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HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 SIGNS ON HORIZONTAL CURVES Introduction Prior to a change in the horizontal alignment of a roadway, information about this change should be conveyed to drivers via roadway signs. This information should be communicated in a concise and efficient manner such that drivers have time to process the information and adjust their speed as well as alter the vehicle path appropriately. Notification of an upcoming curve is typically conveyed using curve warning signs, which indicate whether the curve is to the right or the left; they are sometimes accompanied by advisory speed signs. The use of dynamic warning signs to alert drivers of a curve and/or their vehicle speed has also gained acceptance as an effective means of communication. Researchers disagree as to how advance warnings should be presented to drivers, i.e., through text or through symbols. But all agree that the key to effective warning is to notify the driver of the upcoming curve so that the driver can change the speed or path of the vehicle--or both. Individual studies on the effectiveness of advance warning signs vary considerably with respect to sign placements, sign messages, horizontal curve radii, and driver populations. Designers should consider such variables when making design decisions. Also, any information considered for use in curve signs should not be in conflict with current design standards in publications such as the MUTCD. Design Guidelines The tables below show the guidelines for advance placement of curve warning signs related to advisory/85th percentile speed, as well as spacing for chevrons--both are presented as a function of posted or advisory speeds (Adapted from McGee and Hanscom (1)). Advance Placement Distance (ft) for Advisory Posted or 85th Percentile Chevron Advisory Speed of the Curve (mi/h) of Speed Limit Speed (mi/h) Spacing (ft) (mi/h) 10 20 30 40 50 60 70 20 n/a 1 15 40 25 n/a1 n/a1 20 80 30 n/a1 n/a1 25 80 35 n/a1 n/a1 n/a1 30 80 40 n/a1 n/a1 n/a1 35 120 45 125 n/a1 n/a1 n/a1 40 120 50 200 150 100 n/a 1 45 160 55 275 225 175 100 n/a1 50 160 60 350 300 250 175 n/a1 55 160 65 425 400 350 275 175 n/a1 60 200 70 525 500 425 350 250 150 65 200 NOTE: The above spacing distances 75 625 600 525 450 350 250 100 1 apply to points within the curve. No suggested distance is provided for these speeds, as the placement location Approach and departure spacing depends on site conditions and other signing to provide an adequate advance distances are twice those shown warning for the driver. above. Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 6-12
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HFG CURVES (HORIZONTAL ALIGNMENT) Version 1.0 Discussion Numerous studies have shown the effectiveness of advanced warning signs for curves (2, 3, 4, 5). Typical improvements in driving performance are reductions in speed, fewer lane excursions, and generally fewer crashes--see also the table below. From a driver's perspective, the key advantage of advance warning signs is a notification that a (possibly) unexpected change in the horizontal alignment of the roadway is imminent. Signing can be used to notify the driver of an upcoming curve in many ways, including proper positioning along a driver's line of sight, fluorescent illumination, flashing beacons (5), or dynamic warnings. In this regard, designers are cautioned to avoid overloading the driver with extraneous information that might distract him or her from the primary task of maintaining safe control of the vehicle (6). Improvement Reference Findings Fluorescent Yellow Weighted average decrease in speeds at the curve point of curvature of about 1 mi/h for both the Microprismatic 2 mean and 85th percentile versus the existing standard yellow ASTM Type III signs. 38% overall Chevron Treatments reduction in edge line encroachments. Fluorescent Yellow 2 Speeds reduced slightly. Chevron Posts Fluorescent Yellow The overall number of vehicles initiating deceleration before reaching the curve warning sign Microprismatic Curve 2 was increased by 20%. However, the study found small and inconsistent effects on speeds Warning Signs approaching curves. Standard Red The red border had the greatest effect on speeds during the day for both passenger vehicles and Reflectorized Border 2 heavy trucks. Daytime mean and 85th percentile speeds of heavy trucks were found to decrease on Speed Limit Sign by 4 mi/h. Addition of Flags, The changes made to roadway surface included more reflective centerlines (CLs), more Flashers on Existing 3 reflective edge lines (ELs), wider ELs, the additional of raised retroreflective pavement Warning Signs markers, and the inclusion of horizontal signing warning of approaching curves. Dynamic Advance Curve Warning 4 Results found decreases in mean speeds from 2 to 3 mi/h. System Different Pavement Nighttime average speed reductions for the warning sign with flashing lights (5.1%), the Markings and Raised 5 combination horizontal alignment/advisory speed sign (6.8%), and flashing lights on both Retroreflective warning signs (7.5%). Pavement Markers Design Issues In a literature synthesis of the knowledge and practice, the physical and performance characteristics of heavy vehicles that interact with highway geometric design criteria and devices were examined (7). The synthesis notes that dynamic curve warning systems for trucks--especially highly accurate, sophisticated systems that incorporate vehicle parameters such as speed and weight--may help warn drivers of curves ahead and mitigate rollover crashes. Cross References Speed Selection on Horizontal Curves, 6-6 Key References 1. McGee, H. W., and Hanscom, F. R. (2006). Low-Cost Treatments for Horizontal Curve Safety. (FHWA-SA-07-002). Washington, DC: FHWA. 2. Gates, T.J., Carlson, P.J., and Hawkins, H.G., Jr. (2004). Field evaluations of warning and regulatory signs with enhanced conspicuity properties. Transportation Research Record, 1862, 64-76. 3. Molino, J., Donnell, E. T., and Opiela, K.S. (2006). Field ratings of nighttime delineation enhancements for curves. Transportation Research Board 85th Annual Meeting Compendium of Papers [CD-ROM]. 4. Monsere, C. M., Nolan, C., Bertini, R. L., Anderson, E. L., and El-Seoud, T. A. (2005). Measuring the impacts of speed reduction technologies: Evaluation of dynamic advanced curve warning system. Transportation Research Record, 1918, 98-107. 5. 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. 6. Zwahlen, H. T. (1987). Advisory speed signs and curve signs and their effect on driver eye scanning and driving performance. Transportation Research Record, 1111, 110-120. 7. Harwood, D. W., Potts, I. B., Torbic, D. J., and Glauz, W. D. (2003). CTBSSP Synthesis of Safety Practice 3: Highway/Heavy Vehicle Interaction. Washington, DC: Transportation Research Board. 6-13