Horizontal Curve Safety Gregory J. Taylor, P.E.

Transcription

1 Horizontal Curve Safety by Gregory J. Taylor, P.E.

2 INTRODUCTION In the U.S., the average crash rate for horizontal curves is about three times that of other highway segments. A Guide for Reducing Collisions on Horizontal Curves reports that nearly 25% of U.S. traffic fatalities are killed in vehicle crashes at curves annually with: 76% - single vehicles leaving the roadway 75% - rural roads 70% - two-lane secondary road crashes 11% - head-on crashes. Due to these statistics, the Federal Highway Administration (FHWA) identified Roadway Departure as one of its program emphasis areas and developed practical information publications designed for local road agencies. The main reference for this course, Low-Cost Treatments for Horizontal Curve Safety* is a result of these Roadway Departure program goals. *This reference may also be referred to as LCTHCS throughout this document The AASHTO Strategic Highway Safety Plan (2005) identified a set of goals for reducing highway crash fatalities. These included: Goal 15 - Keeping Vehicles on the Roadway; and Goal 16 - Minimizing the Consequences of Leaving the Road. Two main objectives were developed for improving horizontal curve safety 1. Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve and 2. Minimize the adverse consequences of leaving the roadway at a horizontal curve. Several crash emphasis areas evolved: head-on crashes, curve-related crashes, crashes with trees in hazardous locations, and run-off-road (ROR) crashes. Copyright 2012 Gregory J. Taylor, P.E. Page 2 of 43

3 Course Contents This course provides practical information on where, when, or how to apply a safety treatment or design feature (including cost and examples). The purpose of these treatments is to improve the safety of a single curve or winding section. Some of these traffic control devices or treatments may not comply with the MUTCD and are considered experimental. Anyone wanting to use this type of device on a public road must get FHWA approval for testing. Estimates of the effectiveness for treatments in reducing crashes are included if available. The actual observed effectiveness of a treatment may vary depending on location. The purpose of this course is to encourage readers to use this information to evaluate problems and implement appropriate treatments for problem curve locations. These treatments should help reduce roadway departure crashes, injuries, and fatalities. Numerous strategies or treatments can be applied to a single horizontal curve or road section to address a safety problem. This course includes only relatively low cost engineering treatments, as compared to major improvements, such as reconstructing the curve or road section. Manual on Uniform Traffic Control Devices (MUTCD) By law (23 CFR 655, Subpart F), the Manual on Uniform Traffic Control Devices (MUTCD) is recognized as the national standard for all traffic control devices installed on any street, highway, bikeway, or private road open to public travel. It is the definitive authority for traffic signs and pavement markings. Nationwide consistency is the goal of the MUTCD. The MUTCD is published by the Federal Highway Administration (FHWA) to promote safety and efficiency on our public roads by establishing uniform standards for traffic control devices. It defines the nationwide standards for the installation and maintenance of the devices on all streets and highways. The MUTCD allows us to drive anywhere in the U.S. using the same basic traffic control devices. Drivers who see a particular pavement marking or traffic sign should expect it to mean the same thing and be prepared to take the same action regardless of location. Copyright 2012 Gregory J. Taylor, P.E. Page 3 of 43

4 All States are required to adopt either the MUTCD or a State MUTCD that conforms to the Federal version of the MUTCD. In some cases, States have adopted the national MUTCD with an added State Supplement. BASIC TREATMENTS FOR HORIZONTAL CURVES The MUTCD contains several basic traffic control devices that can be installed at horizontal curves, especially locations where a safety problem has been identified. These include: Centerline Edge line Delineators Horizontal Alignment signs: Turn (W1-1), Curve (W1-2), Reverse Turn (W1-3), Reverse Curve (W1-4), Winding Road (W1-5), Hairpin Curve (W1-11), or Loop (W1-15) Advisory Speed Plaque (W13-1) (with any of the Horizontal Alignment signs) One-Direction Large Arrow (W1-6) sign Combination Horizontal Alignment/Advisory Speed (W1-1a or W1-2a) sign Curve Speed (W13-5) sign Chevron Alignment (W1-8) sign Many curves may require only standard centerlines and edge lines or a basic horizontal alignment warning sign. Adding one or more of the listed devices should be determined by engineering judgment that considers: The difference in the posted speed limit and the 85th percentile speed. Geometric features (length, radius, shoulders and roadside features) Sight distance for the curve Unexpected geometric features within the curve (intersection or change in curve radius) Traffic volume This course provides guidelines for the use of each device. All example signs, markings, figures, and tables are from the MUTCD, except where noted otherwise. Copyright 2012 Gregory J. Taylor, P.E. Page 4 of 43

5 Centerline Markings Horizontal Curve Safety Longitudinal markings (long lines) guide roadway traffic by providing visual clues to the travel path. These are commonly applied in white and yellow. Dashed lines (broken lines) permit vehicles to pass or change lanes. These white or yellow markers are four to six inches wide, applied with ten foot painted dashes and thirty foot spacing. The distance from the beginning of one dash to the beginning of the next dash is 40 feet. These lines provide an excellent way to estimate distances along roadways. For example, if there are three dashes between two side roads; then the estimated distance equals 120 feet separating the roadways (3 x 40). The centerline pavement marking is the minimal treatment for paved curves with sufficient width and volume. Centerline markings help keep vehicles on the correct side of the road and delineate the roadway alignment. The MUTCD allows using a centerline at specific locations, such as around a curve rather than the entire roadway. Short roadway sections may be marked with center line pavement markings to control traffic at specific locations (curves, over hills, grade crossings, bridges, etc.) for roadways without continuous center line pavement markings, Options for the center line markings on two-lane, two-way roadways* include: Two-direction passing zone markings - a normal broken yellow line - passing with care is permitted for traffic traveling in either direction One-direction no-passing zone markings - a double yellow line, one of which is a normal broken yellow line and the other is a normal solid yellow line - passing with care is permitted for the traffic traveling adjacent to the broken line, but is prohibited for traffic traveling adjacent to the solid line Two-direction no-passing zone markings - two normal solid yellow lines - passing is prohibited for traffic traveling in either direction. *Please note that a single solid yellow line shall never be used for center line marking on a two-way roadways. Copyright 2012 Gregory J. Taylor, P.E. Page 5 of 43

6 For undivided two-way roadways with four or more lanes for moving motor vehicle traffic, the centerline markings shall be the two-direction no-passing zone markings (solid double yellow line). Yellow center line pavement markings are used to separate traffic lanes with opposite directions of travel on a roadway. These markings can also be installed at sites that are not the geometric center of the roadway. Center line pavement markings are required on: Paved urban arterials & collectors Traveled way 20 feet or wider ADT = 6000 vehicles/day or more** Paved two-way streets or highways Three or more lanes Rural arterials & collectors Traveled way 18 feet or wider ADT = 3000 vehicles/day or more Where engineering judgment indicates a need **May be used for a minimum ADT of 4000 vehicles/day Center line markings may be placed on paved two-way roads with a minimum width of 16 feet but engineering judgment should determine any potential for traffic encroachment on the pavement edges, effects of parked vehicles, and traffic drifting into opposing lanes. For curves without centerlines that have been identified as a potential safety problem, this should be the first minimal treatment applied. For low traffic volume areas (less than 200 vehicles per day) and/or a pavement width less than 16 ft, post delineators, chevrons, or curve warning signs may be considered appropriate. Longitudinal lines have the following widths and patterns: Normal line - 4 to 6 inches wide. Wide Line - at least twice the width of a normal line. The width of the line indicates the degree of emphasis. There is no known safety benefit to having a wider centerline other than it provides a larger visual marking for motorists. Double Line - two parallel lines separated by a discernible space (approximately equal to the width of a single line 4 in to 6 in.). Copyright 2012 Gregory J. Taylor, P.E. Page 6 of 43

7 Broken Line - normal line segments (10 feet) separated by (30 feet) gaps. Dotted Line for extensions - noticeably shorter line segments (typically 2 feet) separated by shorter gaps (typically 2 to 6 feet). The width of a dotted line shall be at least the same as the width of the line it extends. Dotted Line for lane lines - 3 feet line segments with 9 feet gaps. A 1996 Kentucky study of entire roadway sections estimated that centerline markings reduced crashes by 35 percent. Other studies suggest that municipalities can expect safety benefits from Copyright 2012 Gregory J. Taylor, P.E. Page 7 of 43

8 installing centerline markings with the results varying based on roadway and traffic characteristics. The costs of these pavement markings depend on several factors: material; installer; geometric design, location, etc. Edge Line Markings Edge line pavement markings delineate the edges of streets and highways. They help guide road users during adverse weather and visibility conditions. Edge lines may also prevent vehicles from drifting onto the shoulder and roadside area. For horizontal curves, edge lines are a solid white line at the roadway s right edge. Along with the centerline or lane lines, edge lines define the travel lane for the driver. These markings can be installed before or within curves, but are typically applied to an entire roadway section. Warrants for Use of Edge Lines Edge line markings should be placed on the following paved streets or highways: Freeways, Expressways, and Rural arterials with a traveled way of 20 feet or more in width and an ADT of 6,000 vehicles per day*. * May be used for rural arterials and collectors with an ADT of 3,000 vehicles per day or greater. The standard edge line width is 4 to 6 inches, with the 4-in line being the most common. Freeways and some lower class roads have used six-inch edge lines. Wide edge lines of 8 to 12 inches (while not typical) can be used to emphasize curves and provide better visibility. This wide edge line should be avoided for roadways narrower than 20 feet in order to prevent head-on collisions. Results from a New York study showed that standard edge lines on two-lane roads with curves reduced total crashes by 5 percent and fixed object crashes by 17 percent. The Low Cost Local Road Safety Solutions reports several studies citing safety benefits from wider edge lines. Copyright 2012 Gregory J. Taylor, P.E. Page 8 of 43

9 Horizontal Alignment Signs Horizontal Curve Safety Curves are the second most likely locations for serious roadway crashes (intersections are number one) that often result in injuries and deaths. These types of incidents are due to lane departures, with head-on incidents for curves to the right, and run-off-road crashes for curves to the left. Proper signing can assist motorists through curves without leaving their lane. Curve warning sign usage is dependent on the roadway geometry of the first curve, the advisory speed of the sharpest corner, and whether it is a single curve or multiple curves. An alignment warning sign may be placed a maximum distance of 100 feet in advance of the curve, and a minimum distance of 100 feet from any other signs. The Low-Cost Treatments for Horizontal Curve Safety booklet prescribes four Horizontal Alignment signs to warn drivers of a single horizontal curve: Turn (W1-1) Curve (W1-2) Hairpin Curve (W1-11) 270-degree Loop (W1-15) Three advance warning signs are recommended for sections with multiple curves in close proximity: Reverse Turn (W1-3) Reverse Curve (W1-4) Winding Road (W1-5) Copyright 2012 Gregory J. Taylor, P.E. Page 9 of 43

10 For curves with the advisory speeds of 30 mph or less, W1-1 Turn signs should be used. If the advisory speed is greater than 30 mph, W1-2 Curve signs should be used. The type of sign to use is subject to engineering judgment that considers roadway geometry, traffic volume, road type, and other factors. TURN CURVE < 30 mph > 30 mph Copyright 2012 Gregory J. Taylor, P.E. Page 10 of 43

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12 Curves meeting the following conditions may not require a Horizontal Curve sign: gentle to moderate curvature adequate curve sight distance adequate pavement markings and delineation For curves of 135 degrees or more, the Hairpin Curve (W1-11) sign should be installed. For 270-degree loops (cloverleaf interchange ramps), use the Loop (W1-15) sign. For two curves in opposite directions separated by a maximum tangent distance of 600 feet, they may be signed using a Reverse Turn (W1-3) or Reverse Curve (W1-4) sign. At locations where the lower advisory speed of two curves is 30 mph or less, a Reverse Turn sign should be used. If the lower advisory speed is greater than 30 mph, Reverse Curve signs may be used. However, this is subject Traffic signs use retroreflective sheeting to ensure low light and nighttime visibility. Recently, municipalities have gone from using engineering grade (Type I), to high-intensity grade (Type III), and microprismatic sheeting (Type V). As the engineering grades increase from Type I to V, the signs become brighter and longer lasting but with increasing unit costs for each. A 1968 evaluation of curve warning signs reported a 18% crash reduction from installing warning signs. Although no recent research is available, a beneficial safety effect seems likely from proper horizontal alignment signage to engineering judgment that considers the traffic volume, type of road, and other factors. For road segments of three or more curves less than 600 feet apart, Winding Road (W1-5) signs may be appropriate. Where continuous roadway curves occur within a specific distance, NEXT XX MILES (W7-3aP) supplemental distance plate may be installed below the Winding Road sign. Depending on the roadway, place the appropriate warning sign the distance in advance of the point of curvature, as shown in Table 2C-4. For example, using a posted speed of 50 mph and a posted advisory speed of 30 mph, then the sign should be placed 125 ft before the point of curvature. Copyright 2012 Gregory J. Taylor, P.E. Page 12 of 43

13 Advisory Speed Plaque An Advisory Speed plaque (W13-1) should be installed below a Horizontal Alignment sign to advise drivers of the safe operating speed through the curve. These safe speeds are advisory and not the legal speed limit. The MUTCD requires an engineering study for determining the necessity of an advisory speed Copyright 2012 Gregory J. Taylor, P.E. Page 13 of 43

14 plaque. The advisory speed may be the 85 th -percentile speed, the 16-degree ball bank indicator reading, or the speed otherwise determined due to unusual circumstances. The Traffic Control Devices Handbook provides further guidance on when to use an Advisory Speed Plaque whenever the difference between the advisory speed and the posted speed is 6 mph or greater. One-Direction Large Arrow Sign For curve locations with curve warning signs that are still experiencing crashes, One-Direction Large Arrow (W1-6) signs have proven to be helpful. This sign defines changes in horizontal alignment. Arrow signs provide critical information on the location and sharpness of the curve plus it helps guide the driver. Only one of these signs is typically placed on the outside of the horizontal curve directly in line with the approaching tangent section, and at approximately a right angle to approaching traffic. Typically, either the One-Direction Large Arrow sign or a Chevron Alignment sign is installed with the Hairpin Curve sign or the Loop sign. The Arrow sign should be is limited to sharper curves and be used with an advisory speed plaque. Combination Horizontal Alignment/Advisory Speed Sign The Turn (W1-1) sign or the Curve (W1-2) sign can be combined with the Advisory Speed (W13-1) plaque to form a combination Turn/Advisory Speed (W1-1a) sign or a combination Curve/Advisory Speed (W1-2a) sign. These signs may supplement but not replace the advance Horizontal Alignment sign and Advisory Speed plaque, and should be placed at the beginning of the turn or curve. The sign is intended to advise drivers to slow down as they negotiate any roadway alignment changes. Copyright 2012 Gregory J. Taylor, P.E. Page 14 of 43

15 The Curve Speed (W13-5) sign should be used when the distance between the advance horizontal alignment sign and the point of curvature is 200 feet or less. Otherwise, the signs would be too close. Curve Speed Sign The Curve Speed (W13-5) sign is used to remind motorists of the advisory speed and where the recommended (advisory) speed changes with the road curvature (compound or spiral curve design). This sign should be installed within the curve either on the inside or outside of the curve to increase its visibility. For locations that may call for both the Curve Speed sign and the Combination Horizontal Alignment/Advisory Speed sign, use only one or the other. Chevron Alignment Sign The Chevron Alignment (W1-8) sign is intended to guide drivers through any changes in the roadway s horizontal alignment. Due to their pattern, size, and location, they are the best devices at defining the direction and sharpness of the curve. One of the advance curve warning signs will need to be installed along with any Chevrons. Copyright 2012 Gregory J. Taylor, P.E. Page 15 of 43

16 The Traffic Control Devices Handbook advises installing Chevrons at locations with differences between the advisory and posted speeds of 25 mph or greater. Chevrons need to be highly visible to provide adequate driver perception-reaction time. These signs should be installed on the outside of a turn or curve, in line with or at a right angle to approaching traffic. A minimum of two chevrons should always be visible by the road user. For two-lane, two-way roads, two-sided Chevron signs can be positioned to guide traffic traveling both directions. At locations with limited sight distance or decreased visibility, using higher posts for chevrons may be helpful. The mounting height for chevrons is 5 ft above the surface for rural areas, and 7 ft in urban areas. Chevrons have been shown to reduce vehicle encroachments onto the centerline for locations with the degree of curvatures over 7 degrees. The cost to apply a typical installation of about 10 chevrons is approximately $500. Delineators Delineators are helpful for long continuous sections of highway or through short stretches where the alignment might be confusing or unexpected (lane-reduction transitions, horizontal curves, Copyright 2012 Gregory J. Taylor, P.E. Page 16 of 43

17 etc.). It is a mounted retroreflective device that is placed along the roadway in a series to show its alignment. These are effective guidance devices at night and during adverse weather since they remain visible when wet or snow covered. Delineators consist of retroreflective devices (minimum of 3 inches) that normally retroreflect light from a distance of 1,000 feet when illuminated by standard automobile high beam headlights. The device is typically either circular or rectangular (3-in minimum) and usually mounted on posts (which can be retroreflective) 4 ft above the pavement. They can be placed either on barriers or on lightweight breakaway posts. (Source: Gregory J. Taylor) Single delineator: One retroreflective element for a given direction of travel at a specific location Double delineator*: Two identical retroreflective elements mounted together for a direction *An appropriately sized vertically elongated delineator may be substituted for a double delineator. Copyright 2012 Gregory J. Taylor, P.E. Page 17 of 43

18 A series of single delineators shall be provided on the right side of freeways and expressways and on one side of interchange ramps, except for either of the following two conditions: 1) On tangent sections of freeways and expressways with both of the following conditions: a. Continuous raised pavement markers are used to supplement pavement markings on lane lines throughout all curves and on all tangents, and b. Roadside delineators are used to direct traffic into all curves. 2) On sections of roadways with continuous lighting between interchanges. Delineators may also be used on other classes of roads. Plus, single delineators may be installed on the left-hand side of roadways where needed Delineator Colors White Red Left-hand side of a two-way roadway Wrong direction of ramp or roadway Truck escape ramp The colors of delineators should match the color of the adjacent edge line. Delineators of the appropriate color may be used to indicate where either an outside or inside traffic lane merges into an adjacent lane. Delineators should be installed adjacent to the lane reduced for the full transition length and should be installed to show the reduction. Red delineators may be used on the reverse side of any delineator where it would warn a road user traveling in the wrong direction on a roadway. Delineators should also be used on both sides of truck escape ramps. These delineators should be spaced at 50-foot intervals to identify the ramp entrance. Spacing beyond the entrance should be adequate for the length and design of the escape ramp. Due to their relatively low cost, delineators should be considered for curves (less than 7 degrees) where advance curve warning signs are used. Delineators should be mounted at a mounting height of approximately 4 feet measured vertically from the bottom of the device to the elevation Copyright 2012 Gregory J. Taylor, P.E. Page 18 of 43

19 of the edge of the pavement, They may be mounted at a lower elevation mounted on the face or top of guardrails or other barriers. Delineators should be placed at a constant distance from the edge of roadway 2 to 8 feet outside the outer edge of the shoulder; or in line with roadside barriers that are a maximum distance of 8 feet outside the outer edge of the shoulder. Where an obstruction is between the pavement edge and the line of the delineators, the delineators should be transitioned to the innermost edge of the obstruction. For guardrail or other longitudinal barrier, the delineators should be transitioned just behind, directly above, or on the barrier s innermost edge. The spacing of delineators for horizontal curves should be arranged so several delineators are always visible to the road user. Delineators should be spaced 200 to 530 feet apart on mainline tangent sections and 100 feet apart on ramp tangent sections. Delineators of the appropriate color may be mounted closely-spaced on guardrails or other longitudinal barriers to form a continuous or nearly continuous ribbon of delineation where needed. Double or vertically elongated delineators should be installed at 100-foot intervals along acceleration and deceleration lanes. In 1966, a Ohio Department of Transportation Study which found that delineators on rural two-lane curves reduced run-off-road crashes by 15 percent. Copyright 2012 Gregory J. Taylor, P.E. Page 19 of 43

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21 ENHANCED BASIC TREATMENTS Most basic treatments for horizontal curves can be improved for better visibility. The key concept is to provide the driver with adequate perception-response time to negotiate the section. The following seven treatments have proved effective in attracting motorists attention: o Larger devices o Doubling up of devices o High retroreflective intensity and fluorescent yellow sheeting o Flashing beacons o Profile Thermoplastic Markings o Raised Pavement Markers Larger Devices For typical roadway locations, the MUTCD prescribes using conventional road sign sizes. However, it allows minimum sizes for low-speed road where the reduced letter size is sufficient or where conditions do not permit larger sizes. Oversized and larger signs may be used for horizontal curves that have been identified as a safety problem where speed, volume, or other factors result in conditions where increased emphasis, improved recognition, or increased legibility is needed. Copyright 2012 Gregory J. Taylor, P.E. Page 21 of 43

22 Doubling-Up of Devices Horizontal Curve Safety Doubling-up simply refers to locations where a second, identical sign is installed on the left side of the roadway. Doubling-up increases the opportunity for sign visibility, plus, it increasing driver perception-response time. Doubling-up may be helpful at locations where visibility of the single right-hand side sign is inadequate. High Retroreflective Intensity and Fluorescent Yellow Sheeting High-intensity retroreflective sheeting and fluorescent-yellow sheeting are other ways to make signs more visible or noticeable. Retroreflective sheeting is available in different intensity grades. High-intensity sheeting can help signs to be seen from longer distances at night. Fluorescent yellow increases sign visibility (both day and night) of higher intensity sheeting makes the sign more visible to motorists for earlier recognition and response. (Source: LCTHCS) Research has shown that upgrading Chevrons to fluorescent yellow improves driver perception of the signs and suggests a potential safety effect. Sheeting Estimated Costs Unit Costs Type III Fluorescent Microprismatic $1.20/ft² $4.00/ft² Copyright 2012 Gregory J. Taylor, P.E. Page 22 of 43

23 Flashing Beacons Using flashing beacons with one of the advance Horizontal Alignment signs for a horizontal curve is another way to attract driver attention. They should be used only at locations where other treatments have not solved a safety problem. A limiting factor to their use is the availability of accessible power. Reliable solar power panel systems can be used as a possible power supply. These beacons are standard traffic signal circular yellow sections. One or more beacons that can be flashed either alternately or simultaneously. Locating the beacon signal housing at least 12 in outside of the nearest edge of the sign will prevent the flashing light from masking the sign message (Source: LCTHCS) A 1970s study evaluated the effects of signing to warn drivers of wet weather skidding hazards at horizontal curves and concluded that vehicle speed could be significantly reduced by adding flashing beacons to the curve warning sign. Profile Thermoplastic Markings Thermoplastic markings may be used to create a profile marking, which produces a rumble effect and enhances marking visibility. Since there has been limited use of this treatment with good results, there is no firm evaluation of its results. (Source: LCTHCS) (Source: LCTHCS) Copyright 2012 Gregory J. Taylor, P.E. Page 23 of 43

24 The California Department of Transportation (Caltrans) has used two types of profile thermoplastic markings raised and inverted profile patterns (shown above). Raised Pavement Markers Raised pavement markers (RPMs) can supplement or substitute for pavement markings. These come in a variety of types and can be reflective, nonreflective or snow-plowable. For snowy locations, the reflective material is encased in a metal casting or recessed below the pavement surface to prevent snow plow damage. Markers are typically applied to long roadway sections to provide a longer visible delineation. RPMs also work well when at single curved roadway sections by providing an auditory warning to anyone who travels on them. (Source: LCTHCS) The color of raised pavement markers under both daylight and nighttime conditions shall conform to the color of the marking for which they serve, supplement or substitute. Red used to alert wrong-way traffic Blue used to assist emergency personnel in locating fire hydrants Retroreflective or internally illuminated raised pavement markers may be used in the roadway bordering curbed approach ends or on top of raised medians and curbs of islands. These are available in mono-directional and bidirectional (capable of displaying the applicable color for each direction of travel). Internally illuminated markers should be steadily illuminated and not flash. Non-retroreflective raised pavement markers should not be used alone as a substitute for other types of pavement markings without supplemental retroreflective or internally illuminated markers. Copyright 2012 Gregory J. Taylor, P.E. Page 24 of 43

25 Directional configurations should be used to maximize correct information and to minimize confusing information from visibility of markers that do not apply to the road user. The spacing of RPMs should correspond with the pattern of broken lines for which the markers serve, supplement or substitute. For additional emphasis, retroreflective raised pavement markers may be spaced closer than described in the MUTCD if determined by engineering judgment. Studies of operational effects have shown that RPMs reduce the variation in lane placement and move vehicles away from the centerline. Safety Benefits Roadway curvature High traffic volume < 3.5 degrees > 5000 vehicles/day Safety Disbenefits Roadway curvature > 3.5 degrees (under all traffic volumes The Traffic Control Devices Handbook contains details pertaining to the spacing of raised pavement markers on longitudinal markings. OTHER TRAFFIC CONTROL DEVICE TREATMENTS Reflective Barrier Delineation Retroreflective material (reflectors, panels, etc.) has been proven to be a highly effective treatment for delineating curves, especially at nighttime. Reflective strips can be applied to either concrete barriers or metal guardrail to alert drivers of approaching curves. The color of the delineation should always match the color of the adjacent edge lines. For a two-lane, twoway road, this delineation would be white on both sides of the road. Reflective sheeting panels should be spaced 18 to 36 in apart and run parallel to roadway traffic. These panels can be anchored to concrete barriers with drilled holes and caulking compound. For metal guardrail, adhesive can be used to attach the panels. Copyright 2012 Gregory J. Taylor, P.E. Page 25 of 43

26 Individual reflectors should be spaced similar to post delineators. For curved guardrail, it is important to ensure the individual reflectors are perpendicular to the angle of oncoming headlights. (Source: LCTHCS) (Source: LCTHCS) Oregon DOT (ODOT) used 3M Linear Delineation System to apply reflective barrier treatments. Results have shown that retroreflective panels provide a good alternative to traditional concrete barrier delineation methods. The panels could be removed from the barrier for reuse on future projects. The results led ODOT to consider future implementation where crash histories show the need for additional safety measures. The panel installation process proved to be more challenging due to its time-intensive process. There were also maintenance concerns about road grime and dirt preventing optimal retroreflectivity levels. COSTS Individual reflectors Linear Delineation System $3.00 each $2.33 per linear foot 4 inch wide white Roadside Object Delineation Installation of an object marker, reflectorized tape, or other simple delineation device is a lowcost procedure to reduce the associated crash potential for trees, utility poles, and other obstructions located in close proximity to the roadside (within the designated clear zone). Depending on the roadside obstruction location and the severity of the hazard, there are 3 main options for objects with a history of being struck: o Removal Municipalities may discover the more difficult challenge of overcoming public resistance to removing specific objects. Copyright 2012 Gregory J. Taylor, P.E. Page 26 of 43

27 o Shielding Options include modifying the roadside to shield the object from being struck, flattening or grading sideslopes, regrading ditch sections, or improving shoulders. o Delineation When cost rules out other alternatives, adequate delineation should be installed first. (Source: LCTHCS) Object Marker Installation Guidelines (Type 2 or 3) Objects 8 or less from shoulder/curb Marker bottom a min. of 4 above roadway surface Objects over 8 from shoulder/curb Marker bottom a min. of 4 above ground Use reflective tape (6 min.) as a supplement or substitute Although there is no required color for the retroreflective tape, yellow is most commonly used. At locations with a history of daytime crashes, and in aesthetically sensitive areas, brown retroreflective tape may be used. The Pennsylvania Department of Transportation (PennDOT) has an experimental program for delineating hazards on road segments where it is not feasible to remove an object. Each tree or utility pole is marked with one round of reflective tape with two rounds for poles at intersections. Dynamic Curve Warning System Dynamic curve warnings use a speed measuring device (such as loop detectors or radar) combined with a flashing beacon and a variable message sign. The system is designed to slow Copyright 2012 Gregory J. Taylor, P.E. Page 27 of 43

28 vehicles approaching curves by measuring their speeds and providing messages to speeding drivers. These systems can be produced using off-the-shelf technology and have a much greater impact on speeding vehicles than typical curve warning signs. (Source: LCTHCS) Due to their cost, dynamic curve warning systems should be limited to locations with high crash rates, where other devices have failed. DYNAMIC CURVE WARNING SYSTEM GUIDELINES or 10 or more reported accidents* Within 24 months 7 or more reported accidents* Within 12 months *Occurring within curve limits and 1000-ft downstream A simple treatment without major reconstruction may consist of a radar speed detection device with warning signs and activated flashing beacons. Speeds can be measured and displayed using a speed display sign stating: YOUR SPEED IS.... TYPICAL DYNAMIC SYSTEM LAYOUT Turn Sign (W1-1) 625 in advance of the curve Overhead Sign Point of curvature Radar Detector 300 before overhead sign The equipment and installation costs of a typical system consisting of radar speed detection and associated flashing beacon, is approximately $18,000. The total system cost for a California Copyright 2012 Gregory J. Taylor, P.E. Page 28 of 43

29 interstate system was $61,000 (including traffic control). Although accident effectiveness may vary by location, research has shown that dynamic curve warning systems can significantly reduce vehicle speeds for horizontal curves. Caltrans reported a 44% reduction in accidents in the first year and a 39% in the second year of operation. Speed Limit Advisory Marking in Lane Advance horizontal curves pavement markings provide highly visible, supplementary warning information to motorists. These markings provide essential information for the safe negotiation of curves. Pavement markings are vital for speed reduction at curves where signs have proved ineffective. Speed limit advisory markings are probably more appropriate for higher speed roads and for locations where speed studies indicate excessive speeding. (Source: LCTHCS) The advance distance for such markings depends on both the approach and design speeds of the curve. Advance placement distances should be based on specific approach and curve speeds, which should be the same as advance distances prescribed for warning signs. A Texas evaluation of advisory speed pavement markings which based its findings on speed reductions at curves concluded that the markings were worthy of further exploration. For present specifications regarding the design and placement of these pavement markings, see the MUTCD and the Standard Highway Signs (SHS) manual. Copyright 2012 Gregory J. Taylor, P.E. Page 29 of 43

30 RUMBLE STRIPS Rumble strips are either a series of rough-textured or slightly raised or depressed road surfaces intended to warn drivers through vibration and sound of the edges of the travel lane. These can be installed by milling grooves into the road surface or by placing strips above it. Possible Rumble Strip Locations for Horizontal Curves Longitudinally with roadway centerline With edge line or shoulder Transversally across road in advance of curve Milled rumble strips are the preferred type for centerlines and shoulders due to its higher levels of noise and vibration. Raised rumble strips can be used for transverse applications with noise and vibration produced from raised bars (1/4 to ½ inch) of asphalt. Centerline Rumble Strip The centerline rumble strip (CLRS) may be used to alert drivers who cross over the centerline through horizontal curve sections and avoid head-on or opposite direction crashes. The use of CLRS just along a curve section has not been identified as an actual practice since the installation cost of would not justify their use for a relatively short section. Therefore, CLRS should be used on a considerable section of roadway. Copyright 2012 Gregory J. Taylor, P.E. Page 30 of 43

31 Suggested guidelines for applying CLRS include: Large number of head-on or sideswipe crashes Posted speed limit - 50 mph or greater Minimum ADT 1500 vehicles/day Minimum pavement width - 20 ft Asphalt surface in good condition (minimum depth of 2.5 in) (Source: Gregory J. Taylor) (Source: LCTHCS) The two most common types of milled centerline rumble strip patterns are: Continuous 12 to 24 apart Alternating Paired 12 to 24 apart alternated with paired 24 or 48 apart The greatest benefit from using CLRS is the reduction in vehicular crashes. In Delaware, a reduction of 90% for head- on collisions was reported for a two-lane rural highway after installing centerline rumble strips. Research results have shown that drivers normally stay farther away from CLRS while the milled strips help to delineate the centerline during adverse conditions. Centerline rumble strips are not recommended for the following locations: Bridge decks Intersections with local roads or short distances between access points Copyright 2012 Gregory J. Taylor, P.E. Page 31 of 43

32 Existing concrete pavements (overlay depth less than 2.5 in) Roadways with residences in close proximity The potential negative effects from installing CLRS or any other rumble strips include: disruption to motorcyclists and bicyclists; roadside noise; and pavement deterioration. Typical costs for CLRS are approximately $0.40 per linear foot, depending on the length of roadway, paving, pattern, location, etc. Shoulder Rumble Strip An edge line or center line may be installed over a longitudinal rumble strip to create a rumble stripe. However, edge lines shall not be used in addition to a shoulder rumble stripe. Rumble strips should be considered for locations with a high number of run-off-road (ROR) crashes. The shoulder rumble strip (SRS) can be used to alert drivers who veer onto the shoulder and thus avoid crashes. The shoulder rumble strip (SRS) would likely not be used for a single curve due to high installation costs. Many of the guidelines suggested for CLRS can be applied for SRS use as well. (Source: Gregory J. Taylor) (Source: LCTHCS) To reduce its potential negative effect on bicyclists, municipalities can change the SRS design to allow gaps in the pattern where bicyclist could travel. Copyright 2012 Gregory J. Taylor, P.E. Page 32 of 43

33 While SRS has proven safety benefits for large volume, high speed roads, their effectiveness for lower volume roads may still prove to be cost efficient for high ROR crash sites. Shoulder rumble strips on the New York State Thruway produced an 88% reduction in ROR crashes, and a 95% reduction in fatalities. Roadway Rumble Strip Roadway (transverse) rumble strips (RRS) consist of intermittent narrow, transverse areas of roughly textured or slightly raised (maximum of ½ inch) or depressed road surface that extend across the travel lanes. Through noise and vibration, they attract driver attention to reduce speed or pay attention as they negotiate the curve. (Source: LCTHCS) For instances where the color of a transverse rumble strip within a travel lane does not match the color of the pavement, the color of the strip will be either black or white. In this case, white transverse rumble strips should not be placed in locations where they could be confused with other transverse markings (stop lines, crosswalks, etc.). Guidelines for Using RRS on Horizontal Curves Locations with documented crash problem where conventional treatments have failed Road segment with differences between roadway speed limit and curve advisory speed of 20 mph Proximity of another geometric feature (side street with limited sight distance, second, sharper curve, downgrade leading into the curve, etc.) Long tangent section ending with a curve. Copyright 2012 Gregory J. Taylor, P.E. Page 33 of 43

34 Municipalities can prevent drivers who are tempted to go around the rumble strip by driving onto the shoulder or into an adjacent lane by extending the rumble strip over part of the shoulder in addition to the traveled way or by using a discontinuous rumble strip configuration. Concerns with using Roadway Rumble Strips include: Noise complaints avoid use in residential areas. Motorist use opposing lanes to avoid rumble strips use a discontinuous pattern design Maintenance problems grooved RRS is more durable and longer lasting Motorist worries keep depth of groove or height of bar to ½ in or less and use a warning sign RUMBLE STRIPS AHEAD. Bicyclist and motorcyclist concerns install a discontinuous pattern design and warning sign RUMBLE STRIPS AHEAD. Overuse of rumble strips leads to crying wolf attitude and therefore should be used sparingly. Roadway rumble strips have proven to reduce speed but not to a practical level. MINOR ROADWAY IMPROVEMENTS The following section presents four curve treatments that involve physical changes to the roadway and are relatively minor in costs. Paved Shoulder Treatment Paved shoulder treatments can be applied to unstable or narrow shoulders to increase safety and usable width. Using different colorations for the surface can also produce a safety benefit. Upgrading to paved shoulders should result in cost benefits with fewer crashes. A typical paved shoulder treatment involves reconstructing shoulders by removing and recompacting the shoulder base (commonly 6-in.). Asphalt surfaces can be texturized by applying a larger, uncoated seal coat on the shoulder while using a smaller aggregate seal coat on the roadway to provide visual, audible, and tactile clues to a driver leaving the travel lane. Texas DOT (TxDOT) believes this treatment to be operationally effective and reports reductions in single ROR crashes due to the visual, audible, and tactile cues alerting drivers straying onto the shoulder. The public also responded positively to the strong visual effect of the paved Copyright 2012 Gregory J. Taylor, P.E. Page 34 of 43

35 shoulder treatment, which should prove useful for nighttime travel. Cost estimates of seal-coating a gravel shoulder is approximately $1.00/yd 2. Shoulder Drop-Off Elimination Pavement shoulder drop-off is the result of erosion of the unstabilized pavement edges, which creates the vertical height difference between the paved surface and the unpaved shoulder.. Horizontal curves are especially prone to shoulder drop-offs. Drivers crossing a paved surface onto an unpaved shoulder tend to overcorrect to return to the road. This can cause the rear tire to catch on the pavement edge and lead to sideswipe, head-on, or opposite lane crashes. The shapes (vertical, rounded or tapered edge), pavement depths, vertical angle, and/or operational speed can affect the severity of pavement shoulder drop-offs. A simple and cost-effective solution is the use of a 45-degree angle asphalt fillet that provides a safer roadway edge and a stronger interface between the roadway and the shoulder along each side of the roadway. Methods of creating a 45-degree wedge include using a steel wedge or the Safety Edge Maker (a commercial device). The device s leading edge provides a smooth finished compaction and can be adjusted for varying drop-off heights. (Source: LCTHCS) Research has shown that drop-off shapes can reduce the rate of driver overcorrection, head-on, Copyright 2012 Gregory J. Taylor, P.E. Page 35 of 43

36 and run-off- road crashes. Horizontal Curve Safety Benefits of Shoulder Drop-off Elimination o Fewer crashes o Fewer injuries o Reduced tort liability GDOT evaluated the Safety Edge and determined that it can be placed on any type of roadway facility as part of asphalt preventative maintenance. Using the Safety Edge neither affects finished pavement smoothness or increases shoulder erosion. The Safety Edge can be implemented on any type of roadway as part of the asphalt paving process.costing approximately less than one percent of hot-mix asphalt material. Widen Shoulder The shoulder is designed as a safe space for drivers to get off the roadway and avoid crashes. This is a particularly important safety feature for horizontal curves where vehicles may use more of the travel lane than other sections. (Source: LCTHCS) Roadway shoulders may be stabilized or paved with widths ranging from 2 feet (minor roads) to 12 feet (major roads). All shoulder stabilization and roadside slope development must comply with guidelines established by AASHTO. The table below shows estimated reductions in related crashes (single vehicle run-off-road, multiple vehicle head-on, sideswipe) resulting from widening roadway shoulders. These estimated reductions apply only when roadside characteristics (side slope and clear zone) are Copyright 2012 Gregory J. Taylor, P.E. Page 36 of 43

37 rebuilt to existing conditions prior to shoulder widening. The table results are not limited to horizontal curves, it is reasonable to expect major benefits to other roadway sections as well. (Source: LCTHCS) Skid-Resistive Pavement Surface Treatment Maintaining adequate roadway friction is crucial for safe vehicle operation under both dry and wet conditions. When frictional demand exceeds the friction force between the tire and the road surface, a vehicle will skid (especially on horizontal curves). Pavement overlays and grooving have proven successful for roadway locations with histories of skidding crashes Pavement surfaces containing voids from which aggregate without specific particle gradations produces better drainage and skid resistance. A 1-in, open-graded asphalt concrete can help reduce wet pavement crashes due to its increased number of surface voids than the maximum open-graded asphalt concrete standard mix. Copyright 2012 Gregory J. Taylor, P.E. Page 37 of 43

38 (Source: LCTHCS) Pavement grooving is the installation of longitudinal or transverse cuts on rigid concrete surfaces to increase skid resistance. The resulting rougher pavement should improve the roadway s drainage characteristics and reduce the number of wet-weather crashes. Transverse grooves cut into the pavement are most effective at locations with frequent stops. Longitudinal grooving has proved most effective for improving safety on horizontal curves by increasing directional control of vehicles. Research has shown greater accident reductions at 50 mph curves than at lower speed curves due to reduced hydroplaning (the major benefit of grooving).. TYPICAL PAVEMENT GROOVING DIMENSIONS Depth 5/32 to 5/16 Width 3/16 to 3/8 Spacing 8 grooves/foot (random) The New York State DOT (NYSDOT) implemented a statewide program to treat low skid resistance sites with overlays as part of the maintenance program. During a two-year period, NYSDOT treated 36 sites which resulted in reducing the annually recurring wet road crashes by more than 800. Their results showed that using this treatment helped reduce this type of crash by 50 % and total crashes by 20 %. The Florida DOT (FDOT) treated a curved ramp with Tyregrip, a high-friction material consisting of a highly modified exothermic epoxy resin top with calcinated bauxite (Polish Stone Value of 70 percent plus). This increased the skid resistance value from 35 to 104. Although Copyright 2012 Gregory J. Taylor, P.E. Page 38 of 43

39 used at a specific location, this material may be applicable to a higher volume curve with a history of wet pavement crashes. Results from a California grooved pavement study showed a 72% reduction for wet pavement crashes and a 7% reduction for dry pavement crashes on a two-lane road with sharp curves. Although pavement grooving is thought to accelerate roadway wear, it has proved to affect neither ride quality nor drainage. The costs for applying skid-resistant overlays are relatively moderate. A typical Caltrans 2-mile section cost $200,000 in INNOVATIVE AND EXPERIMENTAL TREATMENTS Two treatments presented in this section are considered experimental and do not have FHWA approval for general use. Optical Speed Bars Optical Speed Bars are transverse stripes gradually spaced with decreasing distances to increase drivers perception of speed and causing them to reduce speed (4-bar/sec spacing). The Optical Speed Bar name comes from the intended visual effect to the spacing of the treatment s painted lines (18 in long and 12 in wide) with the preferred material being thermoplastic. (Source: LCTHCS) Optical Speed Bars are applied to locations where traffic speeds should be reduced. This treatment has been limited to known problem locations requiring traffic to significantly reduce Copyright 2012 Gregory J. Taylor, P.E. Page 39 of 43

40 speed. Overusing Optical Speed Bars just to reduce traffic speed could jeopardize the visual effect of this treatment. The total length of the paving-marking segment is dependent on the desired speed difference (between approach and lower curve speed). The lengths listed in the following table are based on producing a comfortable speed reduction and providing a minimum of 4 seconds driving time within each segment. (Source: LCTHCS) Research by New York, Mississippi, and Texas show transverse pavement markings can effectively reduce mean speeds, 85 th percentile speeds (initial reductions from 0 to 5 mph), and speed variance. A typical estimate to install Optical Speed Bars in two directions is approximately $2,000 (labor and materials). PennDOT Curve Advance Marking The PennDOT Curve Advance Marking is an innovative pavement marking designed to alert motorists of an upcoming curve. This treatment contains two transverse bars, a SLOW legend, and an arrow indicating the upcoming curve s direction. PennDOT developed this treatment for two-lane locations with a high number of curve-related crashes. The device objective is to Copyright 2012 Gregory J. Taylor, P.E. Page 40 of 43

41 reduce the number of run-off-road crashes by reducing the upper percentile speed. (Source: LCTHCS) The PennDOT Advanced Curve Warning marking should not be installed where there is any potential for driver confusion due to intersecting roadways or driveways. All existing signs, delineation, and pavement markings should meet standards with consideration of posting an advisory speed limit. Marking locations may be adjusted for insufficient sight distance due to vertical geometry or other similar sight distance problems. Research has shown that the PennDOT Advanced Curve Warning may reduce overall roadway speeds by 6 to 7 percent with some reductions in high-speed traffic in curves. When using this treatment or any other traffic control device for horizontal curve safety, always remember to address the most hazardous curve first. Copyright 2012 Gregory J. Taylor, P.E. Page 41 of 43