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HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 TASK ANALYSIS ON CURVE DRIVING Introduction This guideline identifies the basic activities that drivers would ty pically perform while trying to safely navigate a single horizontal curve. This information is useful because (1) it can help identify segments of the curve driving task that are more demanding and require the driver to pay closer attention to basic vehicle control and visual information acquisition, and (2) it identifies the key information and vehicle control requirements in different parts of the curve driving task. This information has design implications because workload is influenced by design aspects such as design consistency, degree of curvature, and lane width. In particular, identifying high workload components of the curve driving task provides an indication of where drivers could benefit from having their driving tasks made easier to perform (e.g., clearer roadway delineation, wider lanes, longer radius), or benefit from the elimination of potential visual distractions. Design Guidelines Because drivers have higher visual demands during curve entry and navigation--especially with sharp curves--curves should be designed to minimize additional workload imposed on drivers. Driver visual demands are greatest just before and during curve entry and navigation because drivers typically spend most of their time looking at the immediate roadway for vehicle guidance information. Some General Implications for the Design of Horizontal Curves · Avoid presenting visually complex information (e.g., that requires reading and/or interpretation) within 75 to 100 m or 4 to 5 s of the point of curvature, or within it. · Key navigation and guidance information, such as lane markings and delineators/reflectors, should be clearly visible in peripheral vision, especially under nighttime conditions. · Minimize the presence of nearby visual stimuli that are potentially distracting (e.g., signage/advertisements that "pop out" or irregular/unusual roadside scenery/foliage). · Visual demands appear to be linearly related to curve radius and unrelated to deflection angle. Curves with a curvature of 9 degrees or greater are highly demanding relative to more gradual curves. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data The figure and table below show the different curve segments, as well as key driving tasks and constraints. 1. Approach 2. Curve Discovery 3. Entry and Negotiation 4. Exit Tangent Point 75 -100 m (~ 4 sec) Point of Curvature Expectancy Effects 1. Approach 2. Curve Discovery 3. Entry and Negotiation 4. Exit Key Driving Tasks 1.1 Locate bend 2.1 Determine curvature 3.1 Adjust speed based on 4.1 Accelerate to 1.2 Get available speed in- 2.2 Assess roadway conditions curvature/lateral acceleration appropriate formation from signage 3.2 Maintain proper trajectory speed 2.3 Make additional speed 1.3 Make initial speed adjustments 3.3 Maintain safe lane position 4.2 Adjust lane adjustments position 2.4 Adjust path for curve entry Visual Demands & Low/Flexible Med. Increasing to High High Low Info Sources · Primarily environment · Curvature perception cues · Most fixations to tangent point · Vehicle position driven · Observing roadway conditions information Effective Info · Advisory/message signs · Non-verbal (e.g., chevrons) and · Direct info only (lane markings; · No constraints Modes direct info (e.g., delineators) raised markers) Vehicle-Control · None · Anticipatory positioning · Continuous heading adjustments · Lane position Demands · Curve cutting adjustments Primary Speed · Previous roadway · Expectations & curvature cues · Expectations & lateral acceleration Posted speed or · Influences elements & signage expectations 6-2
HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 Discussion The information about driving tasks in the previous page is taken from the task analysis described in Tutorial 3 that breaks down curve driving into its perceptual, cognitive, and psychomotor components. A key concept for understanding the curve driving task is the visual and vehicle-control demand, which refers to the amount of time that drivers are required to focus their attention on curve driving activities, such as acquisition of visual information and maintaining vehicle control, to the exclusion of other activities they could otherwise be doing while driving (e.g., scanning for hazards, viewing scenery, changing the radio station, etc.). Visual demands: During the Approach segment, the time and effort that drivers typically spend acquiring information needed to safely navigate a curve is low and driven primarily by the driving environment (e.g., other vehicles, scenery). During Curve Discovery, visual demands increase to high levels at the point of curvature, as drivers scan the curve for information that they need to judge the degree of curvature. Visual demands are highest just after the point of curvature (Entry and Negotiation segment) and drivers spend most of their time looking at the tangent point to keep their vehicle aligned with the roadway (1, 2, 3). For more gradual curves (e.g., 3 degrees), drivers spend more time looking toward the forward horizon than the tangent point (3). Vehicle-control demands: The driver workload imposed by the need to keep the vehicle safely within the lane is minimal up through the end of the Curve Discovery segment, at which point many drivers will adjust their lane position to facilitate curve cutting. Demands are highest during the Entry and Negotiation segment as drivers must continuously adjust the vehicle trajectory to stay within the lane. Moreover, these demands are higher for curves with a shorter radii and smaller lane width (1). During the Exit segment, drivers may adjust their lane position with minimal time pressure, unless there is another curve ahead. Effective information modes: The type of curve-related sign/delineator information that is most likely to be useful to drivers differs in each curve segment. During the Approach, drivers have fewer visual demands and have more time available to read more complex signs, such as speed advisory signs. During the Curve Discovery segment, conspicuous non-verbal information, such as chevrons, are more effective because drivers spend more time examining the curve and have less time available to read, comprehend, and act on text-based information. During Entry and Negotiation, drivers spend most of their time looking at the tangent point, and only direct information presented where they are looking (e.g., lane markings) or information that can be seen using peripheral vision (e.g., raised reflective marking at night) should be relied upon to communicate curve information. Speed selection: Driver expectancy and speed-advisory sign information form the primary basis for speed selection; however, the effectiveness of advisory information may be undermined by expectancy and roadway cues (4). Curve perception also plays an important role in speed selection and inappropriate curvature judgments (e.g., in horizontal curves with vertical sag). Once drivers are in the curve, lateral acceleration felt by drivers and likely vehicle handling workload provide the primary cues for adjusting speed. Expectancy effects: Driver expectations about a curve and, more broadly, design consistency are important factors in drivers' judgments about curvature and corresponding speed selection during the Curve Discovery segment (1). While direct cues, such as lane width and the visual image of the curve, influence speed selection, expectations based on previous experience with the curve and roadway (e.g., previous tangent length) also significantly influence speed selection (4). Mitigations to recalibrate driver expectancies (e.g., via signage) would likely be most effective prior to the Curve Discovery segment. Design Issues Visual demands appear to be related linearly and inversely to curve radius, but not to deflection angle. Curves sharper than 9 degrees are significantly more demanding than shallower curves or tangents, however, there is no clear, unambiguous threshold regarding what constitutes a sharp curve based on workload data (1, 2). Also, curve direction does not seem to affect workload (2). Additionally, it is unclear whether the 75 to 100 m length of the Curve Discovery segment is based on distance or time. The primary studies that investigated visual demand used the same fixed 45 mi/h travel speed, so it is currently unknown whether the 75 to 100 m fore-distance applies with other speeds (1, 2). Cross References The Influence of Perceptual Factors on Curve Driving, 6-4 Speed Selection on Horizontal Curves, 6-6 Countermeasures for Improving Steering and Vehicle Control Through Curves, 6-8 Countermeasures to Improve Pavement Delineation, 6-10 Signs on Horizontal Curves, 6-12 Key References 1. 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 Consistency for Rural Two-Lane Highways. Final Report. (Report FHWA-RD-94-034). McLean, VA: FHWA. 2. 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. (Report FHWA-RD-99-172). McLean, VA: FHWA. 3. Serafin, C. (1994). Driver Eye Fixations on Rural Roads: Insight into Safe Driving Behavior. Interim Report. (Report No. UMTRI-94-21). Ann Arbor: University of Michigan Transportation Research Institute. 4. Fitzpatrick, K., Carlson, P., Brewer, M. A., Wooldridge, M. D., and Miaou, S.-P. (2003). NCHRP Report 504: Design Speed, Operating Speed, and Posted Speed Practices. Washington, DC: Transportation Research Board. 6-3
