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Task Analysis of Lane Changes on Tangent Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-2 Overview of Driver Alertness on Long Tangent Sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8-4 8-1 C H A P T E R 8 Tangent Sections and Roadside (Cross Section) Tangent sections and related design aspects are not topics that are typically examined separately in human factors or driver behavior research. Consequently, the current chapter only has a few guidelines on this topic. However, as a basic roadway element, tangents are often a relevant contributing aspect to a variety of driver factors. The HFG reflects this in other chapters with several guidelines directly relevant to tangent sections. The table below provides an index of where to find these guidelines within the HFG. Page Number Guideline Title Relevance 5-2 Key Co mp onents of Sight Distance Deter mi ning si ght distance requirem ents for tangent sections. 5-4 Deter mi ning St opping Sight Distance 5-8 Determining When to Use Decision Sight Distance 5-10 Deter mi ning Passing Sight Distance Passing sight distance recommendations based on design speed and assum ed vehicle speeds. 9-2 Perceptual and Physical Elements to Support Rural-Urban Transitions Recommended measures for controlling driver speed in rural-urban transition zones. 16-2 Passing Lanes Recommended values of length and spacing by ADT and terrain. 16-4 Countermeasures for Pavement/shoulder Drop-offs Vertical drop-off heights warranting traffic control for various lane widths. 16-6 Rumble Strips Recommended sound levels for ru mb le strips, and effects of different rum ble strip characteristics. 17-4 Speed Perception and Driving Speed Factors that affect speed perception and speed selection. 17-6 Effects of Roadway Factors on Speed 17-8 Effects of Posted Speed Lim its on Speed Decisions 21-2 Countermeasures for Mitigating Headlam p Glare Counterm easures for addressing glare from oncom ing vehicles in tangent sections.
HFG TANGENT SECTIONS AND ROADSIDE (CROSS SECTION) Version 2.0 8-2 TASK ANALYSIS OF LANE CHANGES ON TANGENT SECTIONS Task analysis of lane changes provides a general description of the tasks involved with making a lane change (LC) on a tangent section of roadway. These tasks are primarily cognitive, motor, visual, or some combination thereof. The tasks apply to many roadway conditions, but the likelihood of their occurrence and manner in which drivers perform them vary based on the individual driver. This task overview has implications for interchange design, and particularly sign placement. Design Guidelines Avoid presenting drivers with sign reading or decision-making tasks at locations where lane changes are likely, i.e., entrances, exits, merges, weaving. The figure and table below show the different segments in a lane change as well as the cognitive, motor, and visual workloads associated with the tasks (adapted from Lee, Olsen, and Wierwille (1)). 1. Decision 2. Preparation 3. Execution Segment Goal Decide if a lane change is possible Prepare vehicle position and turn signals Steer the lane change maneuver Key Tasks 1.1 Scan traffic (L,C,R) and TCDs 1.2 Check mirrors (D,C) 1.3 Check memory and assumptions 2.1 Scan forward view (L,R) to verify vehicle is centered 2.2 Maintain safe gap in original lane 2.3 Arrange safe gap in destination lane 2.4 Activate turn signal 2.5 Perform final glances to mirrors (D,C) and blind spots 3.1 Initiate LC maneuver 3.2 Steering (D, beside C) 3.3 Deactivate turn signal 3.4 Check rearview mirror Driver Factors â¢ 73-88% of participants underestimate time required â¢ Approximate probability of: â Turn signal activation: 77-78% â Directional mirror glance: 87% (L), 49% (R) â Inside mirror glance: 42% (L), 78% (R) â Blind spot glance: 31% (L), 16% (R) N/A Cognitive Load High Low Low Motor Load Low Low to Medium High Visual Load High High Medium Legend: L = Left, C = Center, R = Right, D = LC Direction Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data Destination lane gap SV POV POV POV POV SV Origin lane gap SV POV POV 1. Decision 2. Preparation 3. Execution Direction of travel DD D L L C CCC C C RR Direct Glance Mirror Glance
8-3 HFG TANGENT SECTIONS AND ROADSIDE (CROSS SECTION) Version 2.0 Di scu ssi on The key tasks outlined in the table on the previous page ( 1 ) depict a general progression of tasks as drivers execute lane changes. The tasks are shown in a vertical colum n, corresponding to the lane change segment in which they occur. Each segm ent also has driver factors, which are relevant behavioral inform ation, further described below. The cognitive, motor, and visual workloads are rated and color coded depending on the demand of those resources during the corresponding segment. The first segment is the decision of whether a lane change is possible. This segm ent places the highest dem ands on cognitive activity, mainly related to the decision to initiate the activity. One subtask for drivers involves checking their memory and assumptions related to the lane change task. In an on-road study, most drivers underestimated the am ount of tim e that a freeway lane change actually takes ( 2 ). The underestimation was more common for lane changes to the right (88% of drivers) than to the left (73% to the left). After deciding that a lane change is possible, the preparation for the lane change begins. In this second segm ent, visual and mo tor tasks dominate. The tasks are centered around arranging a safe gap for the maneuver and prepari ng for the actual m ovement. In a study of dr iv er-side blind alert systems, the probability of turn signal activation was 77% and 78% for left and right lane changes, respectively ( 3 ). In the same study, for left lane changes, the approximate probability of drivers performing a left-mirror glance was 87%, an inside-mirror glance was 42%, and an over-the-left-shoulder glance was 31%. For right lane changes, the approximate probability of drivers executing a right- mi rror glance was 49%, an inside-m irror glance was 78% and an over-the-right-shoulder glance was 16%. In their study of naturalistic lane changes, Lee, Olsen, and Wierwille ( 1 ) found somewhat lower probabilities for the driver glance and turn signal behavi ors; however, their analysis wa s of a set of critical and urgent lane changes, during which drivers may be less inclined to take the ti me to perform their usual routine. Once the driver has prepared the vehicle and its position to allow for the lane change, they execute the motor com ponent. In this segm ent, drivers perfor m the actual steering involved in the maneuver. Following this step, drivers return to their norm al driving behaviors. The distribution of tasks among the modalities has implications for interchange design, particularly for sign placem ent. Some areas of interchanges are mo re likely to systematically involve a higher frequency of lane changes due to geom etric features such as entrances, exits, me rge/ weave sections, and upcom ing divergences. Fro m this task analysis, it appears that the cognitive load is highest at the beginning. This load is followed by a series of uni nterruptable visual activities to arrange appropriate gaps an d monitor surrounding traffic. Since the traffic flow is dyna mic, drivers need to resolve these visual activities at the point when mo tor activities are initiated (i.e., if interrupted, they may need to visually re-check their surroundings to determine if the conditions have changed). One of the implications of these conditions is that signs near high-frequency merge locations may be less likely to be read by merging drivers who are otherwise occupied with visual activities. Therefore, key signs should not be located in such locations. De si gn Is su es None. Cr os s Re fe re nc es Determining Passing Sight Distance, 5-10 Ke y Re fe re nc es 1. Lee, S.E., Olsen, E.C.B., & Wierwille, W.W. (2004). A Comprehensive Examination of Naturalistic Lane-Changes (DOT HS 809 702). Washington, DC: National Highway Traffic Safety Administration. 2. Lerner, N.D., Stein berg, G.V., & Hanscom , F.R. (2000). Development of Countermeasures for Driver Maneuver Errors (FHWA- RD- 00-022). McLean, VA: FHWA. 3. Kiefer, R.J., & Hankey, J.M. (2008). Lane change behavior with a side blind zone alert system . Accident Analysis and Prevention, 40 (2), 683-690.
HFG TANGENT SECTIONS AND ROADSIDE (CROSS SECTION) Version 2.0 8-4 OVERVIEW OF DRIVER ALERTNESS ON LONG TANGENT SECTIONS Introduction This guideline addresses driver fatigue and alertness on long segments of straight roadways. Driver fatigue is defined as a general psycho-physiological state that diminishes an individualâs ability to perform the driving task by reducing alertness and vigilance (1). Since long monotonous tangents have low levels of physical driving demand and reduced visual stimulation, they may induce driver fatigue and boredom, and reduced alertness. The key design issue in this case is that these types of roads may be âfatigue inducingâ because they demand drivers do relatively little in terms of vehicle control and visual scanning. Design Guidelines From Stutts et al. (2): If long tangent sections are used, consideration should be given to adding countermeasures to prevent crashes or reduce crash severity, including: â¢ Install shoulder and/or centerline rumble strips â¢ Eliminate shoulder drop-offs â¢ Widen and pave shoulders â¢ Widen two-lane roads and include a narrow âbufferâ median between opposing lanes â¢ Install median barriers for narrow medians on multi-lane roads â¢ Minimize overturning by designing safer slopes/ditches and removing hazardous roadside obstacles â¢ Reduce severity of run-off-road crashes through improved roadside hardware and barrier/attenuation systems EXAMPLES OF COUNTERMEASURES FOR LONG TANGENT SECTIONS Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data Wider Shoulder Buffer Median Barrier Shoulder Paved Shoulder Shoulder Rumble Strips
8-5 HFG TANGENT SECTIONS AND ROADSIDE (CROSS SECTION) Version 2.0 Di scu ssi on Vol um e 14 of NCHRP Report 500 ( 2 ) provides a helpful overview of the problem associated with drowsy drivers, as well as several roadway design strategies that can be used to reduce the problem . The guidelines presented here prim arily reflect this recent data source. Overall, there is a lack of on-road or crash data studies that directly exam ine the link between tangent length/monotony with fatigue-related safety risk. The issue of tangent length and fatigue is difficult to parse out from general fatigue related to mo re driver-specific causes such as sleep disruption, time of day, etc. Overall, the em pirical data are not definitive regarding the relationship between length or m onotony of a tangent, and fatigue - related crash risk. Nonetheless, in addition to countermeasures aimed at reducing the negative consequences of fatigue-related crashes, another logical type of counterm easure involves increasing driver stim ulation, such as adding visual co mp lexity or vehicle-control requirements (e.g., a horizontal curve). Most of the pertinent data in this area have been generated fro m driving si mu lator research. For example, in an exploratory study, drivers were found to be more likely to make large steering wheel movements on a visually monotonous roadway, which was interpreted as being fatigue related ( 1 ). In another study, the mo st apparent fatigue sy mp toms occurred on straight roads, or straight sections of roads with a mi x of straight and curved elem ents ( 3 ). Tying these findings in with on - road data, a 100-car naturalistic driving study found that 56% of run-off-road events occurred on straight segments of roadway ( 4 ), suggesting countermeasure treatments would be desirable. De si gn Is su es This guideline focuses on roadway design interventions to address driver fatigue on long tangent sections of roadway and task-induced fatigue rather than sleepiness. Driver alertness due to the im pact of m onotonous roadways is just a small part of the larger topic of driver fatigue as caused by sleepiness or drowsiness. There are a number of data sources related to th is broader issue, especially in the context of long-haul truck and other commercial drivers. There are also a num ber of sources that describe risk factors associated with driver fatigue and countermeasures that can be used to reduce driver fatigue ( 5 ). Cr os s Re fe re nc es Rumble Strips, 16-6 Ke y Re fe re nc es 1. Thiffault, P., & Bergeron, J. (2003). Monotony of road environ m ent and driver fatigue: A sim ulator study. Accident Analysis & Pr evention, 35, 381-391. 2. Stutts, J., Knipling, R.R., Pfefer R., Neum an, T.R., Slack, K.L., & Hardy, K.K. (2005). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 14: A Guide for Reducing Crashes Involving Drowsy and Distracted Drivers . Washington, DC: Transportation Research Board. 3. Oron-Gilad, T., & Ronen, A. (2007). Road char acteristics and driver fatigue: A sim ulator study. Traffic Injury Prevention, 8 (3), 2 81-289. 4. McLaughlin, S.B., Hankey, J .M., Klauer, S.G., & Dingus, T.A. (2009). Contributing Factors to Run-Off-Road Crashes and Near-Crashes. (DOT-HS-811-079). Washington, DC: National Highway Traffic Safety Administration. 5. McCallu m, M., Sanquist, T., Mitler, M., & Krue ger, G. (2003). Commercial Transportation Operator Fatigue Management Reference . Washington, DC: U.S. Depar tm ent of Transportation, Research and Special Progr am s Ad mi nistration.