Roadway Departure Crashes: How Can They Be Reduced?

Transcription

1 Roadway Departure Crashes: How Can They Be Reduced? Roadway departure safety is a high priority in most state strategic highway safety plans. Several states also recognize that roadway departure crashes mostly occur on two-lane local highways and that alcohol, fatigue, distraction and speed are factors. A substantive decrease in these crashes will require a comprehensive plan developed through dedicated participation at all levels of highway agencies with integrated safety activities. By geni B. bahar, P.E., P.Eng. Definition Roadway departure crashes result from a vehicle crossing an edgeline or centerline or leaving the roadway. The definition of roadway departure (referred to by some as lane departure) is still under discussion by state agencies. It is generally accepted that single-vehicle run-off-road (ROR); head-on; opposite direction sideswipe; and opposite direction front-to-side collisions at non-intersection locations are the most typical and form the majority of roadway departure crashes. Magnitude National Statistics Fifty-eight percent (24,806 people) of all fatalities in 2006 were attributed to drivers who left their path of travel, mostly inadvertently, and hit another vehicle, a roadside object, or a feature of the road or roadside. 1 The breakdown among major crash types was: 17,241 persons killed in single-vehicle ROR crashes; 4,720 persons killed in head-on crashes; 2,335 persons killed in opposite-direction front-to-side crashes; and 510 persons killed in opposite sideswipe crashes. The total number of fatalities in 2006 is similar to the 5-year average of fatalities between 2001 and Human Factors and Other Roadway Departure Crash Contributory Factors It is key to learn about the road and human-related contributory factors to roadway departure crashes so that relevant and effective solutions can be selected. A study of 200 ROR crashes where the first harmful event occurred off the roadway indicated that...runoff-road crashes are primarily caused by the following six factors (in decreasing order of importance): Excessive speed (32.0 percent): traveling too fast to maintain control Driver incapacitation (20.1 percent): typically drowsiness or intoxication Lost directional control (16.0 percent): typically due to wet or icy pavement Evasive maneuvers (15.7 percent): driver steers off the road to avoid an obstacle Driver inattention (12.7 percent): typically due to internal or external distraction Vehicle failure (3.6 percent): typically due to tire blow-out or steering system failure. 2 A smaller sample of 17 head-on crashes was analyzed, and the findings indicated that only one crash occurred as a result of overtaking, matching the U.S. annual crash statistics, which show that a small amount of head-on crashes are due to misjudgements during overtaking. 3,4 The head-on crashes were associated with the following factors: Excessive speed (47 percent): traveling too fast in relation to road geometry, surface condition and/or speed limit Driver incapacitation (47 percent): alcohol or fatigue Driver inattention (18 percent) Driver inexperience (18 percent) It is also noted that 18 percent of the head-on crashes occurred when drivers entered a sharp curve at too high a speed, and 18 percent occurred when drivers encroached onto the right shoulder (perhaps over-correcting and/or facing an edge drop condition). 5 Based on these two studies, it can be concluded that the most frequent event prior to a ROR or head-on crash is that drivers unintentionally encroach into the shoulder/median/opposing lane because of speed, incapacitation, inattention, or inexperience. 6,7 Speed-related crashes seem to be more closely linked to curves. In curves, driver workload is high due to steering and visual search demand. Drivers are willing to tolerate higher lateral 44 ITE Journal / December 2008

2 acceleration at tighter curves, thus entering these curves without decelerating to a safer condition. The greater the curve deflection angle, the larger the reduction in speed and the more edgeline encroachments for right curves and centerline encroachments for left curves. The tendency of drivers to undercut the curve increases the risk of ROR crashes on right curves and head-on crashes on left curves. 8,9 Driver perception of speed comes from peripheral vision; the geometric elements and related driving demands dictate the speed choice. Drivers limitations that contribute to a ROR or head-on crash include the perception-reaction time to a lane encroachment, assessment of appropriate curve entry speed, loss of steering control on unpaved shoulder and difficulty in assessing closing velocity during overtaking maneuvers. Furthermore, drivers are surprised when their expectations are violated; driver response is delayed and errors occur. Some highway examples are unexpected speed changes such as a left-side exit ramp from a freeway, a stopped vehicle in the through lane to turn left, unexpected horizontal changes blocked by a vertical curve, an abrupt change in horizontal alignment, a signalized intersection at the end of an access-controlled highway or freeway, or a community s commercial area (with accesses, parked vehicles, pedestrians and bicycles) within a high-speed rural highway corridor. tools The need to apply substantive safety (i.e., quantification of safety effects in terms of future or expected crash frequency and severity) is being introduced into highway design and operational studies. 10 One such supporting design tool is the Interactive Highway Safety Design Model (IHSDM), a suite of software analysis tools developed by Federal Highway Administration (FHWA) Safety Research and Development for evaluating safety and operational effects of geometric design decisions. IHSDM results support decision-making in the highway design process. IHSDM includes six evaluation modules (crash prediction, design consistency, intersection review, policy review, traffic analysis and driver/vehicle). The current release is applicable for two-lane IT IS KEY To learn about THE road and Human-rElaTED contributory FacTorS To roadway DEParTurE crashes So THaT relevant and EFFEcTIVE SoluTIonS can be SElEcTED. rural highways, with plans for future crash prediction capabilities matching the first edition of the Highway Safety Manual, and the inclusion of multilane rural highways and urban and suburban arterials. 11 The release of this upgraded version is planned for September SafetyAnalyst provides analytical tools for use by state and local highway agencies when identifying site-specific improvements. 12 This computerized tool, under development by FHWA and 27 state and local agencies, will be released in fall SafetyAnalyst can identify crash patterns at specific locations and determine whether those accident types are overrepresented, assisting agencies in the selection of sites for cost-effective improvements. In addition, SafetyAnalyst can determine the frequency and percentage of a particular accident type on the whole network system or a portion of the system, leading to possible system-wide or corridor treatments such as shoulder rumble strips, centerline rumble strips, median barriers, shoulder widening, enforcement campaigns, etc. Another supporting design tool is the American Association of State Highway and Transportation Officials (AASHTO) Roadside Design Guide. It provides a hierarchy in improving the roadside: 13 Remove the object or obstacle. Redesign the roadway, object, or obstacle so that it can be safely traversed. Relocate the object. Reduce the impact severity. Shield drivers from the object. Delineate the obstacle (if above are not appropriate). An associated software, the Roadside Safety Analysis Program (RSAP), was developed under National Cooperative Highway Research Program (NCHRP) Project 22.9 (an upcoming NCHRP Project will update the RSAP software). RSAP is intended as a tool for economic analysis. Appendix A states: The basic concept behind benefit/cost analysis is that public funds should be invested only in projects where the expected benefits exceed the expected direct costs of the project. 14 When considering the different strategies (such as providing guardrails or removing trees in hazardous locations), this software would be a useful decision-aid tool. ResouRces strategies and their expected safety effects The results of safety evaluations are expressed as accident modification factors (AMFs), which quantify safety effects in terms of the expected change in number, type and severity of crashes at a site caused by implementing a particular treatment or countermeasure. AMFs are used to estimate the safety effects of a particular treatment or to compare possible safety outcomes of different treatments. The comparison involves evaluating the safety of a site with or without a particular treatment or the safety of a site with one treatment or another. The Highway Safety Manual will provide a source of reliable AMFs. A present source of safety effects can be found in the Desktop Reference for Crash Reduction Factors. 15 Nearing completion are four evaluation studies under the ongoing Evaluations of Low Cost Safety Improvements Pooled Fund Study by FHWA. They will provide the safety effects of lane width/shoulder width combinations; offset left-turn lanes; advance street name signings; and curve treatments. Another joint ITE Journal / DEcEmbEr

3 study by the Center for Transportation Research and Education, the Iowa State Department of Transportation and some counties is evaluating the effect on ROR crashes post-installation of 4-6-inch-wide rumble strips milled in 11-foot lane, lowvolume roads with no shoulders. In support of the national safety agenda, AASHTO s Strategic Highway Safety Plan (SHSP) and Tools for Life Program have developed 21 volumes of implementation guides. 16 The following volumes are directly relevant to roadway departure safety: Volume 3, A Guide to Addressing Collision with Trees in Hazardous Locations 17 Volume 4, A Guide for Addressing Head-on Collisions 18 Volume 6, A Guide for Addressing Run-Off-Road Collisions 19 Volume 7, A Guide for Reducing Collisions on Horizontal Curves 20 Volume 8, A Guide for Reducing Collisions Involving Utility Poles 21 Volume 13, A Guide for Reducing Collisions Involving Heavy Trucks 22 Volume 14, A Guide for Reducing Crashes Involving Drowsy and Distracted Drivers 23 Volume 16, A Guide for Reducing Alcohol-Related Collisions 24 Volume 20, A Guide for Reducing Head-on Crashes on Freeways 25 Volume 3 reports more than 3,000 fatal crashes with trees, mostly located on local two-lane rural roads. One of the SHSP emphasis areas is to eliminate tree crashes or reduce the harm that can result from colliding with a tree when the driver fails to keep the vehicle on the road. The following strategies reduce fatal tree crashes: 1. Develop, revise and implement planting guidelines to prevent placing trees in hazardous 2. Mowing and vegetation control guidelines. 3. Shield motorists from striking trees. 4. Modify the roadside clear zone in the vicinity of trees. 5. Delineate trees in hazardous Volumes 4 and 20 focus on the reduction of head-on crashes. Volume 4 reports that, of the 18 percent of non-intersection fatal head-on crashes, about 91 percent of the vehicles involved in these crashes were going straight or negotiating a curve, indicating that these crashes may have resulted from unintentional maneuvers due to fatigue, distraction, alcohol, speeding and failing to negotiate a curve. This finding is consistent with the human factors studies described earlier. The following strategies reduce head-on crashes: 1. Install centerline rumble strips for two-lane roads. 2. Install profiled thermoplastic strips for centerlines. 3. Provide wider cross-sections on twolane roads. 4. Provide center two-way, left-turn lanes for four- and two-lane roads. 5. Reallocate total two-lane roadway width to include a buffer median. 6. Use alternative passing lanes or fourlane sections at key 7. Install median barriers for narrowwidth medians on multilane roads. Volume 6 lists 13 strategies to reduce the number of ROR collisions: 1. Install shoulder rumple strips. 2. Install profile marking edgeline rumble strips or modified shoulder rumble strips on a section with narrow or no paved shoulder. 3. Install midlane rumble strips. 4. Provide enhanced shoulder or in-lane delineation and marking for sharp curves. 5. Provide improved highway geometry for horizontal curve. 6. Provide enhanced pavement markings. 7. Provide skid-resistant pavement surfaces. 8. Apply shoulder treatments (eliminate drop-offs; widen and/ or pave shoulders). 9. Design safer slopes and ditches to prevent rollover. 10. Remove/relocate objects in hazardous 11. Delineate trees or utility poles with retroreflective tape. 12. Improve design of roadside hardware. 13. Improve design and application of barrier and attenuation systems. Volume 7 reports that about 25 percent of people who die each year in crashes are killed on curves. About 75 percent of all fatal crashes occur in rural areas, and more than 70 percent are on two-lane secondary highways, many of which are local roads. Three-quarters of curve-related fatal crashes involve single vehicles leaving the roadway, overturning, or hitting trees, utility poles, or other fixed objects. More than 10 percent of curverelated fatal crashes are head-on crashes, the result of one vehicle drifting into the opposing lane when a driver tries to cut the curve or redirect the vehicle after having run onto the shoulder. The two objectives identified to reduce crashes at curves are: reducing the likelihood of a vehicle leaving its lane and either crossing the roadway centerline or leaving the roadway at a horizontal curve; and minimizing the adverse consequences of leaving the roadway at a horizontal curve. Twenty strategies are listed: 1. Provide advance warning of unexpected changes in horizontal alignment. 2. Enhance delineation along the curve. 3. Provide adequate sight distance. 4. Install shoulder rumble strips. 5. Install centerline rumble strips. 6. Prevent edge drop-offs. 7. Provide skid-resistant pavement surfaces. 8. Provide grooved pavement. 9. Provide lighting of the curve. 10. Provide dynamic curve warning system. 11. Widen the roadway. 12. Improve or restore superelevation. 13. Modify horizontal alignment. 14. Install automated anti-icing systems. 15. Prohibit/restrict trucks with very long semitrailers on roads with horizontal curves that cannot accommodate truck off-tracking. 16. Design safer slopes and ditches to prevent rollovers. 17. Remove/relocate objects in hazardous 18. Delineate roadside objects. 19. Add or improve roadside hardware. 20. Improve design and application of barrier and attenuation systems. 46 ITE Journal / December 2008

4 A companion document to this guide provides practical information on where, when and how to apply low-cost safety treatments or design features, with examples. 26 Volumes 14 and 16 of the NCHRP 500 Series and a National Highway Traffic Safety Administration report focus on reducing crashes involving drowsy and distracted drivers, a very serious contributory factor in crashes. 27 The strategies to reduce collision due to driver inattention comprise engineering, education, enforcement, policy and judicial measures. In July 2008, FHWA prepared a Guidance Memorandum on Consideration and Implementation of Proven Safety Countermeasures, of which some are also applicable to roadway departure crashes. 28 Some recent strategies are as follows: Safety edge treatments have been implemented in recent years. The objective of this treatment is to ease the ability of an errant driver to re-enter the travel way safely by eliminating the vertical edge to be traversed by the tires. Research has found that a vertical edge greater than 2.5 inches increases the severity of crashes that occur. This will avoid steep-angle entry, which leads to oversteering and head-on or ROR opposite roadside crashes. The Georgia Department of Transportation and Fremont County, MN, USA, are some of those agencies that have added safety edge treatments as a preventative maintenance procedure in their resurfacing projects. 29 A median cable barrier is a system in which three or four strands of tensioned heavy-duty wire are tightened between the anchors while steel posts keep them at the correct height. These barriers are installed in the median to prevent vehicles from crossing into the opposing lane and are more forgiving at impact than concrete barriers. Proper positioning and installation continue to be tested and improved based on the observations of present installations. North Carolina, South Carolina, Washington, Missouri and Iowa are states where the installation of the cable barriers has become an integral part of system improvement projects. Recent information for other effective treatments for ROR crashes include: transverse rumble strips or in-lane rumble strips; 30 snowplowable permanent raised paved markers; 31 and centerline rumble strips. Conclusions More scientifically-based screening methods are being used by highway agencies. Also very positive is the systemic installation of shoulder and centerline rumble strips, cable barrier, safety edge and rumble strips in other road types. Newer, experimental applications such as 4-inch rumble strips on local narrow roads with no shoulder also hold promise. Roadway departure crashes are over-represented on horizontal curves, and states are attempting to identify locations and gather their characteristics to enable a more effective selection of treatments, in particular avoiding treatments that may lead to more crashes, such as raised pavement markers at curves with tighter radii (more than 3.5 degrees of curvature) and low traffic (less than 5,000 vehicles per day). 32 We know how to reduce roadway departure crashes, and the actions required to ensure data-driven decisions and costeffective implementation are taking place in the United States. 33 First, research is being funded to find out about the relationship among safety and operational and design elements, and these results will be found in an authoritative document by AASHTO and the Transportation Research Board as the Highway Safety Manual. Second, practitioners are provided training (such as ITE Webinars and the National Highway Institute s New Approaches to Highway Safety Analyses course). Third, political guidance is provided to designers and other practitioners by means of the specified challenging state goals to reduce crashes signed off and shown in state SHSPs. The Safe, Accountable, Flexible, Efficient Transportation Equity Act A Legacy for Users has demonstrated the commitment to increasing highway safety. The importance of empowering local agencies with funding, data and knowledge has been recognized. Our continued focus will ensure that roadway departure crashes and their tragic consequences will be reduced. Acknowledgments I thank Mr. Tom Welch and Mr. Tom MacDonald of the Iowa Department of Transportation; Ms. Cathy Satterfield of the FHWA Office of Safety; Mr. Jeff Shaw of the FHWA Resource Center; Mr. Mike Dimaiuta of the FHWA Geometric Design Lab; LENDIS Corp.; and Dr. Alison Smiley of Human Factors North for their cooperation during the writing of this article, and Maya Bahar for her proficient editing. n References 1. Fatality Analysis Reporting System. Accessible via www-fars.nhtsa.dot.gov/. 2. Pomerleau, D. Run-Off-Road Collision Avoidance Using IVHS Countermeasures. Task 6 Interim Report, Contract No. DTNH22-93-C Washington, DC, USA: U.S. Department of Transportation (U.S. DOT), National Highway Traffic Safety Administration (NHTSA), Office of Collision Avoidance Research, September Larsen, L. and P. Kines. Multidisciplinary In-Depth Investigations of Head-On and Left- Turn Road Collisions. Accident Analysis & Prevention, Vol. 34, No. 3 (2002). 4. Neuman, T.R., J.J. Nitzel, N. Antonucci, S. Nevill and W. Stein. A Guide for Reducing Head-on Crashes on Freeways. Guidance for Implementation of American Association of State Highway and Transportation Officials (AASHTO) Transportation Research Board (TRB), Larsen and Kines, Pomerleau, September Larsen and Kines, Ritchie, M.L., W.K. McCoy and W.L. Welde. A Study of the Relation between Forward Velocity and Lateral Acceleration in Curves during Normal Driving. Human Factors, Vol. 10, No. 3 (1968). 9. Good, M.C. and P.F. Sweatman. Driver Strategies on Road Curves. Washington, DC: TRB, Hauer, E. Safety in Geometric Design Standards. Toronto, Ontario, Canada, December Accessible via Task Force on the Development of the Highway Safety Manual. Accessible via highwaysafetymanual.org. 12. SafetyAnalyst. Accessible via safetyanalyst. org. 13. Roadside Design Guide. Washington, DC: AASHTO, ITE Journal / December

5 14. AASHTO Bookstore. Accessible via bookstore.transportation.org/item_details. aspx?id= Bahar, G., M. Masliah, R. Wolff and P. Park. Report No. FHWA-SA U.S. DOT, Federal Highway Administration, September Accessible via Implementing the AASHTO Strategic Highway Safety Plan. Accessible via safety.transportation.org. 17. Neuman, T.R., R. Pfefer, K.L. Slack, K. Lacy and C. Zegeer. A Guide to Addressing Collision with Trees in Hazardous Locations. Guidance for Implementation of AAHSTO Strategic Highway Safety Plan. Washington, DC: 18. Neuman, T.R., R. Pfefer, K.L. Slack, K.K. Hardy, H. McGee, K. Eccles and F. Council. A Guide for Addressing Head-on Collisions. Guidance for Implementation of AASHTO 19. Neuman, T.R., R. Pfefer, K.L. Slack, K.K. Hardy, F. Council, H. McGee and K. Eccles. A Guide for Addressing Run-Off-Road Collisions. Guidance for Implementation of AAHSTO 20. Torbic, D.J., D.W. Harwood, D.K. Gilmore, R. Pfefer, T.R. Neuman, K.L. Slack and K.K. Hardy. A Guide for Reducing Collisions on Horizontal Curves. Guidance for Implementation Washington, DC: TRB, Lacy, K., R. Srinivasan, C.V. Zegeer, R. Pfefer, T. Neuman, K.L. Slack and K.K. Hardy. A Guide for Reducing Collisions Involving Utility Poles. Guidance for Implementation of AASHTO Strategic Highway Safety Plan. Washington, DC: TRB, Knippling, R.R., P. Waller, R.C. Peck, R. Pfefer, T. Neuman, K.L. Slack and K.K. Hardy. A Guide for Reducing Collision Involving Heavy Trucks. Guidance for Implementation of AAHSTO Strategic Highway Safety Plan. Washington, DC: TRB, Stutts, J., R.R. Knippling, R. Pfefer, T. Neuman, K.L. Slack and K.K. Hardy. A Guide for Reducing Crashes Involving Drowsy and Distracted Drivers. Guidance for Implementation Washington, DC: TRB, Goodwin, A., R. Foss, J. Hedlund, J. Sohn, R. Pfefer, T.R. Neuman, K.L. Slack and K.K. Hardy. A Guide for Reducing Alcohol- Related Collisions. Guidance for Implementation Washington, DC: TRB, Neuman, Nitzel, Antonucci, Nevill and Stein, McGee, H.W. and F.R. Hanscom. Low- Cost Treatments for Horizontal Curve Safety. Report No.FHWA-SA Washington, DC: Federal Highway Administration, Accessible via safety.fhwa.dot.gov/roadway_dept/ pubs/sa07002/horizontalcurves.pdf. 27. Hedlund, J.H., B. Harsha and K.R. Hull. Countermeasures that Work: A Highway Safety Countermeasure Guide for State Highway Safety Offices. U.S. DOT, NHTSA, Governors Highway Safety Association, Lindley, J.A. Guidance Memorandum on Consideration and Implementation of Proven Safety Countermeasures. Accessible via safety. fhwa.dot.gov/policy/memo htm. 29. Roadway Departure Safety: The Safety Edge. Accessible via safety.fhwa.dot.gov/roadway_ dept/docs/sa Bahar, G. and M. Parkhill. Synthesis of Practices for the Implementation of Centreline Rumble Strips. Ottawa, Canada: Transportation Association of Canada, July 2005; and Bahar, G., T. Erwin, M. MacKay, A. Smiley and S. Tighe. Best Practice Guidelines for the Design and Application of Transverse Rumble Strips. Ottawa: Transportation Association of Canada, July Bahar, G., C. Mollett, B. Persaud, C. Lyon, A. Smiley, T. Smahel and H. McGee. National Cooperative Highway Research Program (NCHRP) Report 518: Safety Evaluation of Raised Pavement Markers. Washington, DC: TRB, Ibid. 33. Hauer, Additional Resources Bahar, G., M. Masliah, C. Mollett and B. Persaud. NCHRP Report 501: Integrated Safety Management Process. Washington, DC: TRB, Campbell, J.L., C.M. Richard and J. Graham. NCHRP Report 600A: Human Factors Guidelines for Road Systems. Collection A: Chapters 1, 2, 3, 4, 5, 10, 11, 13, 22, 23, 26. Washington, DC: TRB, Council, F.M., D.L. Harkey, D.L. Carter and B. White. Model Minimum Inventory of Roadway Elements. Report FHWA-HRT Federal Highway Administration, Office of Safety Research and Development, Virginia, August Council, F.M., D.W. Harwood, I.B. Potts, D.J. Torbic, J.L. Graham, J.M. Hutton, B. Hilger Delucia, R.C. Peck and T.R. Neuman. Safety Data and Analysis in Developing Emphasis Area Plans. Guidance for Implementation of AASHTO TRB, Harwood, D.W. Research Results Digest 329: Highway Safety Manual Data Needs Guide. Washington, DC, TRB, June Hauer, E. Observational Before-After Studies in Road Safety: Estimating the Effect of Highway and Traffic Engineering Measures on Road Safety, Oxford, England: Pergamon Press, Taylor, H.W. Preventing Roadway Departure. Public Roads (July/August 2005). Accessible via Theeuwes, J. and J. Godthelp. Self-Explaining Roads (IZF 1992 C-8 (in Dutch)). Soesterberg, The Netherlands: TNO Human Factors Research Institute, Geni B. Bahar, P.E., P.Eng., is the president of NAVIGATS Inc., a company focused on highway safety engineering. She has more than 28 years of transportation experience and was named the 2007 Transportation Person of the Year by the Transportation Association of Canada (TAC) and Transport Canada. She has successfully led FHWA, NCHRP, ITE and TAC research projects and has also worked closely with federal, state, provincial and municipal agencies in the development of multi-disciplinary and multiagency strategic highway safety plans and programs. Additionally, she has created and facilitated practitioners training using state-of-the-art knowledge. She is an active member of several Transportation Research Board committees, TAC committees and the ITE Transportation Safety Council. 48 ITE Journal / December 2008