A Guide for Reducing Collisions on Horizontal Curves (2004) / Chapter Skim
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Pages 10-55
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From page 10... ...
V-1 SECTION V Description of Strategies Objectives The objectives for reducing the frequency and severity of curve-related crashes are to • Reduce the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve and • Minimize the adverse consequences of leaving the roadway at a horizontal curve. Exhibit V-1 presents these objectives and their related strategies for improving safety at horizontal curves.
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Explanation of Strategy Types The strategies in this guide were identified from a number of sources, including the literature, contact with state and local agencies throughout the United States, and federal programs. Some of the strategies are widely used, while others are used at a state or even a local level.
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SECTION V -- DESCRIPTION OF STRATEGIES crashes. In some cases, the effectiveness of treatments may have been shown to have a desired impact on one or more presumed surrogates for crashes (e.g., speed)
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grams that enforce an effective law against driving under the influence (DUI) or driving without seatbelts.
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SECTION V -- DESCRIPTION OF STRATEGIES horizontal curve improvements. However, programs such as this require dedicated funding to be developed.
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and possibly the speed, of the vehicle downstream of the warning to negotiate the curve safely. Advance warning of alignment changes should be provided to a driver when changes in alignment are unexpected.
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SECTION V -- DESCRIPTION OF STRATEGIES Summary of Effectiveness of Nontraditional Treatments at Horizontal Curves This section provides a brief summary of what is known about the safety effectiveness of nontraditional treatments that provide advance warning to horizontal curves, as presented in the ROR guide. Several variations of nontraditional pavement marking treatments have been experimented with to improve safety at horizontal curves.
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curve. The MUTCD states that the Advisory Speed plaque shall be used where an engineering study indicates a need to advise road users of the advisory speed for a condition.
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SECTION V -- DESCRIPTION OF STRATEGIES accidents by 22 percent. Leisch (1971)
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Second, an important key to success is identifying sites where treatments of this nature have the potential to improve safety. This strategy targets curves where changes in alignment are unexpected and drivers may need to reduce their speeds to negotiate the curve safely.
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SECTION V -- DESCRIPTION OF STRATEGIES Summary of Effectiveness of Better Pavement Markings This section provides a brief summary of the safety effectiveness of better pavement markings, as presented in the ROR guide. "Better pavement markings" are pavement markings that are more durable, are all-weather, or have a higher retroreflectivity than traditional pavement markings.
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favorably with operations when post-mounted delineators were present. Vehicle operations were not significantly affected on the inside lane of the curve, but significant differences were observed on the outside lane of the curve.
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SECTION V -- DESCRIPTION OF STRATEGIES Strategy Attributes The ROR guide presents attributes common to Strategies 15.2 A2 and 15.1 A6. The reader is directed to the ROR guide for more detailed information related to this strategy.
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SECTION V -- DESCRIPTION OF STRATEGIES V-14 EXHIBIT V-3 Strategy Attributes for Providing Adequate Sight Distance (T) Technical Attributes Target Expected Effectiveness Keys to Success Potential Difficulties Appropriate Measures and Data Associated Needs Organizational and Institutional Attributes Organizational, Institutional and Policy Issues Issues Affecting Implementation Time Drivers of vehicles approaching a curve with limited sight distance.
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SECTION V -- DESCRIPTION OF STRATEGIES Strategy 15.2 A4: Install Shoulder Rumble Strips (P) General Description This strategy focuses on issues related to the safety effectiveness of shoulder rumble strips at horizontal curves (Exhibit V-4)
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horizontal curves, or both. Regardless, the research results indicate that ROR crashes typically occur at shallow departure angles.
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SECTION V -- DESCRIPTION OF STRATEGIES 1. The proportion of vehicles that run off the road is expected to be significantly greater on a curve than on a tangent section of road.
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It should be noted that the ROR guide addresses the use of mid-lane rumble strips (Strategy 15.1 A3) , which serve a similar purpose to shoulder rumble strips except that mid-lane rumble strips are installed in the center of the travel lane instead of on the shoulder.
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SECTION V -- DESCRIPTION OF STRATEGIES V-19 EXHIBIT V-6 Centerline Rumble Strips (Photo Provided by Caltrans) EXHIBIT V-7 Centerline Rumble Strips (Photo Provided by MnDOT)
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Summary of Safety Effectiveness of Centerline Rumble Strips The head-on guide identifies two studies that showed centerline rumble strips to be effective in reducing head-on crashes. Centerline rumble strips were installed on a two-lane, undivided rural highway in Delaware (Perrillo, 1998)
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SECTION V -- DESCRIPTION OF STRATEGIES analysis used 44 months of before data and 44 months of after data. The resulting crash data and associated percent changes are shown in Exhibit V-8.
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Other potential disadvantages of centerline rumble strips include decreased visibility of centerline pavement markings, potential drainage problems, and snow removal difficulties. However, experience has not proven these potential disadvantages to be significant or insurmountable.
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SECTION V -- DESCRIPTION OF STRATEGIES or vehicle-related settlement and can occur whether the shoulder is paved or not. Edge dropoffs may be more common on curves than on tangents.
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are based upon the standard curve formula that provides that a portion of the lateral acceleration developed by the vehicle will be resisted by superelevation and the remainder by tire-pavement friction. A vehicle will skid during braking and maneuvering when frictional demand exceeds the available friction at the tire-pavement interface.
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SECTION V -- DESCRIPTION OF STRATEGIES minimum it should include the highest-volume roadways. Caltrans operates an Office of Pavement Rehabilitation, which includes a program of pavement friction inventory (http://www.dot.ca.gov/hq/esc/Translab/opr.htm)
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upon accident data from a 3-year before period and a 1-year after period, Wong found a 72-percent reduction in wet-pavement accidents, while only finding a reduction of about 7 percent in dry-pavement accidents. Wong concluded that pavement grooving was effective in reducing wet-pavement accidents.
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SECTION V -- DESCRIPTION OF STRATEGIES longitudinal grooving, partially because most grooving equipment lends itself more readily to placing grooves parallel to the roadway. Grooves cut in the longitudinal direction have proven most effective in increasing directional control of the vehicle, while transverse grooving is most effective where vehicles make frequent stops, such as intersections, crosswalks, and toll booths.
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SECTION V -- DESCRIPTION OF STRATEGIES V-28 EXHIBIT V-10 Strategy Attributes for Providing Lighting of the Curve (T) Technical Attributes Target Expected Effectiveness Keys to Success Potential Difficulties Appropriate Measures and Data Associated Needs Organizational and Institutional Attributes Organizational, Institutional and Policy Issues Issues Affecting Implementation Time Costs Involved Drivers of vehicles approaching a horizontal curve who have difficulty seeing the curve during non-daylight hours.
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SECTION V -- DESCRIPTION OF STRATEGIES Strategy 15.2 A10: Provide Dynamic Curve Warning System (T) General Description The purpose of this strategy is to reduce the speed of high-speed vehicles on their approach and as they navigate through a horizontal curve.
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Several dynamic curve warning systems have also been deployed specifically to reduce the likelihood of a truck rollover crash. In 1998, 207 trucks were involved in fatal rollover accidents on the U.S.
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SECTION V -- DESCRIPTION OF STRATEGIES V-31 Key to Success Potential Difficulties in-motion detectors, speed loop detectors, height detectors, fiber-optic signs, and controllers to operate the systems. Based on an analysis of speed data, it was concluded that all three systems significantly impacted truck speeds and that the systems caused truck drivers to reduce their speeds before entering ramps, when their speed was exceeding the maximum safe speed.
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SECTION V -- DESCRIPTION OF STRATEGIES V-32 Appropriate Measures and Data Associated Needs Organizational and Institutional Attributes Organizational, Institutional and Policy Issues Issues Affecting Implementation Time Costs Involved Training and Other Personnel Needs Legislative Needs Other Key Attributes for radar equipment and possible video equipment. Weigh-in-motion devices may need to be installed as well.
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SECTION V -- DESCRIPTION OF STRATEGIES V-33 Strategy 15.2 A11: Widen the Roadway (P) General Description It is common practice to widen the traveled way on horizontal curves to make operating conditions on curves comparable to those on tangents.
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Finally, there is concern that widening the roadway may increase operating speeds. Because speed is such a critical factor related to safety at horizontal curves, roadway widening may worsen safety.
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SECTION V -- DESCRIPTION OF STRATEGIES These relationships indicate that there is no effect on safety until the superelevation deficiency reaches 0.01, which is consistent with the Zegeer work. Strategy Attributes During routine pavement projects, deficiencies in superelevation should be addressed (Zegeer et al.)
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Safety Effectiveness of Increasing the Radius of a Horizontal Curve Increasing the radius of a horizontal curve can be very effective in improving the safety performance of the curve. This strategy is also covered under Strategy 15.1 A5 (improved highway geometry for horizontal curves)
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SECTION V -- DESCRIPTION OF STRATEGIES Safety Effectiveness of Eliminating Compound Curves Compound circular curves are sometimes advantageous in providing desirable shapes of curves. However, although no quantitative comparisons have been made between the safety at simple circular curves and the safety at compound curves, agencies should be cautious of using compound curves, particularly if the radius of the first curve is significantly greater than the radius of the following curve.
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storm, when ice may already have formed. The most common automated anti-icing system now in use is one that uses a series of spray-nozzles connected to a chemical storage tank.
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SECTION V -- DESCRIPTION OF STRATEGIES V-39 Potential Difficulties Appropriate Measures and Data Associated Needs Organizational and Institutional Attributes Organizational, Institutional and Policy Issues In a report by Barrett and Pigman (2001) , certain difficulties were listed from an automated anti-icing system installed on a Kentucky Interstate bridge in October 1997.
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SECTION V -- DESCRIPTION OF STRATEGIES V-40 Issues Affecting Implementation Time Costs Involved Training and Other Personnel Needs Legislative Needs Other Key Attributes The major issues affecting the time required to implement automated anti-icing systems are the time required to prioritize roadway segments for installation of the systems and the time required for installation. Selection of locations for automated anti-icing systems should be based on consideration of wintertime accident rates, traffic volumes, horizontal alignments, and distances from maintenance yards to determine which areas are in the greatest need for such systems.
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SECTION V -- DESCRIPTION OF STRATEGIES The magnitude of vehicle offtracking is the amount of radial distance displaced from the center of the first axle to the center of the rear axle as a vehicle is making a turn (see Exhibit V-17)
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From page 51... ...
for trucks are permitted, it may be desirable to prohibit or restrict trucks with kingpin-to-rearaxle distances that exceed a specified threshold from operating on specific facilities. California has an active program of identifying roadways with horizontal curves that cannot accommodate trucks with longer kingpin-to-rear-axle distances and establishing appropriate truck advisory restrictions on particular roads (Caltrans, 1989)
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SECTION V -- DESCRIPTION OF STRATEGIES V-43 T R A C T O R - S E M I S O V E R _ _ F E E T K I N G P I N T O R E A R A X L E N O T A D V I S E D EXHIBIT V-19 California Truck Advisory Sign (http://www.dot.ca.gov/hq/traffops/trucks/trucksize/fs-trkrouts.htm) EXHIBIT V-20 Strategy Attributes for Prohibiting/Restricting Trucks and/or Other Large Vehicles on Roadways with Horizontal Curve Geometrics that Cannot Accommodate Vehicle Offtracking (T)
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SECTION V -- DESCRIPTION OF STRATEGIES V-44 Organizational and Institutional Attributes Organizational, Institutional and Policy Issues Issues Affecting Implementation Time Costs Involved Training and Other Personnel Needs Legislative Needs Other Key Attributes EXHIBIT V-20 (Continued) Strategy Attributes for Prohibiting/Restricting Trucks and/or Other Large Vehicles on Roadways with Horizontal Curve Geometrics that Cannot Accommodate Vehicle Offtracking (T)
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SECTION V -- DESCRIPTION OF STRATEGIES Five strategies are designed to reduce the consequences of leaving the roadway: • Strategy 15.2 B1: Design safer slopes and ditches to prevent rollovers (P) • Strategy 15.2 B2: Remove/relocate objects in hazardous locations (P)
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The guides for addressing collisions with trees in hazardous locations and utility poles provide supplemental information on reducing the harm done by collisions after vehicles have left the roadway. Strategy 15.2 B2 in this guide directly relates to Strategy 16.1 B1 in the tree guide.
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