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HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 COUNTERMEASURES TO IMPROVE PAVEMENT DELINEATION Introduction This guideline describes countermeasures that support improvements in curve detection and driver performance through the use of pavement surface markings, such as edge lines, raised retroreflective pavement markers (RRPM), transverse stripes, etc. These markings provide primarily non-verbal cues that promote improved vehicle control through earlier detection and recognition of curves, reductions in speed, and adjustments to lateral position. Design Guidelines General Use surface delineations that are characterized by small gaps, long dashes, and short repetition cycles. Use combinations of treatments wherever practical to increase overall effectiveness. Edge line/ Use edge lines when curves are sharp or frequent, on narrow roads, or in the vicinity of crossing Centerline roadways or major driveways. Use the widest possible edge lines and centerlines to maximize visible surface area. When possible, use striping materials with highly retroreflective characteristics to implement edge lines and centerlines. RRPM Combine RRPM with edge lines/centerlines. Use pairs of RRPM on the outside edges of the centerline for very sharp curves ( 12 degrees); for flatter curves, single RRPMs are sufficient. Place RRPMs 244 m in advance of the curve. Space markers at 40 m intervals for sharp curves and 80 m intervals for flatter curves. Transverse When practical, implement transverse stripes as graduated rumble strips. Stripes Space stripes to achieve 0.5 s intervals at the desired deceleration rate (e.g., 0.9 m/s2) "SLOW" text Use "SLOW" with arrow surface markings in the tangent section approximately 70 m before the with arrow curve to augment treatments in high-hazard areas or at sharp curves. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data The following table indicates various pavement marking treatments and their strengths for enhancing speed reduction, lane-keeping, and curve detection and recognition. Treatment Type Strengths General Surface markings Strongest curvature cues and short-range steering control (compensatory control) General Post-mounted chevrons Strongest guidance cues and long-range guidance (anticipatory control) Treatment Combinations Superior effectiveness compared with individual treatments Edge line/Centerline Strongest for curve recognition, curvature perception, and reduction of lateral variability. Discontinuities in edge line aid in recognizing upcoming intersections, driveways, etc. RRPM Improving visibility of edge lines and centerlines. Reducing lane encroachments. Both visual and rumble effects provide encroachments cues. Transverse Stripes Speed reduction. May be more effective at reducing higher (> 85th percentile) speed driving than lower speed driving. "SLOW" Text with Arrow Speed reduction and curve ahead warning. 6-10
HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 Discussion Road delineations provide cues that assist drivers in detecting curves and assessing the level of curvature. Road surface markings provide the strongest curvature cues and are best for providing short-range steering control cues (compensatory control--see "Countermeasures for Improving Steering and Vehicle Control Through Curves"), while chevron designs on post-mounted panels give the strongest guidance cues and are best for long-range guidance (anticipatory control). Under conditions of reduced visibility, steering performance improves in the presence of road surface delineations that are characterized by small gaps, long dashes, and short repetition cycles. Edge lines improve perception of curvature, curve recognition distance, and lane-position stability. Roads with edge lines exhibit fewer crashes than those without edge lines, particularly in combination with narrow widths, wet pavement, and/or high-hazard areas (1). Surface area has the greatest effect on edge line (and centerline) visibility-- effectiveness increases with wider edge lines. Also, the effectiveness of these stripes increases with the level of retroreflectivity. Raised reflective pavement markers are highly effective at improving curve visibility and reducing crashes, especially when used in combination with centerlines and edge lines (2). They can be particularly useful as a cue for warning of lane encroachment because the raised marker provides tactile as well as visual stimulus. As with edge lines, the effectiveness of RRPMs increases with retroreflectivity. Transverse stripes refers to painted or taped stripes that are applied perpendicularly across the roadway alignment. Typically, these stripes are separated by decreasingly graduated spacings in order to encourage speed reduction by creating a sensation of increased speed when the vehicle is traveling at constant speed. The effectiveness of transverse stripes has been mixed; while some studies report reductions in speed at curve entry (3), others report either no reduction or a slight increase in speed (4). Transverse stripes are most effective when implemented as rumble strips because they provide both visual and tactile stimuli. "Slow" text with arrow refers to the word "Slow" marked in elongated letters with an arrow above it pointing in the direction of the curve and transverse lines before and after the symbols. This treatment may be effective at speed reduction, especially in late night driving when drivers are more likely to be impaired by fatigue or alcohol (5). Combinations of treatments are generally more effective than any single treatment, especially when the combination includes rumble strips. Curve recognition, lane position, and number of encroachments are improved when RRPMs are used in conjunction with edge line/centerline markings compared with single treatments. Design Issues In general, centerline treatments tend to cause drivers to shift lateral position away from the centerline, while edge line treatments result in a lateral shift toward the centerline. RRPMs may reduce nighttime corner cutting in left-hand curves but increase corner cutting in right-hand curves (6). Several treatments, such as transverse stripes and widening of inside edge markings at the curve, may have a greater effect on driver performance for high-speed drivers (above 85th percentile speeds) than for lower-speed drivers. These treatments should be considered in hazard areas where speed is a prevalent factor in elevated crash rates (3). Cross References Speed Selection on Horizontal Curves, 6-6 Countermeasures for Improving Steering and Vehicle Control Through Curves, 6-8 Key References 1. Tsyganov, A. R., Machemehl, R. B., and Warrenchuk, N. M. (2005). Safety Impact of Edge Lines on Rural Two-Lane Highways. (CTR Research Report 0-5090-1). Austin: University of Texas Center for Transportation Research. 2. Nemeth, Z. A., Rockwell, T. H., and Smith, G. L. (1986). Recommended Delineation Treatments at Selected Situations on Rural State Highways. Part 1 (Revision). (Report EES-627-PT-1, FHWA/OH-86/009). Columbus: Ohio State University, College of Engineering. 3. Vest, A., and Stamatiadis, N. (2005). Use of warning signs and markings to reduce speeds on curves. Proceedings of the 3rd International Symposium on Highway Geometric Design [CD ROM]. Washington, DC: Transportation Research Board. 4. Macaulay, J., Gunatillake, T., Tziotis, M., Fildes, B., Corben, B., and Newstead, S. (2004). On-Road Evaluation of Perceptual Countermeasures. (Report CR 219) Civic Square, ACT: Australian Transport Safety Bureau. 5. Retting, R. A., and Farmer, C. M. (1998). Use of pavement markings to reduce excessive traffic speeds on hazardous curves. ITE Journal, 68(9), 6. 6. Zador, P., Stein, H. S., Wright, P., and Hall, J. (1987). Effects of chevrons, post-mounted delineators, and raised pavement markers on driver behavior at roadway curves. Transportation Research Record, 1114, 1-10. 6-11
