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OCR for page 12
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
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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.
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