1E: Measure Existing Geometric Sight Distances Existing geometric sight distance limitations along the approach to the hazard must be measured in accordance with AASHTO crite- ria. Specifically, sight distance observations should be made from an elevation above the pavement that equals the design driver eye height (i.e., 3.5 ft) to a point ahead that is 2.0 ft above the pavement. This step will yield the length of specific roadway subsections along the approach in which drivers must observe and process available information (e.g., roadway features and other vehicles). References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

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. Documentation and general awareness of these factors are important because subse- quent minor highway improvement proj- ects may result in higher highway speeds, thus producing increased sight distance requirements. Procedure Product/Application Step 1G: Note Visual Distractions at Hazard Location Certain environmental conditions are known to produce “visual clutter” (i.e., distractions that make hazards more difficult for drivers to perceive). Examples include (1) off-roadway lighting, (2) commercial signing in driver field of view, (3) complex urban intersection designs, (4) high volumes of vehicular/ pedestrian movement (including bicycles), and (5) proliferation of intersec- tion traffic control devices. Observations should document drivers’ field of view at SSD from hazard (e.g., AASHTO (2011)). This inventory of visual distractions will be subsequently applied in a human factors analysis to determine the applicable sight distance criterion (e.g., DSD, to address driver perception and information- processing time requirements at the hazard location). References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application Step 1H: Note Visual Distractions Along Approach Roadway As in Step 1G, visual environmental condi- tions along the approach to the hazard also may produce driver distractions. These need to be included in the field data collec- tion process. Observations should document drivers’ field of view at DSD from hazard (e.g., AASHTO (2011)). This inventory of visual distractions will be subsequently applied in a human factors analysis to determine the applicable sight distance criterion to address driver infor- mation-processing time requirements on the approach to the hazard location. References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

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. A—Point of required action. The location where an intended maneuver (e.g., hazard avoidance) is to be completed. SDTCD—Sight distance to a traffic control device. The point at which the device is first detectable to an approaching motorist. TCD—Traffic control device. The location of the device that warns of the hazard, measured as a distance from the location or object about which information is provided. The inclusion of uniform symbols on the site diagram will facilitate the subsequent sight distance analysis (see Figure 22-4). Procedure Product/Application Approach Distance to Hazard (ft) Estimated Operational Speed (mi/h) Visually Cluttered Environment Visually Non-Cluttered Environment Additional, when TCDs Present 25 360 180 95 30 440 220 110 40 580 290 150 50 730 370 185 60 880 440 220 70 1030 520 260 Table 22-1. Recommended approach distance to hazard for collection of field data. ASDHAZSDTCD TCD (SDTCD) (TCD) (SDHAZ)) Figure 22-4. Example symbol diagram: A two-lane 55-mi/h roadway approaches a 35-mi/h curve.

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. In addition, the placement of traffic control devices needs to be examined in terms of MUTCD requirements. Step 2A: Examine Hazard Location with Respect to AASHTO Design Criteria To ensure a valid engineering diagnosis of sight distance to a hazard, it is necessary to first assess whether the hazard location itself has any inherent design shortcomings. One geometric deficiency potentially associated with a hazard location might be roadside that fails to meet requirements of the AASHTO Roadside Design Guide. Other examples are (1) a high-crash intersection may be deficient with respect to existing corner sight distance (AASHTO, 2011) and (2) in the case of a high incidence of run- off-road crashes, observed operational speeds (from Step 1A above) may differ significantly from the design speed upon which the curve radius and super elevation of the curve under consideration were based (AASHTO, 2011). The resulting analytical steps ensure that the hazard location itself is free of any inherent design shortcomings that have the potential for confounding the intended sight distance diagnosis. References: AASHTO (2002). Roadside Design Guide. Washington, DC. AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application Step 2B: Examine Approach with Respect to AASHTO Design Criteria As with the procedure noted in Step 2A, to ensure the integrity of the overall sight dis- tance diagnosis, it is necessary to assess whether the approach to the hazard loca- tion has any inherent design shortcomings. (For example, a substandard lateral clear- ance to a roadside object along the approach may create a visual obstruction, thus producing an unintended sight distance limitation.) Likewise, crest vertical sight dis- tances along the approach should be consis- tent with observed operational speeds gathered during Step 1B. The resulting analytical steps ensure that the approach to the hazard is free of any inher- ent design shortcomings that have the potential for confounding the intended sight distance diagnosis. Procedure Product/Application

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 distrac- tions and/or complex TCDs at intersections along the approach. Visual clutter along an approach to a hazard detracts from drivers’ perception of the haz- ard. When DSD-warranting conditions are found to exist along an approach to a hazard, the distraction is sufficient such that available sight distance to the hazard must be restricted to that distance beyond the distraction. Procedure Product/Application Step 2E: Examine Traffic Control Devices with Respect to MUTCD Criteria The MUTCD (FHWA, 2009) prescribes device placement criteria for signs, signals, and markings. Devices at both the hazard location and along the approach need to be examined for MUTCD compliance. Note that the MUTCD establishes manda- tory, recommended, and optional require- ments for the application of TCDs. The examination conducted in this step (as well as Steps 4 and 5) should reflect these MUTCD criteria. The output of this step will reveal whether in- adequate traffic control device application (e.g., insufficient warning distance or inap- propriate warning message) constitutes possi- ble sources of driver confusion. Inappropriate or inadequate TCD information can result in longer information processing times, thereby creating an artificial sight distance problem. References: FHWA (2009). Manual on Uniform Traffic Control Devices (MUTCD). Washington, DC. Procedure Product/Application Step 2C: Examine Hazard Location with Respect to Possible DSD Warrants AASHTO (2011) (e.g., section on DSD) notes a distinction between typical stopping sight distances and those in which drivers are required to make complex decisions (i.e., in which drivers require PRT beyond the design value [which is typically 2.5 s]). The DSD criterion applies to a difficult-to- perceive information source in a roadway environment that may be visually cluttered. Therefore, the hazard location needs to be examined for conditions of “visual noise” from competing sources of information (e.g., roadway elements, traffic, TCDs, and advertising signs). Specific sources of visual clutter were also noted in Step 1E. When DSD-warranting conditions are found to exist, apply the sight distance requirements noted in AASHTO (2011), rather than con- ventional stopping distances based on a 2.5-s PRT. References: AASHTO (2011). A Policy on Geometric Design of Highways and Streets. Washington, DC. Procedure Product/Application

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. The practitioner is cautioned that the absence of crashes does not rule out the existence of a sight distance problem, as crashes are probabilistic events and reporting requirements are variable. Step 3A: Establish Typologies and Frequency by Spot Locations A review of crash data will reveal the occur- rence of various types in close vicinity to the hazard under study. The associated pre- collision paths and their proximity to high- way features may suggest the existence of a sight distance problem. Certain crash types are typically associated with specific sight distance problems: • Run-off-road, fixed-object crashes (SSD) • Side-swipe, rear-end crashes (PSD) • Right-angle, rear-end crashes (ISD) A collision diagram is used to summarize crash types by location. For examples, see Robertson et al. (2000) and Hostetter and Lunenfeld (1982). References: Robertson, H. D., Hummer, J. E., and Nel- son, D. C. (Eds.) (2000). Manual of Trans- portation Engineering Studies. Washington, DC: ITE. Hostetter, R. S., and Lunenfeld, H. (1982). Planning and Field Data Collection (FHWA- TO-80-2). Washington, DC: FHWA. Procedure Product/Application Step 3B: Assess Suitability of Crash Sample While well-documented procedures exist to statistically establish crash causation (see Council et al. (1980)), this level of sophistica- tion is not necessary for the diagnosis of a sight distance problem. It is desirable (to the extent possible based on available crash data) to establish causation inferences based on crash patterns and to rule out non-sight-distance causal effects. A reasonable level of confidence (albeit logic- based rather than statistically rigorous) regarding crash causation is possible based on the following: • Inferences based on crash patterns rather than a single event • Occurrences whereby non-sight-distance factors can be logically ruled out. References: Council, F. M., Reinfurt, D. W., Campbell, B. J., Roediger, F. L., Carroll, C. L., Dutt, A. K., and Dunham, J. R. (1980). Accident Research Manual (FHWA-RD-80-016). Washington, DC: FHWA. Procedure Product/Application

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 sugges- tive of sight distance problems: for exam- ple, 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 condi- tions of more limited visibility (e.g., dark- ness). These patterns need to be examined to determine whether sight distance is a potential causal factor (i.e., adequate night- time sight distance conveyed by TCDs). A collision diagram can be descriptive of the location and nature of a sight distance haz- ard, thus supporting a hypothesis regarding the effect of a sight distance problem. Procedure Product/Application

22-19 HFG TUTORIALS Version 2.0 Step 4A: Establish and Plot Action Points Along Approach Segment Specific locations within the study road- way section requiring a driver action (e.g., maneuver) will be identified and plotted. For example, the hazard under study is the key point where action (e.g., driving at the posted speed) is likely required. Where a maneuver (e.g., deceler- ating) is necessary prior to reaching the hazard, the “compliance point” is the point where the maneuver is initiated (e.g., start of the deceleration distance). In the event that the approach roadway section requires some intermediate action (e.g., merging from a dropped traffic lane), this action also needs to be identified and plotted. Action points on the site diagram prepared in Step 1 should be indicated on the dia- gram by the symbol A. A series of sequen- tial action points may be designated as A1, A2, etc. The developed site diagram will indicate specific points where vehicle actions are required. Examples are as follows: • Approach maneuver (such as slowing) as required by the hazard under study • Any intermediate actions (e.g., required lane change) on the approach to the haz- ard under study Procedure Product/Application Step 4: Establish Roadway Segments The practitioner specifies component roadway approach segments in a manner to support the detailed human factors analysis in Step 5. Separate approach roadway segments are theoretically required for driver PRT and hazard avoidance maneuver functions. The product of this section is a series of driver task diagrams that depict the point where driver actions are required to avoid a potential hazard, information sources that warn of the hazard, and drivers’ available sight dis- tances to perform the necessary information-processing and maneuver tasks.

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. In this step, drivers’ information sources that inform an intended action must be located and documented. Informa- tion to the driver should be available from (1) detection of the hazard and/or (2) traffic control devices pertaining to the hazard. The following information/detection sources were noted on the site diagram in Step 1I: • Initial point of sight distance to the hazard identified by the symbol SDHAZ • Location of TCD providing information regarding the hazard identified by the symbol TCD • Initial point of sight distance to the appli- cable TCD identified by the symbol SDTCD In this step, separate plots of component information-processing segments may be helpful. The developed site diagram will indicate specific points where information pertain- ing to the hazard is available to the driver. Examples are as follows: • Point of initial detection opportunity on an approaching of both the hazard and any traffic control device warning of the hazard. • Specific locations of any TCDs advising of the hazard. NOTE: In the event that the hazard under study is not detectable (i.e., defined in the visual field), the symbol SDHAZ would not appear on the diagram. In such instances the required sight distance to action point (A) will be determined in Step 5. Procedure Product/Application

22-21 HFG TUTORIALS Version 2.0 Step 4C: Define Component Driver Response Sections Within Approach Segment Distinctly different driver information- processing 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 per- form these varied information-processing and maneuver tasks. Depending upon physical characteristics of the roadway section under study, four dis- tinct driver response cases are possible: Case 1: Direct line of sight to hazard SDHAZ ➞ A Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Case 3: Intervening (e.g., distracting) hazard (A2) within sight line of first hazard (A1) SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Case 4: Intervening traffic control device and dis- tracting hazard SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 The product of this step is a diagrammed set of roadway component sections, each corre- sponding to specific information-processing and maneuver driver tasks. The distance over which a driver can react to a detectable hazard is the roadway sec- tion SDHAZ ➞ A. In this roadway section, the driver would detect the hazard and per- form any required preparatory maneuver (e.g., decelerating). Likewise, the distance over which a driver reacts to an advance traffic control device is the roadway section SDTCD ➞ TCD. In this roadway section, the driver has the opportunity to detect the sign and compre- hend the sign’s message. The message becomes readable at the point LDTCD (i.e., the legibility distance from the sign), which will be computed and located during Step 5. In the final approach section to the hazard, TCD ➞ A, the driver would complete the decision-making and maneuver tasks. Procedure Product/Application

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 accommo- date both the information-processing and maneuver tasks) approaching action points (A) will fall into one of the following cate- gories (depending upon roadway environ- ment condition), which were identified in Section 5.2: • Stopping sight distance (SSD) • Intersection sight distance (ISD) • Decision sight distance (DSD) • Passing sight distance (PSD) The result of this task is the specification of the applicable procedure (e.g., engineering design formula) for the computation of SDHAZ corresponding to each identified haz- ard or action point. The required sight dis- tance based on application of the appropriate design formula is applied to determine the required length of the roadway segment under study. Procedure Product/Application Step 5: Analyze Component Driving Task Requirements In this step, the practitioner applies human factors principles (comprising information- processing and decision-making criteria) to ensure the adequacy (or to quantify the shortcoming) of the approach roadway to allow for time/distance hazard avoidance requirements.

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) SDTCD ➞ LDTCD ➞ TCD ➞ A 1. Driver must detect traffic control device: SDTCD ➞ LDTCD 2. Driver must read or otherwise compre- hend message and may begin decision process: LDTCD ➞ TCD (Legibility distance will be determined in Step 5C.) 3. Decision and maneuver must be completed: TCD ➞ A Case 3: Intervening, distracting hazard at A2 within sight line of first hazard at A1 SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 1. Driver requires longer PRT due to com- plex visual scene ahead: SDHAZ1 ➞ SDHAZ2 ➞ A Consider DSD application. 2. Driver may require longer MT due to complexity of maneuver and visual scene: SDHAZ2 ➞ A2 ➞ A Case 4: Intervening traffic control device and dis- tracting hazard at A2 within sight line of first hazard A1 SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 1. Driver must detect traffic control device: SDTCD ➞ LDTCD 2. Driver must read or otherwise compre- hend message and may begin decision process: LDTCD ➞ TCD 3. Driver may require longer MT due to complexity of maneuver and visual scene: SDHAZ2 ➞ A2 ➞ A Procedure Driver information-processing demands vary as a function of environmental factors, according to the four cases indicated below. Identify separate PRT and MT components of the driving task for each of the four cases. Specific values of PRT and MT will be deter- mined subsequently. 22-23 HFG TUTORIALS Version 2.0

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). TCDs present: SDTCD ➞ LDTCD Drivers should be able to detect a TCD prior to time required to comprehend its message; 2.5 s is desirable, although less time may be adequate (e.g., second, third, etc. in a sequence). LDTCD ➞ TCD LDTCD is the “legibility distance” or approach distance at which a traffic control device mes- sage is comprehended. A detailed discussion in the following paragraphs addresses the LDTCD for signs. In the case of pavement markings, LDTCD is the advance distance at which the marking is visually recognized. The LDTCD for a sign is the distance at which its legend is read or its symbol message is comprehended. PRT requirements for signs consist of reading times for the message leg- end and symbol as follows (Smiley, 2000): Reading Time = 1*(number of symbols) + 0.5*(number of words and numbers) [s] The minimum reading time is 1 s. For mes- sages exceeding four words, the sign requires multiple glances; the driver must look back to the road and at the sign again. Therefore, for every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time. TCDs present (continued): This segment must be sufficient in length to accommodate the reading time noted above. However, its length is constrained by letter height (i.e., limited to 40 ft for every inch of letter height). For example, a 4-in. letter-height sign must be read within a distance of 4 × 40 = 160 ft. On a 40 mi/h (58.8 ft/s) roadway, the driver is limited to a maximum of 160/58.8 or 2.7 s to read the sign. Moreover, the traffic engineer must consider that the driver can not be expected to fixate on the sign. Considering the driver’s alerted state after reading the sign, decision time (i.e., time to make a choice and initiate a maneuver if required) can range from 1 s for common- place maneuvers (e.g., stop, reduce speed) to 2.5 s or more when confronted with a com- plex highway geometric situation. LDTCD ➞ TCD ➞ A While the required MT may be initiated prior to passing the TCD, it must be com- pleted in the above-noted segment. MT val- ues associated with designed sight distance considerations are treated in Chapter 5. Additional literature sources of extensive maneuver time data are available (Lerner, Steinberg, Huey, and Hanscom, 1999). References: Lerner, N. D., Steinberg, G. V., Huey, R. W., and Hanscom, F. R. (1999). Under- standing Driver Maneuver Errors, Final Report (Contract DTFH61-96-C-00015). Washington, DC: FHWA. Smiley, A. (2000). Sign design principles. Ontario Traffic Manual. Ottawa, Canada: Ontario Ministry of Transportation. Procedure The general model to be applied for quantifying driver task requirements (i.e., required PRT and MT) is SDTCD ➞ LDTCD ➞ TCD ➞ A. Driver task requirements are determined for each task as follows.

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 con- trol SDHAZ ➞ A Does the subsection length SDHAZ ➞ A allow sufficient time for the driver to perform any required hazard avoid- ance maneuver? Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Does the subsection length, SDTCD ➞ LDTCD allow sufficient time (minimum 1.5 s) for the driver to detect the traffic control device? Does the subsection length, SDTCD ➞ TCD allow sufficient time for the driver to detect and read the traffic control device? Does the subsection length, TCD ➞ A allow sufficient time for the driver to per- form any required hazard avoidance maneuver? Case 3: Intervening, distracting hazard at A2 within sight line of first hazard at A1. SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Does then subsection length SDHAZ1 ➞ A1 allow sufficient time for the driver to process and respond to the intervening distraction (i.e., apply DSD criteria) and perform any required hazard avoidance maneuver? Case 4: Intervening traffic control device and dis- tracting hazard A2 within sight line of first hazard A1. SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 Does the subsection length, SDTCD ➞ LDTCD allow sufficient time (2.5 s desir- able; minimum 1.0 to 1.5 s) for the driver to detect the traffic control device? Does the subsection length, SDTCD ➞ TCD allow sufficient time for the driver to detect and read the traffic control device? Does the subsection length, TCD ➞ A1 allow sufficient time for the driver to process and respond to the intervening distraction (i.e., apply DSD criteria) and perform any required hazard avoidance maneuver? Procedure

Step 6: Develop Engineering Strategies for Improvement of Sight Distance Deficiencies In this final step, the practitioner recommends improvement (e.g., traffic control device appli- cations or minor design modifications) to correct deficiencies. HFG TUTORIALS Version 2.0 22-26 Step 6A: Apply Traffic Engineering and Highway Design Principles to Component Sight Distance Deficiencies Case 1: Direct line of sight to hazard; no traffic control SDHAZ ➞ A Available sight distance to hazard, SDHAZ, is less than required based on Step 5B results. Case 2: Intervening traffic control device (i.e., warning of hazard) SDTCD ➞ LDTCD ➞ TCD ➞ A Total available sight distance less than the required sight distance from Step 5C. Case 3: SDHAZ1 ➞ SDHAZ2 ➞ A2 ➞ A1 Available sight distance to hazard, SDHAZ, is less than required based on Step 5B results. Case 4: SDTCD ➞ LDTCD ➞ TCD ➞ SDHAZ2 ➞ A2 ➞ A1 Total available sight distance less than the required sight distance from Step 5C. Add warning traffic control device, increas- ing warning distance as shown in Case 2 below. If LDTCD ➞ TCD is inadequate (i.e., infor- mation overload): • Apply “information spreading” by adding more devices, each with less information • Increase legibility distance (e.g., by increasing letter size) If LDTCD ➞ TCD ➞ A is inadequate: • Increase warning distance, SDTCD ➞ LDTCD, via improving the TCD’s legibility distance • Apply larger device, increase letter size • In DSD condition, add conspicuity device (e.g., flashing beacon) or consider ITS application. If SDTCD ➞ LDTCD ➞ TCD is inadequate: • Reduce information load on existing TCDs • Apply additional TCDs (e.g., delineation devices, advance supplemental devices) to convey essential information. Add warning traffic control device, achiev- ing increased warning distance. Apply combination of Case 2 solutions noted above. Procedure Product/Application

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. Driver requirements in this situation are as follows: 1. Reduce speed from 55 to 35 mi/h to negotiate curve 2. Process traffic control device information related to intersection (e.g., destination name sign) 3. Stop for intersection Step 1: Collect Field Data and Prepare Site Diagram The labeled site diagram is shown in Figure 22-5. Step 2: Conduct Preliminary Engineering Analyses This example requires a sight distance analysis to two separate potential hazards. The first is a 35-mi/h curve that requires slowing from 55 mi/h; and the second is an intersection that is heav- ily signed with a stop sign and two guide signs, containing multiple route shields, symbols, and destination names. The approach roadways to each hazard point are separately treated as fol- lows: (1) curve approach and (2) signed intersection approach. Curve 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) do not apply. Step 2E: Examine Traffic Control Devices for Compliance with the MUTCD. The MUTCD specifies requirements for warning signs. The curve warning sign in the example is a “W1-2, Hor- izontal Alignment Sign” with a 35-mi/h advisory speed plate. Section 2C-05 of the MUTCD spec- ifies an advance placement guideline for warning signs. Given the requirement to slow from 55 to 35 mi/h, the minimum recommended distance in Table 2C-4 (located on page 2C-5) is 138 ft (FHWA, 2003). Figure 22-5. Example site diagram.

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) do not apply. Step 2E: Examine Traffic Control Devices for Compliance with the MUTCD. This segment is a stop-controlled intersection approach containing signs to multiple routes and destinations. The MUTCD provides requirements for guide signs on conventional roads. Signs in the example consist of a “directional assembly” with destination name signs and route shields. Required advance distances and spacing of these signs is given in Figure 2D-2 (FHWA, 2009). Typically, when a series of guide signs is placed sequentially along the approach to an inter- section there is a 100- to 200-ft separation between the first two signs. The minimum spacing between signs is 100 ft, which is intended to enable drivers to read the entire message on both signs. Section 2D.06 requires 6-in. letter heights for a 35-mi/h roadway (FHWA, 2009). Specifications for stop sign size and placement are contained in Chapter 2A of the MUTCD. As shown in Figure 2A-2, the stop sign should be set back a minimum of 12 ft from the intersection. The recommended letter height is 8 in. (FHWA, 2009). Step 3: Apply Crash Data Not conducted as part of this example. Step 4: Establish Roadway Segments This example requires a sight distance analysis to two separate potential hazards. The first is slowing from 55 mi/h to 35 mi/h, the posted curve advisory speed; and the second is a stop- controlled approach to an intersection containing signs to multiple routes and destinations. As above, the approach roadways are discussed separately. Curve Approach Segment. The roadway segment requiring the driver to slow from 55 mi/h to 35-mi/h is labeled in accordance with Steps 4A and 4B and is shown below. The two sight dis- tance driver response scenarios follow: • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve): SDHAZ ➞ A • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A This roadway is diagrammed in Figure 22-6. Signed Intersection Approach Segment. On this roadway section, motorists traveling at 35-mi/h are confronted with a stop-controlled intersection and two guide signs containing destination names and route shields. Because sight distance to the intersection is limited by a curve on the approach, a sight distance analysis is critical. The component section diagram is labeled in accordance with Steps 4A and 4B and shown below. The sight distance driver response scenarios follow: HFG TUTORIALS Version 2.0 22-28 Figure 22-6. Curve approach segment diagram.

• 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. Step 5: Analyze Component Driving Task Requirements Curve Approach Segment. The roadway section, requiring the driver to slow from a 55-mi/h speed zone to a 35-mi/h curve, considers sight distance to the curve and legibility distance requirements posed by the advisory speed sign. Step 5A: Determine the Relevant Design Sight Distance Application. The applicable design sight distance is slowing sight distance—the required distance for a driver to observe the curve ahead and adjust speed accordingly. In the event that certain visual noise conditions or other fac- tors are present that would render the curve difficult to perceive, then the practitioner must con- sider applicable DSD criteria (discussed in Chapter 5). Where a traffic control device is present, driver information-processing time is required to observe and comprehend the sign as well as slow to a safe curve negotiation speed. In the current example (i.e., a rural uncluttered environ- ment), DSD criteria are not applied. Step 5B: Determine the Driving Task Requirements. Considering the two possibilities (i.e., Case 1 in which the driver observes the curve ahead without seeing the sign, and in Case 2 whereby the driver observes and comprehends the sign), the requirements for each are as follows: • Case 1, direct line of sight to hazard (i.e., 55-mi/h speed zone to 35-mi/h curve): SDHAZ ➞ A The sight distance requirement in this case is simply that the driver observes the curve ahead and slows to a safe speed. • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A The sight distance requirement in this case is that the driver observes the sign, comprehends the sign message, and slows to a safe speed. 22-29 HFG TUTORIALS Version 2.0 Figure 22-7. Intersection approach segment diagram.

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), the design PRT value of 2.5 s is applied; thus the PRT component of sight distance is 202 ft (i.e., 2.5 s times 80.85 ft/s). The MT require- ment (4.0 s) is derived from the need to slow from 55 mi/h to 35 mi/h at a comfortable decel- eration level (i.e., .23g), which requires 261 ft. Thus the total PRT and MT sight distance requirement is 463 ft. The comfortable deceleration level is derived from Table 2-25 of AASHTO (2011). (For safety purposes, wet weather deceleration is considered.) However, AASHTO (2011) acknowledges that its deceleration data may be outdated and that more rapid (albeit uncomfortable) deceler- ations are common. A typical such deceleration is .35g (Knipling et al., 1993), resulting in an MT of 2.6 s. It is also known that most reasonably alert drivers are able to initiate braking within a PRT of 1.6 s (Chapter 5). Applying these performance parameters to slowing from 55 to 35 mi/h, the total required PRT distance is 129 ft plus 172 ft MT distance, or 301 ft. It is unlikely that the need to slow to 35 mi/h would be visually evident from an advance dis- tance of either 301 or 463 ft. Therefore, the critical sight distance consideration is based on the application of the speed advisory sign. • Case 2, intervening traffic control device (i.e., 35-mi/h advisory speed sign warning of hazard): SDTCD ➞ LDTCD ➞ TCD ➞ A In this case the driver needs to detect the sign, read the sign, and decelerate to the safe curve speed. A critical requirement for sight in advance of a sign (i.e., allowing time to comprehend the sign’s message) is known as legibility distance. There is a considerable body of knowledge regarding sign legibility distance requirements (Smiley, 2000). For simple warning signs, the MUTCD specifies an advance placement guideline, which includes “an appropriate legibility distance” of 175 ft for word legend signs or 100 ft for symbol signs. The MUTCD sign placement requirement to allow for slowing from 55 to 35 mi/h is 350 ft. Driver requirements imposed by the MUTCD rule in this case are as follows: Given that 2.0 s are needed to detect and comprehend (e.g., minimum 1.0 s for detection plus 1.0 s for symbol comprehension) the simple warning sign message prior to the initiation of slowing, the deceleration requirement would be .32 g or approximately the equivalent slowing rate of skidding on wet pavement. In this example the required PRT and MT distances would be 161 and 189 ft respectively, for a total of 350 ft. For signs with complex messages (i.e., sets of destination names or symbols in combination with symbols), message comprehension may require significantly more legibility distance. The next example illustrates such a situation. Signed Intersection Approach Segment. On this roadway section, motorists traveling at 35 mi/h are confronted with a stop-controlled intersection and two guide signs containing des- tination names and route shields. Because sight distance to the intersection is limited by a curve on the approach, a sight distance analysis is critical. Step 5A: Determine the Relevant Design Sight Distance Application. As the driver approaches a stop-controlled intersection, there must be sufficient available stopping sight dis- tance (Chapter 5) to enable stopping at the stop line. (While negotiation of the intersection involves the application of intersection sight distance, the current example is limited to approaching the intersection.) HFG TUTORIALS Version 2.0 22-30

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), the requirements are as follows: • Case 1, direct line of sight to hazard (i.e., 35-mi/h speed zone to intersection): SDHAZ ➞ A The sight distance requirement (to accommodate travel time) in this case is simply that the driver observes the intersection ahead and safely slows to a stop. • Case 2, three intervening traffic control devices, i.e.: – A route shield assembly: SDTCD1 ➞ LDTCD1 ➞ TCD1 ➞ A – A destination name sign: SDTCD2 ➞ LDTCD2 ➞ TCD2 ➞ A – A stop sign: SDTCD3 ➞ LDTCD3 ➞ TCD3 ➞ A TCD1 is a route shield assembly bearing two route designations; TCD2 is a destination guide sign with two destination names and directional arrows; and TCD3 is a stop sign. The sight distance requirement in this case is that the driver detects and comprehends the signs and slows to a safe stop at the stop line. Step 5C: Quantify the Applicable PRT and MT Requirements for Each Driving Task • Case 1, direct line of sight to hazard (i.e., speed reduction from 35 mi/h to stop at the stop line): SDHAZ ➞ A The design stopping sight distance does not accommodate information-processing requirements of the intervening guide signs. The AASHTO design SSD value (AASHTO, 2004) for a 35-mi/h approach is the range of 225 to 250 ft, which accounts for both the PRT and MT tasks. However, this 225- to 250-ft sight distance would barely accommodate the physical placement of the two guide sign assemblies that are shown in the Figure 22-7. Moreover, the information- processing load imposed by the signs requires significant attention in terms of sight distance requirements. Therefore the Case 2 condition is treated below. • Case 2, intervening traffic control device (i.e., guide signs): SDTCD ➞ LDTCD ➞ TCD ➞ A The general model (above) entails the following considerations. First, there must be sufficient sight distance so that the sign is detected prior to the time required to comprehend the sign’s message, thus application of the SDTCD term. This advance distance is not specified in the MUTCD. Nevertheless, 2.5 s is desirable for this sign detection task, although less time may be adequate as motorists who are looking for signs are generally aware of the expected posi- tion in their field of view. The more essential approach sight distance to a traffic control device is that required to comprehend its message. LDTCD refers to legibility distance—the approach distance at which a TCD legend is read or its symbol message is comprehended. The legibility distance of a legend sign is determined by mul- tiplying a legibility index (i.e., the distance at which a given unit of letter height is readable) by the letter height. The applicable legibility index values are shown in Table 22-2. For example, the legibility distance typically associated with 6-in. letter height is 240 ft (40 times 6). 22-31 HFG TUTORIALS Version 2.0

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). For example, the mean legibility distance for the right curve arrow symbol was determined to be 283 m (with a standard deviation of 68 m). Considering that a 55-mi/h approach allowing a 2.5-s advance sight distance and 1.0-s reading time would consume only 86 m, pure symbol signs are not expected to result in an information- processing problem. The required PRT for this example roadway segment consists of three components: detecting the signs, comprehending the sign messages, and detecting the intersection. Each is separately discussed. Sign Detection. Upon a driver’s detection of the first sign, the second and third signs would require minimal detection time. The recommended detection time for the first sign is 2.5 s; however, the second two signs are likely to be detected much more rapidly. “Alerted” PRT responses are known to occur in as little as 1.0 to 1.5 s. Moreover, signs can be quickly detected as drivers know where to look for signs and typically scan toward expected sign locations. Therefore, a conservative sign detection PRT for the example roadway segment is (2.5 + 1.5 + 1.5) or 5.5 s. Sign Comprehension. Sign comprehension consists of reading the sign plus making the result- ant decision (e.g., right or left turn in response to the sign’s information). The PRT require- ment (Smiley, 2000) is based on sign-response reading and decision time, for which general rules are noted in Table 22-3. HFG TUTORIALS Version 2.0 22-32 Metric US Customary 4.8 meters/ centimeter of letter height 40 feet/ inch of letter height Table 22-2. Legibility index. Comprehension Task PRT Requirements Reading Time requirements for reading the sign are 0.5 s for each word or number, or 1 s per symbol, with 1 s as a minimum for total reading time. In the event of the sign’s containing redundant information, the reading time computation should be limited to critical words. The suggested formula for estimating sign reading time is: Reading time = 1(number of symbols) + 0.5(number of words and numbers). For messages exceeding four words, the sign requires multiple glances, which means the driver must look back to the road and at the sign again. Therefore, for every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time. When the driver is sufficiently close to see a sign at an angle, the sign is not visible for the last 0.5 s. Therefore, 0.5 s should be added to the required reading time. An exception applies to signs requiring a maneuver before the sign is reached, as no further reading is required. Deciding Considering the driver’s alerted state having read the sign, decision time can range from 1 s for commonplace maneuvers (e.g., stop or reduce speed) to 2.5 s or more when confronted with a complex highway geometric situation. Table 22-3. General rules for sign comprehension PRT requirements.

The first guide sign assembly contains two numbers and two symbols, requiring 3.0 s of read- ing 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. This estimate is highly conservative, as drivers would likely scan the guide signs seeking only a particular name or route number; however, it is necessary to provide suf- ficient information-processing sight as some drivers may need the entire set of information. An additional 3.0 s is considered for decision time responses to the three signs. Thus the total com- prehension time for the three signs is 10 s. Intersection Detection Distance. As noted above under the Case 1 (SDHAZ ➞ A) discussion, the stopping sight distance requirement considers a 2.5-s PRT. A summary of the above-noted PRT requirements, if separately considered, is shown in Table 22-4. The sum of PRT requirements would apply to a serial task process. However, a realistic assessment of PRT requirements considers that many of the tasks in Table 22-4 are concurrent. For example, stop sign comprehension would not logically entail a separate process of perceiving the intersection, thus conceivably reducing the total PRT by 2.5 s. In addition, following a driver’s 2.5-s detection of the initial sign, the subsequent two signs would likely be detected with a minimum detection time (e.g., 1.0 s rather than 1.5 s), thus conceivably reducing the total PRT by another 1.0 s. Therefore, subtracting 3.5 s from the serial total of 19.5 s, the estimated PRT requirement becomes 16.0 s. The MT requirement (i.e., to slow from 35 mi/h to a stop at the specified AASHTO g-force) calculates to 4.7 s over a distance of 120 ft. The extent to which the deceleration process would occur concurrently with the various sign-response tasks is uncertain. However, it is logical (and best serves liability concerns) to allow time for comprehension of all signs prior to the initiation of the slowing response. Therefore, the overall sight distance requirement is approximately 16.0 s of sign information processing at 35 mi/h (51.45 ft/s) or 823 ft, plus the 120-ft deceleration distance, for a total of 943 ft. (Actual requirements will reflect real-world conditions. If possible, data should be col- lected at the relevant sites.) A final consideration is the necessity that drivers have sufficient time to comprehend a sign’s message during the interval when the message is discernable. Therefore, an essential sight dis- tance diagnostic step is to compare the available sign legibility distance (i.e., available reading distance) with distance traveled during reading PRT (i.e., required reading distance and deci- sion time). Table 22-5 contrasts the distance traveled during PRT with the legibility distance. While the guide signs in this example accommodate both reading time and associated deci- sion time, the decision component of PRT can obviously be accomplished after the driver passes the sign. 22-33 HFG TUTORIALS Version 2.0 Driving Task PRT Requirement (s) Perceive initial guide sign 2.5 Perceive next three signs @ 1.5 s/sign 4.5 Comprehend initial guide sign 4.0 Comprehend second guide sign 4.0 Comprehend stop sign 2.0 Perceive intersection 2.5 Total 19.5 Table 22-4. Summary of PRT requirements.

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) PRT Distance (ft) 1. 6-in. letters: 2 Numbers + 2 Symbols 240 231 2. 6-in. letters: 2 Numbers + 2 Symbols 240 231 3. 8-in. letters: 1 Word 320 51 Table 22-5. Contrast of distance traveled during PRT with legibility distance.

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 qualita- tive information about the various perceptual, cognitive, and psychomotor elements of curve driving. Consistent with established procedures for conducting task analyses (Campbell and Spiker, 1992; Richard, Campbell, and Brown, 2006; McCormick, 1979; Schraagen, Chipman, and Shalin, 2000), the task analysis was developed using a top-down approach that successively decomposed driving activities into segments, tasks and subtasks. The approach used in this tutorial was specifically based on the one described in Richard, Campbell, and Brown (2006); readers interested in additional details about the methodology should consult that reference (available at http://www.tfhrc.gov/safety/pubs/06033/). The curve driving task was broken down into four primary segments, with each segment gen- erally representing a related set of driving actions (see Figure 22-8). The demarcation into seg- ments was primarily for convenience of analysis and presentation and does not imply that the curve driving task can be neatly carved up into discrete stages. Within each segment, the indi- vidual tasks that drivers should or must perform to safely navigate the curve were identified. Moreover, these driving tasks were further divided based on the information-processing ele- ments (perceptual, cognitive, and psychomotor requirements) necessary to adequately perform each task. The perceptual requirements typically refer to the visual information about the curve and the surrounding roadway that drivers need to judge the curvature, determine lane position and heading, etc. The cognitive requirements typically refer to the evaluations, decisions, and judgments that drivers have to make about the curve or the driving situation. The psychomo- tor requirements refer to the control actions (e.g., steering wheel movements, foot movements to press brake, etc.) that drivers must make to maintain vehicle control or to facilitate other information acquisition activities. The task analysis presented in Table 22-6 shows the driving tasks and corresponding information-processing subtasks associated with driving a typical horizontal curve, approach- ing from a long tangent. Drivers must also engage in other ongoing safety-related activities, such as scanning the environment for hazards; they may also engage in in-vehicle tasks such as adjust- ing the radio, using windshield wipers, or consulting a navigation system (just to name a few). However, these more generic tasks are not included in the task analysis in order to emphasize those tasks and subtasks that are directly related to curve driving. 2. Curve Discovery 3. Entry and Negotiation 4. Exit1. Approach Tangent Point Point of Curvature 75 -100 m (˜4 sec) Figure 22-8. The four primary segments of the curve driving task.

HFG TUTORIALS Version 2.0 22-36 Table 22-6. Driving tasks and information-processing subtasks associated with a typical curve. Driving Task Perceptual Requirements Cognitive Requirements Ps yc ho mo to r Re qu ir em en ts 1. Approach 1. 1 Lo ca te be nd In sp ec t fo rw ar d ro ad wa y sc ene fo r ev id en ce of be nd Re co gn iz e vi su al cu es in di ca ti ng depa rt ur e fr om st ra ig ht pa th Ey e mo ve me nt s n eeded fo r sca nni ng 1. 2 Ge t av a ila bl e sp eed in fo rm at io n fr om si gn ag e Vi su a lly sc an en vi ro nm ent fo r si gn ag e Re ad an d in te rp re t si gn in fo rm at io n He ad an d ey e mo ve me nt s n eeded fo r sca nni ng 1. 3 Ma ke in it ia l sp eed ad ju st me nt s L ook at sp eedom eter Re ad sp eedom eter in fo rm at io n an d co mp ar e to po st ed sp eed Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. Curve Discovery 2. 1 De te rm in e cu rv at ur e L ook at ro ad wa y an d en vi ro nm ent fe at ur es at cu rv e lo ca ti on Es ti ma te cu rv e an gl e ba se d on vi su al im ag e an d ex pe ri en ce He ad an d ey e mo ve me nt s n eeded fo r sca nni ng 2. 2 Asse ss ro ad wa y co nd it io ns (e .g ., lo w fr ic ti on , p oor vi si b ilit y) L ook at ro ad wa y in fr ont of ve hi cl e De te rm in e co nd it io ns re qu ir in g (a ddi ti on al ) sp eed re du ct io ns Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. 3 Ma ke a ddi ti on al sp eed ad ju st me nt s L ook at sp eedom eter an d/ or vi ew sp eed cu es fr om en vi ro nm ent Re ad sp eedom eter an d/ or ju dge sa fe sp eed ba se d on cu es an d ex pe ri en ce Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 2. 4 Ad ju st ve hi cl e pa th fo r cu rv e entr y L ook at ro ad wa y/ la ne ma rk in g in fo rm at io n in the i mmedi at e fo rw ar d vi ew De te rm in e the am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d la ne po si ti on He ad an d ey e mo ve me nt s n eeded fo r vi ew in g, an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 3. En tr y an d Ne go ti at io n 3. 1 Ad ju st sp eed ba se d on cu rv at ur e/ la te ra l a cce le ra ti on Pe rc ei ve la te ra l a cce le ra ti on an d l ook at ro ad wa y mo ti on cu es Ju dge sa fe sp eed ba se d on vi su al cu es an d ex pe ri en ce or re ad sp eedom eter Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 3. 2 Ma in ta in pr op er tr aj ec to ry L ook at ta ngent po in t or in tended di re ct io n De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d he ad in g He ad an d ey e mo ve me nt s n eeded fo r sca nni ng , an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 3. 3 Ma in ta in sa fe la ne po si ti on L ook at ro ad wa y/ la ne ma rk in g in fo rm at io n in the i mmedi at e fo rw ar d vi ew De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d la ne po si ti on He ad an d ey e mo ve me nt s n eeded fo r vi ew in g, an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l 4. Ex it 4. 1 Acce le ra te to a ppr op ri at e sp eed L ook at sp eedom eter an d/ or vi ew sp eed cu es fr om en vi ro nm ent Re ad sp eedom eter an d/ or ju dge sa fe sp eed ba se d on cu es an d ex pe ri en ce Ex ec ute ne ce ssa ry f oot mo ve me nt s to ac hi ev e de si re d sp eed ch an ge 4. 2 Ad ju st la ne po si ti on L ook se ve ra l se co nd s ah ea d do wn the ro ad wa y De te rm in e am ount of st eer in g wh eel di sp la ce me nt re qu ir ed to ac hi ev ed de si re d he ad in g He ad an d ey e mo ve me nt s n eeded fo r sca nni ng , an d pr ec is e ar m mo ve me nt s fo r st eer in g co nt ro l The primary source of information for segment tasks was the comprehensive driving task analysis conducted by McKnight and Adams (1970); however, other research more specifically related to curve driving were also used: • Donges, E. (1978). Two-level model of driver steering behavior. Human Factors, 20(6), 691–707. • Fitzpatrick, K., Wooldridge, M. D., Tsimhoni, O., Collins, J. M., Green, P., Bauer, K. M., Parma, K. D., Koppa, R., Harwood, D. W., Anderson, I., Krammes, R. A., and Poggioli, B. (2000). Alternative Design Consistency Rating Methods for Two-Lane Rural Highways. Final Report. (FHWA-RD-99-172). McLean, VA: FHWA.

• Groeger, J. A. (2000). Understanding Driving: Applying Cognitive Psychology to a Complex Everyday Task. Hove, U.K.: Psychology Press. • Krammes, R. A., Brackett, R. Q., Shafer, M. A., Ottesen, J. L., Anderson, I. B., Fink, K. L., Collins, K. M., Pendleton, O. J., and Messer, C. J. (1995). Horizontal Alignment Design Con- sistency for Rural Two-Lane Highways. Final Report. (FHWA-RD-94-034). McLean, VA: FHWA. • McKnight, A. J., and Adams, B. B. (1970). Driver Education Task Analysis. Volume I. Task Description. (DOT HS 800 367). Washington, DC: National Highway Traffic Safety Administration. • Pendleton, O. J., and Messer, C. J. (1995). Horizontal Alignment Design Consistency for Rural Two-Lane Highways. Final Report. (FHWA-RD-94-034). McLean, VA: FHWA. • Richard, C. M., Campbell, J. L., and Brown, J. L. (2006). Task Analysis of Intersection Driving Scenarios: Information Processing Bottlenecks (FHWA-HRT-06-033). Washington, DC: FHWA. Available at http://www.tfhrc.gov/safety/pubs/06033/ • Salvendy, G. (Ed.). (1997) Handbook of Human Factors and Ergonomics (2nd ed.). New York: Wiley. • Serafin, C. (1994). Driver Eye Fixations on Rural Roads: Insight into Safe Driving Behavior. (UMTRI-94-21). Ann Arbor: University of Michigan Transportation Research Institute. • Underwood, G. (1998). Eye Guidance in Reading and Scene Perception. Oxford: Elsevier. • Vaniotou, M. (1991). The perception of bend configuration. Recherche Transports Securite, (7), 39–48. For the most part, these references and the other research provided information about which tasks were involved in a given segment, but not complete information about the spe- cific information-processing subtasks. To determine this information, the details about the information-processing subtasks and any other necessary information were identified by the authors based on expert judgment and other more general sources of driving behavior and human factors research (e.g., Groegor, 2000; Salvendy, 1997; Underwood, 1998). 22-37 HFG TUTORIALS Version 2.0

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 Engi- neers recommends several procedures for determining clearance interval duration in a 1994 informational report (see ITE, 1994) on signal change interval lengths. These methods include: 1. A rule of thumb based on approach speed, such as this one presented in the ITE Traffic Engineering Handbook (Pline, 1999): – Yellow change time in seconds = operating speed in mi/h/10 – Red clearance interval = 1 or 2 s 2. Formulas for calculating interval lengths based on site, vehicle, and human factors charac- teristics, such as this equation (from Pline, 1999): Where: CP = non-dilemma change period (change + clearance intervals) t = perception-reaction time (nominally 1 s) V = approach speed, m/s [ft/s] g = percent grade (positive for upgrade, negative for downgrade) a = deceleration rate, m/s2 (typical 3.1 m/s2) [ft/s2 (typical 10 ft/s2)] W = width of intersection, curb to curb, m [ft] L = length of vehicle, m (typical 6 m) [ft (typical 20 ft)] 3. A uniform clearance interval length—Various studies report that uniform value of 4 or 4.5 s for the yellow change interval length throughout a jurisdiction is sufficient to accom- modate most approach speeds and deceleration rates. Refer to Determining Vehicle Signal Change and Clearance Intervals (ITE, 1994) for more discussion on this. The Manual on Uniform Traffic Control Devices (FHWA, 2007) states that a yellow change interval should be approximately 3 to 6 s, and the Traffic Engineering Handbook (Pline, 1999) states that a maximum of 5 s is typical for the yellow change interval. The red clearance interval, if used, should not exceed 6 s (FHWA, 2007), but 2 s or less is typical (Pline, 1999). The traffic laws in each state may vary from these suggested practices. ITE recommends that the yellow interval not exceed 5 s, so as not to encourage driver disrespect for signals. CP t V a g W L V = + ± + + 2 64 4.

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) describes a process that utilizes these factors and is the basis for the steps described below. This method is mostly focused on guide and informational sign applications. Step 1. Calculate the Reading Distance The reading distance is the portion of the travelling distance allotted for the driver to read the message, based upon the time required to read it (reading time). The Traffic Control Devices Handbook outlines two methods for calculating the reading time. The first method, used by the Ontario Ministry of Transportation, is described in the following three steps: 1. Allocate 0.5 s per word or number and 1 s per symbol, with a 1-s minimum for the total reading time. This time should only include critical words. Drivers do not need to read every word of each destination listed on a sign to find the one they are looking for. For example, assume they are reading a sign with two destinations: Mercer St. and Union St., each with a direction arrow. Drivers only need to read the word Mercer to realize that is not the street they are looking for and the word Union to know that is their destination. They then only need to look at the arrow for Union St. 2. “If there are more than four words on a sign, a driver must glance at it more than once, and look back to the road and at the sign again. For every additional four words and numbers, or every two symbols, an additional 0.75 s should be added to the reading time.” (Ontario Ministry of Transportation Traffic Office, 2001) 3. If the maneuver does not begin before the driver reaches the sign, add 0.5 s to the reading time. This extra time is to account for the extreme viewing angle immediately before the driver passes the sign, which prohibits reading. If the maneuver has already begun, the driver does not need to continue to read the sign, and thus does not need more time. These three steps are summarized in Table 22-7. Step 1 Step 2 Step 3 Base Reading Time (BRT) Are there more than 4 words? Does the maneuver initiate before passing the sign? Yes: Add time based on the BRT 2 < BRT 4 Add 0.75 s 4 < BRT 6 Add 1.50 s 6 < BRT 8 Add 2.25 s …etc Yes: Add 0 s BRT (s) = 0.5x + 1y where: x = the number of critical words/ numbers in the message y = the number of critical symbols in the message No: Add 0 s No: Add 0.5 s Table 22-7. Three-step method for calculating base reading time.

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. Step 2. Calculate the Decision Distance The decision distance is the distance required to make a decision and initiate any maneuver, if one is necessary. After reading the sign, the driver needs this time to decide his/her course of action based upon the sign’s message. Decision times range as follows: • 1 s for simple maneuvers (e.g., stop, reduce speed, choose or reject a single destination from a D1-1 sign) • 2.5 s or more for complex maneuvers (e.g., two choice points at a complex intersection) After finding the decision time, convert it into the decision distance by multiplying by the travel speed. Step 3. Calculate the Maneuver Distance The maneuver distance is the distance required to complete the chosen maneuver. The maneu- ver distance depends on the course of action decided upon by the driver and the travel speed. The sign placement should consider all of the maneuvers that could be chosen based upon the message. An example of required maneuver distances is provided in Table 22-8 for lane changes in preparation for a turn. These distances do not apply to situations in which drivers must stop. For high-volume roadways, more time may be needed to find a gap, while for low-volume roadways, some of the deceleration distance may overlap with the lane change distance. Reading Time (s) (Number of Familiar W= 0 31. ords)+1 94. Table 22-8. Maneuver distances required for preparatory lane changes. Operating Speed (mi/h) Gap-Search Distance (ft) Lane Change Distance (ft) Deceleration Distance (ft) Non-Freeway Maneuver Distance Requirements 25 66 139 77 35 92 195 154 45 119 251 257 55 145 306 385 Freeway Maneuver Distance Requirements 55 218 306 308 65 257 362 462 70 277 390 549 Source: Pline (2001)

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., inter- section) at which the driver needs information. This distance is calculated using the following formula: Step 5. Calculate the Legibility Distance The legibility distance is the distance at which the sign must be legible. This distance is based upon the operating speed and the advance placement of the sign from the choice point. The leg- ibility distance is calculated using the formula below: Step 6. Calculate the Minimum Letter Height The minimum letter height is the height required for the letters on the sign based upon the legibility distance calculated above. It is also based upon the legibility index provided in the MUTCD (30 ft/in.). Another consideration is the minimum symbol size. The minimum symbol size is based upon the legibility distance of the specific symbol that is being used. Table 22-9 contains daytime leg- ibility distances for five types of symbols based upon research (Dewar et al., 1994). From these legibility distances, we can obtain two general trends: (1) legibility distances vary by sign type and (2) legibility distances are greatly reduced for older drivers. Legibility distances for symbols are generally greater than for word messages. Example Application As an example, a driver approaches an intersection on a 35-mi/h (51 ft/s) roadway. The driver needs to read a simple designation sign (D1-1) that contains one destination word and Minimum Letter Height (in.) Legibility Dist= ance (ft) Legibility Index (ft/in.) Legibility Distance Information Presentatio= n Distance Advance Placement− Information Presentation Distance Reading D= istance Decision Distance Maneuver Distanc+ + e Table 22-9. Daytime legibility distances of five symbol types by age group. Daytime Legibility Distances (ft) Symbol Type Number of Signs Young Middle-Aged Old Mean Warning 37 736.4 714.7 581.5 677.6 School 2 573.3 634.7 501.2 569.7 Guide 21 472.3 461.5 366.0 433.3 Regulatory 12 464.4 437.9 367.4 423.1 Recreational 13 321.1 292.6 228.9 280.8

HFG TUTORIALS Version 2.0 22-42 one symbolic arrow. The sign is placed 200 ft in advance of the intersection. The legibility index is assumed to be 30 ft/in. (FHWA, 2009). See Figure 22-9. 1. Reading Distance (ft) = [(1 s/word)(1 word) + (0.5 s/symbol)(1 symbol)](51 ft/s) = 77 ft 2. Decision Distance (ft) = (1 s/simple decision)(1 simple decision)(51 ft/s) = 51 ft 3. Maneuver Distance (ft) = Gap Search + Lane Change + Deceleration = 92 ft + 195 ft + 154 ft = 441 ft 4. Information Presentation Distance (ft) = Reading Distance + Decision Distance + Maneuver Distance = 569 ft 5. Legibility Distance = Information Presentation Distance – Advance Placement = 569 ft – 200 ft = 369 ft 6. Letter Height = (369 ft)/(30 ft/in.) = 12 in. (when rounded to the nearest inch) Figure 22-9. Graphic illustrating the example application of a driver approaching an intersection. Information Presentation Distance (569 ft) Maneuver Distance (441 ft) Reading Distance (77 ft) Decision Distance (51 ft) Gap Search (92 ft) Lane Change (195 ft) Deceleration (154 ft) Legibility Distance (369 ft) Advance Placement (200 ft)

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 legi- bility distance of the sign and the traveling speed of the passing vehicle. The legibility distance is the maximum distance at which a driver can first read a CMS message. According to Dudek (2004), this distance depends upon a number of factors including: • Lighting conditions • Sun position • Vertical curvature of the roadway • Horizontal curvature of the roadway • Spot obstructions • Rain or fog • Trucks in the traffic stream These obstructions and visibility limitations reduce the amount of time that the sign is within view or legible, ultimately requiring a reduction in the amount of information that is displayed on the CMS. The information that can be displayed is measured in information units. An infor- mation unit is a measure of the amount of information presented in terms of facts used to make a decision. For example, the location of the problem, the audience that is affected by the problem, and the recommended action to take are each 1 information unit. To determine the appropriate number of information units for display on a CMS, the following steps should be considered. Step 1. Determine the Legibility Distance for the CMS The maximum legibility distance for a CMS depends on the design characteristics of the sign (Dudek, 2004). These characteristics include the display type, character height, character width, character stroke width, and the font displayed. The base legibility distances found in Table 22-10 are presented in Dudek (2004) and are based on the results of several studies. The distances are based on all uppercase letters, 18 in. character heights, approximately 13 in. character widths, and approximately 2.5 in. stroke widths. Note that all of the information for light-emitting diode signs provided in this tutorial applies only to the newer aluminum indium gallium phosphide (or equivalent) LEDs. Table 22-10. CMS legibility distances for varying lighting conditions. Legibility Distances (ft) Lighting Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk Mid-Day 800 800 700 600 Washout 800 800 700 400 Backlight 600 500 400 250 Nighttime 600 600 600 250 Source: Dudek (2004)

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) and the speed of the passing vehicles. The faster that the passing drivers are going, the less time they have to read the CMS message. Also, because the legibility distance of the sign depends upon the technology used, the number of information units also varies with the technology that is used. Finally, the diverse technologies perform differ- ently under changing conditions. Table 22-11 presents the base maximum number of informa- tion units that can be presented for assorted CMS technologies, under several ambient lighting conditions. Step 3. Adjust for Adverse Roadway and Environmental Conditions There are many roadway and environmental conditions that reduce the visibility of CMSs and thus require a reduction in information units. Dudek (2004) provides further guidance on the exact number of information units that should be used under different conditions. The follow- ing sections describe how various conditions and factors lead to trade-offs in the number of information units that may be displayed. Vertical Curves The reduction in information units required for vertical curves depends on the design speed of the curve as well as the CMS offset from the road and mounting height. The following general relationships apply to CMSs on vertical curves: • As the design speed of the curve decreases, the number of information units that may be used decreases. • As the horizontal offset from the road increases, the number of information units that may be used decreases. • As the mounting height of the CMS decreases, the number of information units that may be used decreases. Table 22-11. Maximum number of information units per message for various technologies at different speeds. Maximum Information Units per Message Light-Emitting Diode Fiberoptic Incandescent Bulb Reflective Disk Lighting 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h 0-35 mi/h 36-55 mi/h 56-70 mi/h Mid-Day 5 4 4 5 4 4 5 4 3 5 4 3 Washout 5 4 4 5 4 4 5 4 3 4 3 2 Backlight 4 4 3 4 3 2 4 3 2 2 1 1 Nighttime 4 4 3 4 4 3 4 3 3 3 2 1 Source: Dudek (2004)

22-45 HFG TUTORIALS Version 2.0 In general, permanent CMSs that are mounted over the roadway are not affected by crest ver- tical curves (Dudek, 2004). Horizontal Curves The main concern with CMSs located on horizontal curves is the obstruction of the sign by roadside objects. Permanent CMSs that are mounted above or adjacent to the travel lanes will likely be high enough to be seen over any roadside obstructions. However, portable CMSs are usually closer to the ground and more likely to be obscured by obstructions. In general, the num- ber of information units that may be used decreases when: • The obstruction gets closer to the roadway • The curve radius decreases (i.e., for tighter curves) Rain Rain does not generally affect CMSs (Dudek, 2004). However, when the intensity of the rainfall increases to 2 in./h or more, the visibility of the sign can be impacted. When the operating speed of the roadway is over 55 mi/h, Dudek (2004) recommends that the number of information units displayed on portable LED CMSs should be reduced by 1 information unit. Portable LED CMSs often use fewer pixels per character, and thus have lower luminance levels per character than permanent CMSs, which are relatively unaffected even in heavy rain- fall. Therefore, signs utilizing other technologies should use fewer information units in heavy rainfall. Fog Fog can affect visibility even more than heavy rain. Generally, Dudek (2004) does not recom- mend a reduction in information units for permanent LED CMSs because of fog. A reduction is not necessary because of the high character luminance and contrast of permanent LED CMSs. However, portable LED CMSs require a reduction. The number of information units that may be used decreases when: • The visibility range decreases • The offset from the road increases Trucks on the Roadway Large trucks pose sight obstructions for other vehicles on the roadway. When a driver’s view of a CMS is obscured by a truck, the driver has the option to change his/her traveling speed or position to see around the truck. However, as the number of trucks on the roadway increases, the amount of space that is available for drivers to do this repositioning decreases. Thus, the more trucks that are on the roadway, the more likely they are to impair the view of a CMS for other drivers. Step 4. Adjust for Blanking Time Greenhouse (2007) found that inserting a 300-ms blank screen between phase 1 and phase 2 of a portable message sign improves comprehension. The study is further discussed in the guideline

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), it is reasonably conceivable that drivers who see a blank between phases 1 and 2, but not between phases 2 and 1, would reverse the order of the phases and possibly have trouble understanding the message. Dudek (1992) recommends that blank time and/or asterisks should be displayed between cycles of a message that contains three or more phases (on one-word or one-line signs). Because one-word and one-line signs are more lim- ited in the amount of information that they can display at one time, the phases may not make sense independently and drivers who read later phases before phase 1 may not understand the message. Thus, giving an indication of where the message is in the cycle gives drivers an idea of their location in the cycle. Overall, drivers may use the blanking time to determine where they are in the message cycle, even before the message is legible to them. There are additional benefits in terms of message com- prehension as shown by Greenhouse (2007). However, the insertion of blanking time reduces the total available time for the driver to read the message, potentially requiring a reduction in information units. Thus, there is a trade-off between the benefits of providing blanking time and the number of information units that may be contained in the message. Step 5. Display the Resulting Number of Information Units After the calculations and adjustments from Steps 1 through 4 are performed, the result will be the number of information units that may be displayed in the message. If there are still more information units in the message than should be displayed, they should be reduced using the fol- lowing steps, until the appropriate number of information units is reached (steps and examples adapted from Dudek (2004)). Step 5A: Omit and Combine Information Units First, attempt to reduce the number of information units without losing content by following the steps below. • Omit unimportant words and phrases Example: Original Message: Shortened Message: ROAD CLOSED AHEAD ROAD CLOSED DUE TO CONSTRUCTION 1 MILE FOLLOW DETOUR ROUTE FOLLOW DETOUR The word “Ahead” is unnecessary as drivers will assume the closure is ahead. The reason is less important than the location of the closure.

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. • Combine base CMS elements Example: Original Message: Shortened Message: TRUCK ACCIDENT FREEWAY CLOSED PAST I-80 EXIT AT I-80 ALL LANES CLOSED FOLLOW DETOUR AT I-80 I-287 NORTH TRAFFIC EXIT AT I-80 FOLLOW DETOUR In the example above, the incident descriptor, incident location, and lanes closed mes- sage elements are combined into the information unit “Freeway Closed”. The location of the closure can be eliminated because the action element “Exit at I-80” describes the location. Step 5B. Reduce the Number of Audiences in the Message Example: Original Message: Shortened Message: I-76 CLOSED I-76 CLOSED BEST ROUTE TO BEST ROUTE TO PHILADELPHIA/I-95 PHILADELPHIA USE RTE-73 NORTH USE RTE-73 NORTH When using this reduction technique, message designers must use their judgment to decide which audience is more important to address in the message. In the previous example, the audience “Philadelphia/I-95” was reduced from 2 information units to 1 information unit, “Philadelphia”.

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. The priority order is derived from the information drivers need the most in order to make driving decisions. In Table 22-12, the information units are listed in priority order, with number 1 being the highest priority information. If the closure is due to roadwork, the effect on travel and good reason for following the action should be eliminated. Even though the incident/roadwork descriptor is useful to driv- ers, it may be replaced with the lanes closed element if necessary. When choosing information units to eliminate, the designer should start deleting units from the bottom of these priority lists first (i.e., element numbers 8 or 6). More examples of the application of these steps can be found in Dudek (2004). Table 22-12. Order of priority for information units. Lane Closures Freeway/Expressway Closures 1. Incident Descriptor 2. Incident Location 3. Lanes Closed 4. Speed Reduction Action (if needed) 5. Diversion Action (if needed) 6. Audience for Action (if needed) 7. Effect on Travel (if needed) 8. Good Reason for Following Action (if needed) 1. Closure Descriptor 2. Location of Closure 3. Speed Reduction Action (if needed) 4. Diversion Action 5. Audience for Action (if needed) 6. Effect on Travel (if needed)