Designing Safer Roads Practices for Resurfacing, Restoration, and Rehabilitation -- Special Report 214 (1987) / Chapter Skim
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SPECIAL REPORT 214 DESIGNING SAT'ER ROADS Practices for Resurfacing, Restoration, and Rehabilitation I I Tlansportation Research Board National Research Council Washington, D.C.
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Tiansportation Research Board Special Report 2f4 mode t highwaytrânsportation subject areas 2l facilities design 5l transportation safety 52 human facton Transporratiør Research Board publications are available by ordering directly from TRB. They may also be obtained on a regular basis through organizational or individual affiliation with TRB; affüiates or library subscribeis are eligible for subsøntial discounts.
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COMMITTEE FOR THE STTJDY OF GEOMETRIC DESIGN STANDARDS FOR HIGHWAY IMPROVEMENTS Peran G Kor-rNow, American Trucking Associations, Alexandria, Virginia Co-Chnirnnn Hnnsrnr H
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Tiansportation Research Board Staff Rossnr E Sxn¡¡æn, Director for Special Projects H¿nnv S
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Preface In response to a provision in the Surface Transportation Assistance Act of 1982, the Secretary of Transportation, acting through the Federal Highway Administration, requested the National Academy of Sciences to study the safety cost-effectiveness of highway geometric design stândards and recommend minimum st¿ndards for resurfacing, restoration, and rehabiliøtion ßRR) projects on existing federal-aid highways, except freeways.
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vl PREFACE the Federal Highway Adminisnation offices in each of the 15 states selected for case studies and conducted telephone interviews with local highway officials representing 16 counties, 20 cities, and 3 metropoliøn planning organizations. Federal, state, and local offlcials provided valuable information on the types of projects funded with federal aid, procedures used to select RRR projects, current design standa¡ds and their use, and the ways in which safety needs a¡e taken into account.
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PREFACE vll accomplish this, the Secretary, acting through the Federal Highway AdminisEation, must either set nationwide RRR standards or approve standards adopted by individual states. In either case, ttre committee's recommendations provide guidance.
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Contents EXECUTIVE SUMMARY 1 Gao* rr:rtrc DFsrcN Sraxn¡.nls ron Rrsrrn¡'¿,crNc, RnsroRATIoNt a¡¡o Rn¡ranrr.rranroN Pno.rÈcrs: BacxcnornD añD IssuEs.
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4 Rrr.arroNsgrps BmwBBx Hrcnwav Cosrs eNn Gnorr-rnrc Drs¡cx ...... : Cost Relationships-Problems and Limiøtions, 110 Typical RRR Project Costs, 113 Added Project Costs for Geometric Improvements, 116 Right-of-Way Requirements, 125 Maintenance Cost Implications, 126 Summary, 129 References, 129 5 Srnrrv Cosr-EFFncrrvENEss op Gpop¡vrnrc DssrcN SraNn¿,nos .,.
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Arrr,Nnrx F Relationship Between Accidents and Speciflc Roadside Featuresr....
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Executive Summary In response to a provision of the Surface Transportation Assistance Act of 1982, the Secretary of Transportation, acting through the Federal Highway Adminisnation, requested the National Academy of Sciences to study the safety cost-effectiveness of geometric design standards and recommend minimum standards for resurfacing, restoration, and rehabilit¿tion (RRR) projects on existing federal-aid highways, except freeways.
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2 DESIGNIT.{G SAFER ROADS Safety organizations, on the other hand, have viewed the federal RRR prograrn as an opportunity to make long-needed safety improvements to older highways at the same time as pavement repairs a¡e made. These organizations have viewed ttre flexible RRR standards proposed by some highway agencies as too lenient and have favored a more rigorous, safety-oriented design process.
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WECWNE SUMMARY improvemenfs at these locations can sometimes be more safety cost-effective than routine cross-section improvements.
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4 orsrcumc sAFER RoADS shifts may decrease RRR project costs; in others they will increase costs. Nationwide, the typical project cost will probably increase slightly but not enough to measurably affect RRR pavement repair and preservation activities.
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ÐGCWNE SUMMARY 5 should øke the necessary steps to implement the recommendations in the ûrst three categories-safety-conscious design process, design practices for key highway features, and other design procedures and assumptions. Taken together, these recommendations comprise a practical national policy on RRR project design that will be more safety-cost effective and comprehensive than an extensive set of rigid minimum standards.
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6 ossrcNl.Ic sAFER RoADs Although many state highway agencies already incorporate one or more of these steps in their design process, most will have to modify their process to include ttrem all. Design Practices for Key Highway Features Designers use minimum RRR geometric design standards to determine whether a particular geometric feature must be upgraded âs paft of a RRR == -- !
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øßCWNE SUMMARY TABLE ES-2 Recommended Minimum Lane and Shoulder Width values for Two-Lane Rural Highways l0 Percent or More Tiuckså Less Than l0 Percent Tiucks Design Year Running Volume Speed,(ADT)
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8 ossrCNS{GSAFERROADS aúe. Nevertheless, highway agencies should evaluate the safety benefits and added costs of curve reconstruction when there is a reasonable possibility that reconstruction will be safety cost-effective.
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UECWNE SUMMARY Bridge Wídth The safety cost-effectiveness of bridge widttr improvements depends on the usable width of the bridge, the width of approach lanes, fraffic volumes, and the length of the bridge. Highway agencies should evaluafe bridge replacement or widening in siruations in which bridge width improv"."nL might bejustified on the basis of safety cost-effectiveness -- åridges less than 100 ft long with usable widths less than the values given in Table ES-3.
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10 DESIGNINGSAFERROADS improvements on RRR projects. These procedures should encourage the following: o Flatten sidesþes of 3:1 or steeper at locations where run-off-road accidents are likely t0 occur (e.g., on the outside of sharp horizontal curves)
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Ð{ECWNE SUMMARY 11 tions and rely heavily on professional judgment and experience. To faciliøte this, st¿te highway agencies should deveþ criæria for identifying int€rsections that warant careful evaluation and checkliss of improvements to be considered.
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12 DESIGNING SAFER ROADS As a result, most highway agencies have not deærmined where geometric improvements to existing highways would have ttre greatest safety payoffs and be the most safety cost-effective. Moreover, when additional right-of-way is needed for a RRR geometric improvement, highway agencies must often delay the project or neglect the improvement.
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Ð(rcWME SAMMARY 13 o special s$ety taskforce.' congress should di¡ect the secretary of Transporøtion to establish a special task force to assess highway safety engineering needs and to esablish research, education, and funding priorities to meet these needs. o Safety compendium; The Federal Highway Administration should develop, distribute, and periodically update a compendium that reports the most probable safety effects of improvements to key highway design featu¡es and identiûes the principal gaps in curent knowledge.
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Geometric Design Standards for Resurfacing, Restoration, and Rehabilitation Projects : Background and Issues INTRODUCTION Since 1976 when the U.S. Congress first authorized federal aid for resurfacing, restoration, and rehabilitation (RRR)
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BACKGROAND AND TSSUES 15 available funds on a small number of projects, leaving unattended many federal-aid highways in need of pavement repair and meeting neither preservation nor safety objectives. Safety organizations, on the other hand, have viewed ttre federal RRR program as an opportunity to make long-needed safety improvements to older highways at the same time as pavement repairs are made.
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16 DESIGNING SAFER ROADS their own RRR standatds subject to federal approval, or to continue to use new construction standards. This action failed to silence the debate over RRR standards as evidenced by the congrcssional mandate for this study, as well as the restated program objectives cont¿ined in the same legislation ".
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BACKGROUND AND ISS¿/AS 17 o Fed¿ral-state relationship; afflrmed the responsibility of state and local governments to construct, own, and maintain highways while committing the federal govemment to share in the financing of highway construction; o Fed¿ral-øid apportionmenr: prescribed distribuûon of federal highway funds through a formula, which initially considered area, population, and rural postal route mileage; o Project cost sharing: on federal-aid projecc, required that federal funds be matched with state (or local) funds initially, with a maximum federal share of 50 percent; .
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18 DESIGNING SAFER ROADS related excise taxes and established additional taxes whose revenues were totally or partly funneled into the Highway Trust Fund. (The rust fund was established as a holding mechanism for tax revenues earmarked for highway purposes.)
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BACKGROUNDIND/SSUES 19 for 77 percent of all traffic fat¿lities (Table 1-1)
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20 DESIGNING SAFER ROADS conditions of the U.S. highway system, including the federal-aid systems, based on a nationwide sample of highway segments.
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BACKGROANDANDTSSUES 2T 23 percent of these (more than 60,000 bridges) are eligible for special federal bridge replacement and rehabilitation funds because of existing deflciencies in design and condition.
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22 DESIGNING SAFERROADS improvements at specific locations, often involving construction or rehabilitation: o Bridge replacement and rehabilitation: Since 1979 this program has funded bridge replacement and rehabiliøtion projects on and off the federalaid system. Before the 1978 Highway Act, bridges on fhe federal-aid system could be partially replaced (e.g., new deck)
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BACKGROUND.A.ND/SSUES 23 Resurfacing, Restoration, and Rehabilitation Under the classiflcation of resurfacing, restoration, and rehabilitation, the federal-aid highway program funds the following types of improvements to existing federal-aid highways: resurfacing, pavement structural and joint repair, minor lane and shoulder widening, minor alterations to vertical gades and horizontal curves, bridge repair, and removal or protection of roadside obstacles. h making RRR work eligible for regular federal aid, Congress made federal funds available for the heavier, more costly t¡Des of maintenance and at fhe same time provided highway agencies the opportunity to use federal funds for incremental geometric and safety improvements short of full reconstruction.
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24 DESIGNINGSAFERROADS RRR projects.z Total spending on RRR projects in 1985 was $5.0 billion, about 10 percent of expenditures for highways by all levels of government combined. Mâny state highway agencies use RRR federal aid as a means of addressing critical pavement preservation and repair needs while making selective improvements to road geometry and roadside features.
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BACKGROUND AND ISSUES 25 Until the late 1920s, state agencies generally adopted ståndards independently of one another, leading to design inconsistencies between adjacent states as well as duplications of effort. To add¡ess these problems, the American Association of State Highway Officials (AASHO)
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26 DFSIGNING SAFERROADS centage of heavy rucks) , and function (local, collector, arûerial, etc.)
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BACKGROUND AND ISSAES 27 topography and the age of highway systems (20)
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28 DESIGNING SAFER ROADS National Transportation Safety Board" Safety organizations generally opposed any regulation that might lead to special standards for RRR projecs and favored the first altemative as least objectionable. Although the FTIWA had granæd a large number of design exceptions under the first alternative, safety organizations believed the process of explicitly considering design exceptions on a project-by-project basis will occasionally result in substantial geometric improvements.
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BACKGROUND AND TSSAES 29 RRR program. A number of questions were raised with respect to the impact of the sønda¡ds on safety.
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30 DESIGNING SAFER ROADS KEY ISSUES Many questions and issues that bear on minimum RRR sønda¡ds were left unresolved during the RRR rulemaking process and relaæd debate. In organizing a study that would respond to the congressional request, the study identifled six key areas of inquiry that add¡ess these issues and, taken together, provide the technical foundâtion necessary for specitc recommendations.
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BACKGROUNDÁilDISSUES 3I studies of accident data, these relationships are not well known, and divergent relationships are suggested by different analyses (28)
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32 DESIGNING SAFER ROADS . What are the cost and safety ûade-offs of making incremental geometric improvements to existing highways?
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2 State and Local Procedures for Selection, Design, and Construction of Highway Improvement Projects State and local (county and municipal) highway agencies are responsible for constructing and maintaining the nation's federal-aid highways.
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36 DESIGNING SAFER ROADS . Develop an understanding of the process in state and local agencies that determines the characteristics of RRR projects.
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STATE AIVD I,OCAL PROCEDURES 37 REVIEW OF RRR PRACTICES: INFORMATION SOURCES To answer the preceding questions, fhe committee first reviewed existing studies and daø that address procedures for developing federal-aid RRR projects or describe resulting projects. Relevant previous studies are principally an FIIWA review of RRR projects in 19 states (l)
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38 DEsrcNrNc sAFER RoADS been updated to reflect conditions in 1986. Other changes summarized in ttre introduction to Ap'pendix B
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STATE AND LOCALPROCEDURES 39 O r g anizational Str uc twe In most state highway agencies, responsibilities are divided between a central office and several disrict offices. The cenEal office is organizeÅ into functionally defined divisions (e.g., design, construction, maintenance)
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FIGURE 2-1 Typical state highway agency organization.
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SIATEAAIDLOCALPROCEDURES 4I changed in 1976 when Congress made RRR eligible for federal aid. Some states responded by using federal aid for projecs conducted under their maintenance programs.
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42 DESIGNING SAFER ROADS In most states, district ftaffrc or maintenance engineers receive reports of high-accident locations from the accident data system and other sources and are expected to investigate the sites and recommend solutions to problems. These investigations lead either to traffic engineering improvements (e.9., signals, signs, pavement marking)
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STATE AIID I.OCAL PROCEDARES 43 federal funds a¡e passed on to local governments. The state agency usually makes funds for federal secondary and urban highways available o local governments by either a formula distribution, a discretionary process, or a combination of the two.
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44 DESTcNINcsAFERRoADS lane)
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STATE Al,lD LOCAL PROCEDURES 45 patching, and overlays on short road segments. These projects have the same general objectives as the heavier resurfacing projects classified in the resurfacing and minor widening category-retarding structural deterioration of the road and improving ride quality-and therefore complement the states' major resurfacing activities.
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lIABLE 2-l State Highway Agency Capital Outlays on Non-Interstate Flderal-Aid Projects, by Type of Improvement and HighwayFunctional Class, 1985 Improvement Type Right-oÊWay New Engineering Construction RÞconstruction lr4ajor Widèning RRR Bridge Sr'ork siafety/Other TotalHighwav $mit- gmr- $-il- s* it.- smit- smir- $imir-¡rrË'wêj Þ rþ S rl- mil- $mil $ il- $ il- fi il- $milFunctional Class lions Vo lipns olo lions olo lions % lions a/o lions o/0 lions % lions %, Rtral iArterials 537 t6.2 549 16.5 sL4 i5.5 2ûS 6.3 813 24.5 5g5 17.6 tt4 3.4 3,320 100.0Çollecors 136 13.6 105 10.5 t32 13..1 30 3.0 294 29.3 264 26.i 41 4.2 1,OOZ 100.0 UrbanArte¡ials 733 zLI 697 20.1 449.
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STATE AI'ID TNCAL PROCEDURES 47 nationwide, federal-aid projects whose main objective is safety improvement are usually funded by the federal categorical safety programs, and projects for which bridge work is the main activity are usually funded by the federal categorical bridge replacement and rehabilitation program. On most federal-aid RRR projects resurfacing for pavement preservation is the principal activit¡ but many of these projects involve improvements to bridges and intersections within ttre project limits, roadside safety improvements, and even reconstruction of short segments to improve alignment or replace failed pavement.
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48 DESIGNING SAFER ROADS were missed because low+ost and relatively simple safety improvements were not considered during design. These missed opportunities principally involved roadside improvements and safety hardware.
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STATE AND LOCAL PROCEDT] RES 49 o Differences in general philosophy among the state highway agencies regarding the pdority and value of safety improvements, reflected in the states' programming decisions and design procedures.
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50 DESIGNING SAFER ROADS extent to which these objectives are actually met in a state's highway program as a whole depends on how they are considered in fhe state's programming decisions. As described in this section, the state case studies revealed that RRR programming decisions a¡e influenced by federal RRR søndards, and an undersfanding of these influences is necessary üo evaluate the effect of RRR standards on safety and preservation.
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STATE AND I.OCAL PROCEDURES 51 decisions of the highway agencies in the case studies. preservation-oriented work, from seal coats to reconstruction, is rapidly becoming ttre dominant component in most states' capital improvement programs.
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52 DESIGNINcSAFERRoADS ries or full staæ funding-to each scheduled project. In this procedure the ståte must ensure that none of its apportioned federal funds are lost because of failure to use them in the required time period.
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STATE AND T,OCAL PROCEDURES 53 projects, comparing existing conditions to RRR standards to identify projects already in or close to compliance. Søæ funds are used for projects for which compliance with federal ståndards would be costly.
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54 DESIGNING SAFER ROADS other states)
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STATE AAID I,OCAL PROCEDURES 55 using it recently, not because of shortages in total capiøl resources but to release state funds needed to undertake urban system reconstuction projects for which federal-aid urban funds were not available. Despiæ these examples, the Eend toward increasing reliance on federal aid for resurfacing and other RRR work appears likely to continue.
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56 DESIGNINGSAFERROADS rail automatically be upgraded in the course of RRR work. The rationale for this arrangement is that priority selection of bridge rail improvements based on accident potential will be substantially more cost-effective ttran the haphazard selection ttrat would result if bridge rail improvements were conducted solely when the bridge happened to fall within tl¡e limits of a resurfacing project.
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STATE AND LOCAL PROCEDURES 57 roadside features, accident history, proposed improvements, anticipated design exceptions, and expecæd project costs. In some states (e.g., New Jersey and souúr Dakoø)
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58 DESIGNINCSAFERROADS FHwA-approved RRR d€3lgn standards FHWA llêld revlew lnlormal FHWA guldance State lleld rovle Fleld revlsw bY deslgne¡s - RevtetsJ o! acclden!
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Ohio Department of Transportalion (nondivided highways) Stato lleld revlew lllinols Department of Transportation AASHTO nèw conslructlon standards FHWA tle¡d revlew; ldentlflcatlon ol nec€ssary roadglde lmprov€ments; lnlorm¿l assossmenl ot posslble d€slgñ êxceptlons Fleld revlew by deslgners Deslgners prepar.
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60 DESIGNINGSAFERROADS geometric and roadside improvements may range from no change to improvements approaching those made on federal-aid projects' F ederal Highway Adninistation Role For federal-aid construction projects, FIIWA normally performs initial reviews of project scope; intermediate reviews of designs; and reviews of frnal plans, specifications, and estimates before authorizing construction. Respon.iUitity ior these reviews is deiegatui io FFi't{A division offices, and division administraúors have ttre latitude to specify format and frequency of review activities.
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STATE AAID LOCALPROCEDURES 6T review the sfatus of all federal-aid projecs. In some cases (e.g., Illinois)
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62 DESIcNING sAFER RoADS shoulder widths and horizontal alignment-values generally less stringent than new construcfion standards (Appendix B, Table B-9)
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STATEAND LOCAL PROCEDURES 63 . Age, design, and right-of-way of existing highways; o Topogaphic and geologic conditions; ¡ Extent and nature of RRR project proposals; o Extent of safety-oriented RRR activities; and o Financial resources of the state.
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& DESICNING SAFER ROADS design exception requests submitted by state highway agencies and approved by FÉIWA. Procedures for evaluating exception requests differ among states, although division administrators must approve formal design exceptions.
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STATE AND LOCALPROCEDT] RES 65 offices have reached informal agreements about when these geomeric design values must be applied rigorously, the roadside improvements that will be required, and the circumstances under which design exceptions will be approved.
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6 DESIcNINc sAFER RoADs sider safety in project selection and programming. This program is not popular wittr some highway agencies because it requires a substantial amount of paperwork in comparison to other progfams and its funding is comparatively small.
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STATE AI,ID I.OCAL PROCEDTJRES 67 ary highways, and incorporated cities and towns are responsible for urban system highways. However, this division of responsibility can valy, particularly in u¡banized areas where county governments may also administer some urban system highways.
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68 DESIGNINGSAFERROADS TABLE2-2 Comparison olExpenditures by Type of Improvement (Excluding New Construction) in Federal-Aid Urban, Secondary, and Primary Programs Federal-Aid Program Category Type of Improvement Secondary Urban(ok)
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STATE AND T.OCAL PROCEDURES 69 coordinating, and approving federal Eansportation investments in urbanized a¡eas with populations greater than 50,000. Local elected officials are responsible for decision making in the metropolitan planning organization (10)
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70 DESIGNINGSAFERROADS local funds to resurface federal-aid highways is a common occurrence. Only in financially pressed rural counties where federal aid is a significant share of total highway funding do stringent design stândards appear to cause difflculties for local offrcials.
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STATEAND LOCALPROCEDURES 7I Design Practices and Standards state high\¡/ay agencies frequently share (or assume) responsibility for design and consfuction oversight on local federal-aid projects.
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72 DESIGNINGSAFERROADS number of local officials in both urban and rural areas indicated that RRR design standards were having negative effects on thei¡ road progfams. In some cases, the stândards precluded needed preservation work, forcing the locality to spend federal aid in less productive \ryays.
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STATE AND I.OCAL PROCEDURES 73 In the last few years, state highway agencies have paid increasing attention to safety on federal-aid RRR projects, and most projects inciude some improvements that enhance safety. Nevertheless, highway agencies still miss opportunities for cost-effecfive safety improvements on Rnn projects.
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74 DESIGNINGSAFERROADS The nature of compleæd RRR projects depends on the process the highway agency follows in project planning, selection, and design. This process ofæn rans o produce a fully safety-conscious design because of a lack of (¿)
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STATEAND LOCALPROCEDURES 75 geometric or roadside conditions and setting st¿tewide priorities for their correction. None of the cæe study states that produce the most safety-conscious RRR designs has comprehensively analyzed the benefrts and costs of this policy.
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4J Relationships Between SafetY and Geometric Design Relationships between safety and highway design features routinely improved on resurfacing, restoration, and rehabilitation S.RR) projects are described in this chapter.
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SAFETY AND GEOMETRIC DESIGN 77 quite limited, sometimes contradictory, and often insuffrcient to establish ûrm and scientifically defensible numerical relationships. Further, in ttrose cases in which relationships can be established with substantial conf,dence, the results are often not known or applied by highway designers.
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78 DESIGNING SAFER ROADS RELATIONSHIPS BETWEEN SAFETY AND KEY ROAD FEATURES Highway features affect safety by o Influencing the ability of the driver to maintain vehicle conFol and identify hazards. Significant features include lane width, alignment, sight distance, superelevation, and pavement surface characteristics; o Influencing the number and types of opportunities that exist for conflicts between vehicles.
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SAFETY AND GEOMETRIC DESIGN 79 . Some factors, such as vehicle performance and crashworthiness, that unde¡lie relationships between safety and road design, change over time so that relationships developed at one time may no longer be representative in later years.
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80 DESIGNING SAFER ROADS ¡ Whether a relationship between safety and the design feanres exists (e.g., is shoulder width related to safety?
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SAFETY AND GEOMETRIC DESIGN 81 o Accident rates decrease with increases in lane and shoulder width; . In terms of accidents eliminated per foot of added width, widening lanes has a bigger payoff than widening shoulders; and¡ Roads with søbilized shoulder surfaces, such as asphalt or portland cement concrete, have lower accident rates than nearly identical roads with unstabilized earth, turf, or gravel shoulden.
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Shquldff wldth (fi)
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SAFEW AND GEOMETRIC DESIGN 83 none existed reduces accidents by 19 percent. If the 3-ft shoulder addition were paved, tlre expected reduction would be somewhat greater-about 22 percent.
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FIGURE 34 Steep sideslope. FIGURE 3-5 Rigid drainage structure.
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SAFETY AND GEOMMRIC DESIGN 85 Past research on the safety of the roadside environment has produced important improvements to roadside hardware, including, for example, the development of barriers ttrat better contain and more safely redirect erant vehicles and sign and luminaire supports that break away on impact, causing little damage to ttre striking vehicle and its occupants. In addition, design stândards occasionally provide for clear recovery areas-borders beginning at the edge of the fravel lanes with Eaversable slopes and free of hazardous obstacles.
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86 DESIGNING SAFER ROADS 024681012141618 CLEAR RECOVERYAREA BEYOND OUTSIDE SHOULDER EDGE (ft) NOTES: Accident relat¡onship covsrs s¡ngl€-veh¡cle, s¡desw¡pe, and oppos¡tedirection accidents on two-lane rural highways.
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SAFETY AND GEOMETRIC DESIGN 87 7,t&.. FIGURE 3-7 Narrow bridge on curve.
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88 DESIGNING SAFER ROÀDS wher€ A wldth oltravel lanes, I = brldg€ wldthl ç = bridse (struaiæ)
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SAFE(Y AND GEOMETRIC DESIGN 89 Horizontal Alignment Accidents ate more likely to occur on horizontal curves than on strâight segments of roadway because of increased demands placed on the driver and the vehicle and because of friction between tires and pavements @igure 3-10)
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90 DESIGNING SAFER ROADS (a) Plan vl€w of slmPle curv€ L where R = radius of curve ln f€€t, I = central angle of curve ln degr€es, D = degree of curve = 5730/R' and L = lêngth of curve ln feet = 100 (l/D)
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SAFETT AND GEOMETRIC DESIGN 9I weaker. For cost-effectiveness analyses of horizontal curve improvements, the study used a numerical relationship based on the FTIWA data (Appendix D)
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92 DEsrcNINc SAFER RoADs . Adequate superelevatio¿.'Horizontal curves on high-speed highways are usually superelevated, or banked, for safety and passenger comfort.
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SAFET! AND GEOMEIRIC DESIGN 93 nated without changes to highway geometry (e.g., by cutting brush or Eees)
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94 DESIGNING SAFER ROADS Profllê of Vertical Crest Curve Slght dlstance LIne of slghl ln, Parabollc curve L wh€tE L = length of curve, g1 = percentgradeofapproachtangent, 92 = percent grads of exlt tangent, a = ls2-s11, hl = heightotdriver'seYe,and hz = helghtofoblect. FIGURE 3-14 Vertical curve geometry, sight distance, and terms.
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SAFETY AND GEOMETRIC DESIGN 95 dix E, increasing ttre design speed from 35 ûo 45 mph by lowering the crest is expected to reduce the frequency of accidents about 15 percent on the 0.6-mi segment containing the curve. Climbing lanes for slow-moving vehicles can improve safety on crest cr¡rves with inadequate sight distance for passing.
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96 DESIGNING SAFER ROADS Also, improvements commonly add¡ess a number of intersection deficiencies simultaneously. Although simple relationships to predict the effects of specif,c intersection improvements are generally unavailable, a substantial body of information exists that designers use in remedying defrciencies at hazardous siæs (see Appendix G for a summary of physical and operational features affecting intersection safety)
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SAFETY AND GEOMETRTC DESIGN 9'] typically increases following resurfacing, likely by an amount less ttran 5 percent.
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98 DESIGNINc sAFER RoADS Deøiled investigations of specifrc accidents have revealed the poæntially deleterious effects ofedge drops on vehicle safety. Although the daø needed to make reliable estimates of ttre frequency with which edge drop problems contribute to highway accidents are not available, some safefy resea¡chers believe that vertical discontinuities, particularly at the edge of a narrow üavel lane, pose a serious haza¡d to driverS who make Otherwise minor encroachments onto the shoulder.
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SAFETY AND GEOMETRIC DESIGN 99 difficulty of successful recovery clearly increases. However, there are no generally accepted standards or guidellnes on the degree of edge drop, as chanctenzeà by height and shape (Figure 3-16)
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100 DESIcNINc sAFER RoADs section. However, Appendix C contains a procedure that can be used to deveþ reasonable estimates of the combined effect of multiple improvements.
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SAFETY AND GEOMETRIC DESIGN 101 FIGURE 3-17 Pavement edge drop, large Eee, and steep sideslope. programming safety improvements for locations where high accident frequencies have been observed.
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From page 114... ...
LOz DESIGNING SAFER ROADS Recognizing the need for caution in the use of published accident-reduction factors, the study committee nonetheless concluded ¡trat the low-cost safety measures described can provide significant reductions in the frequency and severity of accidents. The safety benefits of these measures, coupled with their low costs, are such that the measures can be highly cost-effective on RRR projects.
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From page 115... ...
SAFETY AND GEOMETRIC DESIGN 103 Brøkíng. Possibly increased use of antilock braking systems in new automobiles could ultimately reduce average stopping distånces.
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From page 116... ...
104 DESIcNINc sAFER RoADS t¡affic. Although this trend may well continue into the future, the rate of increase is likely o diminish.
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From page 117... ...
SAFETY AND GEOMEÎRIC DESIGN 105 slow, leading possibly to inappropriate maneuvering and an increase in accident potential. In addition fo inconsistencies that occur at point or spot locations, such as illustrated previously, other situations are found in which clues from the physical environment belie the nature of the roadway hazard.
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From page 118... ...
106 DESIcNINcSAFERRoADS o Pavement edge drops, and ¡ Pavement surface condition. For the first ûve features listed, evidence is sufficient to support quantitative estimates of the reduction in accident rat€s as a result of design improvements on two-lane rural highways.
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From page 122... ...
4 Relationships Between Highway Costs and Geometric Design This chapter contains a discussion of the relationships between cost and key highway design features. More specifically reported are the added costs highway agencies incur on resurfacing, Iestoration, and rehabiliøtion (RRR)
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From page 123... ...
HIGTTWAY COS(S AND GEOMETRTC DESIGN 111 can be important, and neglecting to tâke them into account can result in poor design decisions. For example, placing a guardrail may appear to be an attractive alternative fo flattening sideslopes because the initial project cost is less; however, a different conclusion may be reached when the long-term cost of maintaining the guardrail is taken into account.
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From page 124... ...
tt2 DESIGNING SAFER ROADS mask the effects of economies of scale related to geometric improvements. For example, review of sample RRR projects in \Mashingron S¡ate revealed excavation unit costs 10 to 15 percent less for projects where more than 10,000 yd2 of ea¡th were excavated compared with 3 projects where between 1,000 and 10,000 yd2 were excavated l3)
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From page 125... ...
HIGTIWAY C) STS AND GEoMETRIC DESIGN 113 region.
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From page 126... ...
II4 DESIGNINGSAFERROADS TABLE 4-1 Percent of RRR Costs by Category for Typical Resurfacing and Minor Widening Projects Category Washington Florida Illinois Site preparation and earthwork Drainage Pavement Structures Tiaffic and safety; traffic control Miscellaneous Engineering, mobilization, and other Tota! Shoulder type l0 5 65 I 2 76 9 9 45 ll 2 24 i00Paved 8 l3 T00 Paved 5 6 9 r00Unpaved NorEs: Right,oÊway is excluded.
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From page 127... ...
HIGTIWAY COSTS AI,ID GEOMEÎRIC DESIGN 115 variation results from many of the factors noted previously-including differences in topography, scope of geometric and roadside improvements, and usual pavement and shoulder design practices. Urban system RRR projects are generally the most costly on a per-mile basis, followed by primary syst€m projects, with secondary system projects costing less than the other two.
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From page 128... ...
116 DESIGNINGSAFERROADS projects usually involve little work and cost only about one-tenth as much as federal aid resurfacing and minor widening projects. ADDED PROJECT COSTS FOR GEOMETRIC IMPROVEMENTS The frgures reported in the preceding section illustrate average costs of RRR projects and the variation of costs by staæ and highway system.
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From page 129... ...
HIGTTWAY COSTS AI,ID GEOME-TRIC DESIGN LI1 Resurfacing Only For projects that involve only resurfacing and no geometric improvements, suffrcient daa a¡e generally available ro relare typical project costs within a state to the repaving width. For two-lane rural roads, such relationships are available for the three case study stâtes (24)
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From page 130... ...
118 DESIGNING SAFER ROADS and pavement overlay thicknesses (always greater than % in. and usually in the 1.5- to 3.0-in.
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From page 131... ...
HIGÍMAY COSTS AI,ID GEOMETRIC DESIGN 119 etc.)
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From page 132... ...
TABLE 4-6 Unit Costs for Selected Roadside Obstacle Removal and Protection Strategies (7-10l Unit CostTypeofAction (1985 $) Sourçe Remarks Guardrail Removal 1.65/lincar ft (8)
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From page 133... ...
HIGTIWAY COSTS AND GEOMETRIC DESIGN I21. engineering analyses.
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From page 134... ...
r22 DESIGNING SAFER ROADS Costtf?
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From page 135... ...
HIGTIWAY COSTS AND GEOMETRIC DESIGN I23 qooog 3ooF U' oor- 200(J u¡ oÍ r00 v:¿46910 NEW DEGREE OF CURVATURE ll¡r 70 60 NEW DESIGN SPEED, mph g;*
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From page 136... ...
r24 DESIGNING SAFER ROADS approach and exit grades is 4 percent. The maximum design speed for such a curve is 35 mph (Table 4-7)
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From page 137... ...
HIGHWAY COSIS AI'ID GEOMEÍRIC EESIGN TABLE 4-8 Representative Costs of Intersection Improvements (14) Type olProject Construction Cost (1983 $)
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From page 138... ...
126 DESIGNING SAFER ROADS acquisition. Such a requirement conflicts with RRR project schedules, which are usually geâred toward providing urgent pavement repair within one year.
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From page 139... ...
HIGTTWN COSTS AND GEOMETRTC DESIGN I27 Exístíng Conditíons Requíred Improvemcrús Length = 2.0 mi Lane width = 11 ft Lane width = 12 ft Shoulder width = 3 ft Shoulder width = 4 ft Rolling ærrain Paved shoulders Consfuction costs for hypothetical RRR project ile as follows: Cost andImprovemenÍ Costlft-mi ($) Lane widening 89,200 (22,300)
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From page 140... ...
r28 DESIGNING SAFER ROADS Lanes Shoulders Total State Local Total Existing Surface fft-mì) " 11,681,000 5,146,000 16,827,000 Added Surface Area If Improued Percent(fr-mi)
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From page 141... ...
HIGTTWAT COSTS AND GEOMETRIC DESIGN r29 SIJMMARY Geometric design improvements on RRR projects affect both the initial capital investment required for the project and future maintenance requirements. Although initial project costs tend to be the dominant consideration for highway agencies making decisions about geometric design improvements, maintenance costs can also be important, and neglecting to tâke them into account can result in poor design decisions.
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From page 143... ...
5 Safety Cost-Effectiveness of Geometric Design Standards The safety cost-effectiveness of design standards was analyzed for resurfacing, restoration, and rehabiliøtion @RR) projects on the basis of relationships between safety and geometric design (chapter 3)
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From page 144... ...
I3Z DESIGNINGSAFERROADS EARLIER STI.JDIES OF SAFETY COST.EFFECTIVENESS IN HIGHWAY DESIGN Existing highway design standards-both for new construction and for RRR work-generally are not linked to explicit assessments of cost-effectiveness. Although the standards reflect judgments about the effects of changes in design variables on safet¡ traffic operations, consfuction cost, and other considerations, they afe not the products of formal cost-effectiveness studies in which these effecS are quantified and rade-offs are analyzed.
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From page 145... ...
C O ST.EF F ECTN EN ESS O F DESIGN STAN D ARD S 133 ing a methodology for "tailoring the designs of individual projects rather than developing designs.through rigid application of design standards." Similarly, Graham and Harwood 13) state "the cost-effectiveness of roadside design improvements can vary widely between highway sections based on accident rates, traffic volumes, tenain, required construction quantities, unit construction costs, and right-of-way requirements.
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From page 146... ...
r34 DESIGNING SAFER ROADS . Annualize the added cost, based on an assumed project life and discount rate.
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From page 147... ...
CO ST.EF F ECTN EA¡ ESS O F DESIGN STAN DARD S TABLE 5-l Alternative Estimates of Accident Costs by Severity Cost per Accident ($thousands 1985) , Severity ofAccidents NHTSA NSC "Vy'illingnessApproachó Approach.
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From page 148... ...
t36 DESIGNING SAFER ROADS that the dollar value impuæd to accidents eliminated falls within the $10,000 to $50,000 range. Economic Assumptions For the calculations of cost per accident eliminated presented in this chapter, a discount rate of 7 percent and a project life of 30 years ale assumed.
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From page 149... ...
COST.EFFECTNP¡,IESS OF DESIGN ST}WDARDS 137 Lane and Shoulder lVidths of all the highway geometric features considered in this stud¡ lane and shoulder widths are the most amenable to quantiøtive analysis. Although the caveats presented about uncertainties in accident relationships and costs apply to lane and shoulder widths, more is known about the safety effects and costs of these features.
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From page 150... ...
138 DESIGNING SAFER ROADS " o 1,000 2,000 3,000 4,000 5,000 6,000 7,000 AVERAGE DAILYTRAFFIC NOTES: Exmple assures rolling t€rain, 1o-ft lanes wjth 2-ft shoulders before improverent and 1l-ft lanes w¡th 4-tt shoulders aiter improverent Costs are ¡n 19d5 dollars and wer€ calculated using a dis@unt rate of 7 percenl and a prol'ect l¡fe of 30 Years. FIGURE 5-1 Cost-effectiveness of lane and shoulder widening by ADT.
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From page 151... ...
COST-EFFECTNENESS OF DESIGN STANDARDS 139 6oo &40 o t¡lF Êrof lr¡ Fz ot 2o õ o ffro(\ Fa o o Least Most Hazârdous ROADSIDE HAZARD RATING Hazardous NOTES: Exarìpte assures 2,OOO ADT, rolting tera¡n, 1O_ft lanqs with 2_ft shoutders before improv€rent and 1f _fl lane¡ with 4_ft shoulders afterjmproverent. Costs ar€ in lggs doltars and w€rs calcularea usin!
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From page 152... ...
TABLE5-2IllustrativeCost-EffectivenessoflaneandshoulderWidening, 2,000 ADT Tèrrain Lanes Paved Shoulders Unpaved Shoulders Added Cost pe¡ Mile of Widening (l ft each direction) Flat Rolling Mountainous 31.6 44.6 68.2 14.0 21.2 44.6 4.6 I 1.8 35,2 ¡ -^:r^-+- El:*
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From page 153... ...
CO ST.EF F ECTN EN ESS O F D ES IGN STAN DARD S r4t highway users, as well as improved highway safety. Narrow lanes and shoulders on two-lane rural roads cause motorists to drive closer to vehicles in the opposing lane.
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From page 154... ...
142 DESIGNING SAFER ROADS effect of a given lane and shoulder width improvement on travel time will also vary depending on terrain because curves and grades add to the adverse effects of na¡row lanes and shoulders on vehicle operating speeds. Travel time savings for the first few feet of widening are usually substantially greater than those for further widening (Figure 5-5)
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From page 155... ...
COST.EF FECTMENESS OF DES¡GN STáNDII.RDS In this example, about one-third of ttre travel time savings are offset by greater vehicle operating costs. At greater ADT levels, the net user savings-time savings less added vehicle operating costs -- -can be substantial in relation to the cost of lane and shoulder widening (Figure 5-6)
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From page 156... ...
144 DESIGNING SAFER ROADS Alternative standards examined at the national level include . Application of AASHTO new construction standards to RRR projects; ¡ RRR standards developed by AASHTO in 1977 [Geornetric Design Guide for Resurfacing, Restoration, and Rehabilitation of Highways and Streets (14)
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From page 157... ...
CO ST.EFFECTNENESS OF DESIGN STANDARDS 145 traffic levels are greater, this study assumed for simplicity thât segments not meeting minimum standards would be upgraded to la) minimum standa¡ds if ADT is 2,000 or less and (b)
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From page 158... ...
146 DESIGNINGSAFERROADS gJearer than 4,000 ADT. The breakpoint at 400 ADT was shifted to 750 ADT' and the breakpoint at 4,000 ADT was shifted to 2,000 ADT.
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From page 159... ...
COST.EFFECTMENESS OF DESIGN STANDARDS I47 TABLE 5-5 Cost-Effectiveness of Alternative Lane and Shoulder Width Standards: State-Level Analysis for Florida, Illinois, and Washington Cost per Accidents Accident Lane and Shoulder Cost per Year Eliminated EliminatedState Width Standard ($millions) per year ($thousands)
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From page 160... ...
148 DESIGNING SAFER ROADS Horizontal Curves The cost-effectiveness of reconstructing horizontal curves by decreasing their degtee of curvature was examined at ttre project and system levels using (ø) the safety relationship presented in Chapter 3 and discussed in more detail in Appendix D, (b)
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From page 161... ...
CO ST.EFFECTNENESS OF DESIGN STANDARDS r49 ¡ Trafûc levels; the number of accidents eliminated depends not only on the improvement in the accident rat€, but also on the volume of traffic using the curve. using a hypotfietical curve flattening project, the study examined the effect of design speed before and after improvement, cental angle, and ADT on cost per accident eliminated.
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From page 163... ...
oô @o6 ;Bo l¡¡ k =EsoJ lr.tF z, ¡¡¡o40 oo EHeo F c, oo CENTRALANGLE NOTES: Charactedsìiçs 01 the hypothelicat horÞontal curye on which this æ¡silivlty analys¡9 is based are (a) dsign speed of 30 mph before imlrrovorent,(b)
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From page 164... ...
r52 DESIGNING SAFER ROADS because of the tire side friction required to keep the vehicle on the curve. Second, flaúening a curve provides a small reduction in Eavel disønce.
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From page 165... ...
COST.EFFECTNENESS OF DESIGN ST¿NDINOS I53 TABLE 5-6 Illustrative cost and user Savings for Horizontal curve Flattening Annualized Cost lor Curve Flattening Cost ($) Added construction cost User savings per year I,OOO ADT 3,OOO ADT 5,OOO ADT Net cost (construction cost less user savings)
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From page 166... ...
154 DESIGNING SAFER ROADS 3,000 2,500 2,000 1,500 1,000 s00 0 DESIGN SPEED (mph) NOTES: Des¡gn speeds of ind¡v¡dual curu€s al€ est¡mated from their radi¡ of curuatur€ using the equation pres€nted ¡n AASHTO'S PolÆv o" the Geometric Des¡gn of Highways and Streett, 1984 (l 1, p.
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From page 167... ...
C O ST.EF F ECTNENES S O F DESI GN S? WDARDS 155 project-level analysis showed that cost per accident eliminated for reconstructing curves is very sensitive to the cental angles of the curves.
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From page 168... ...
156 DESIGNING SAFER ROADS 15 mph between the as-built design speed of curves and 85th percentile speeds as breakpoints t0 sepafate situations in which curve flattening is frequently cost-effective from those in which it is not cost effective. Summary of Findings on Horizontal Curves .
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From page 169... ...
COST-ffi FECTNEI'IESS O F DESIGN STAIIDARDS t57 P roj e c t-Lev e I C o st -Effe c tiv e ne s s Important determinants of the cost-effectiveness of improving sight disønce at crest curves are ¡ Difference between the design speed of the curve and the speeds of vehicles as ttrey Eavel through the sight-restricted area, . Degree of hazard in the sight-restricted area, and o Traffic volumes.
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From page 170... ...
158 DESIGNINGSAFERRoADS wittr ADT because the number of accidents eliminated is directly proportional to ADT. længthening vertical crest curves reduces mo¡or vehicle operating costs by , reducing the length of vertical tangents.
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From page 171... ...
C O ST.EF FECTN EN ESS O F DESI GN S? WDARDS 159 o The width of the bridge relative to the widrh of the navel lanes on bridge approaches; o Whether widening is practical or, alternatively, whether it is necessary to demolish and reconstruct the bridge to provide more width; o Whether the bridge requires major rehabiliøtion or reconstruction for other reasons (e.g., inadequate load-bearing capacity)
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From page 172... ...
70øÞoott 60 o uJäso =E =40l¡ll-ñsoÕõ3zo &Hro F 1t8o 1,000 2,000 3,000 4,000 AVERAGE DAILYTRAFFIC NOfES: Example assures a bridge lenglh of 50it, reìatìvsw¡dth before improverent of o, and relative width after ¡mproverent of I fl. Costs âre in 1985 dollats a¡d were calculated using a discount rate of 7 percent and a projêct lile of 3o Years.
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From page 173... ...
COST-EFFECTNENESS OF DESIGN STAI\IDARDS 161 ADDED WTOTH (ft)
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From page 174... ...
162 DFJIGNING SÀFER ROADS Sy stem-Level C os t'Effec rtvene s s The cost-effectiveness of bridge widttr improvements was examined at the system level using data from the FTIWA bridge inventory. Cost per accident e-liminated was calculated under the assumption that widening ttre existing bridge is not practical; therefore, it must be demolished and replaced with a widãr smcture.
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From page 175... ...
COST.EFFECTNENESS OF DESIGN STANDARDS L63 than 6 ft. For bridges greater than 100 ft long with less than 2,000 ADT, the average cost per accident eliminated is more ttran $200,000.
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From page 176... ...
I& DESIGNING SAFER ROADS . Degree of hazard posed by the obstacle, for example, as measured by the probability that a collision with the obstacle will result in an injury or fatality; .
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From page 177... ...
CO ST.EFFECTNENESS OF DESIGN STAI,IDARDS 165 The cost per accident eliminated of removing obstacles increases with distance because there is less chance that errant vehicles will srike obs¿acles farther from the edge of the road (Figure 5-18)
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From page 178... ...
166 DESIGNING SAFER ROADS . The safety cost-effectiveness of removing a roadside obstacle depends on the distance of the obstacle from the roadway edge, the presence of other obstacles nearby, sideslopes on which ttre obstacle are located, and traffic volume.
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From page 179... ...
C O ST-EF FECTN EN ESS O F DES IGN STAN D ARDS TABLE 5- I 1 Cost per Mile Under Alternarive Width Standards 167 Alternative Minimum Lane and Shoulder Width Standard Resurfacing and Minor Widening Cost per Highway Mile (thousands) AASHTO new construction standards AASHTO RRR standards FHWA 1978 proposed standards FHWA proposed standards with modiÍÌcations t74 l3l t44 t45 NorEs: Costs are for all two-lane rural federal-aid highways, as estimated lrom HpMS data.
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From page 180... ...
168 DFSIGNING SAFER ROADS 2,500 2,000 1,500 1,000 500 0 l¡lt5 u.lJ =Fz l¡l ÞL lllo 1999 YEAR IHJ:3;.',,ä""-,'3H¿iiL'i[îå, #ÅXå? :y"ffi""¿';.i'ï"ff iffL".i:["Ii" Washington Stat€ paverent managerent pract¡ces (PSl of approx¡mately 2 5)
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From page 181... ...
1,000 COST.EFFECTNENESS OF DESIGN STANDARDS 169 1999 YEAR G y 800 É u¡ fL fil 600F =5o* t¡J rnFz ä 200 õ o oprion, 3,841 ;iffiålTJi::Í:'S;i:Jlnliååil,:'';,,,T,ffJ.'i?
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From page 182... ...
i70 DESIGNINC SAFER ROADS highways with lane widths of 12 ft tlan do other states, and has somewhat lower shoulder widths than the national average. However, the pavements in Washington State are in better condition than pavements in most states.
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From page 183... ...
CO ST.EF FECTNEI,IESS OF DESTGN STAI,IDARDS 17t be opportunities for cost-effective improvements beyond those called for by standards. On the other hand, there will sometimes be specific improvements called for by standards that are unusually costly (or that have other undesirable consequences)
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From page 184... ...
r72 DESIGNING SAFER ROADS Bridge Width o Bridge width improvements can be cost-effective, particularly when ADT levels are high; the bridge is short (e.g., less than 100 ft) ; and its width is close to or less than that of the travel lanes on bridge approaches.
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From page 185... ...
COST-EFFECTNENESS OF DESIGN STAI\IDARDS I73 Highways. Minnesota Department of Transportation, Report FV/HA/IvIN-79-04.
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From page 186... ...
6 Tort Liability and Geometric Design In recent years highway agency administra¡ors have become increasingly concerned about the growth of ¡ort claims. Such claims allege that highway agencies have committed a legal wrong by improper or negligent highway design, operation, or maintenance that became a cause or partial cause of a highway accident.
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From page 187... ...
TORT T.TABILITY I75 cenEal focus of ort claims. Pavement features, üafûc control devices, and roadside bar¡iers account for the large majority of ¡ort claims in Pennsylvania and Louisiana.
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From page 188... ...
176 DESICNING SAFER ROADS unforeseeable operâting losses. Lending support to this, the U.S.
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From page 189... ...
TORT UABILIT-r T77 developed in-house expertise to litigaæ thek tort claims. The high interest rates in the late 1970s and early 1980s created favorable investment opportunities for insurance companies causing them to lower rates in an attempt to increase volume.
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From page 190... ...
178 DESIGNINGSAFERROADS apportioning fault, opportunities for successful negligence claims against the state have increased. I-awsuits have alleged negligence in virtually every activity of state highway agencies, but maintenance activiúes are more vulnerable to tort suits û)
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From page 191... ...
TORT UABILITT I79 claims alleging a defect in roadway condition (g)
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From page 192... ...
T8O DESIGNINGSAFERROADS TABLE 6-2 Tort Claims Filed in New York and Florida New York (1983- 1985) Florida (1972- 1986)
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From page 193... ...
TOKI UABILITT 181 Susceptibility of RRR Projects and Standards to Tort Ctaims The standards selected for RRR projects, the design process followed, and the scope of the improvements may influence ttre litigation of future tort claims. The issues that might a¡ise in a fort action a¡e o Did the project meet the appropriate design standards?
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From page 194... ...
I8Z DESIGNINGSAFERROADS Deficient roadside signs or pavement markings and pavement edge-drop problems, which are ofæn the bases of tort claims, can be routinely conected on RRR projects. Defense of a RRR Project Design Although planning and design activities are exempt from liability in most states, this immunity has been held not ø apply to decisions made without nrior studv or conscious deliberation.
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From page 195... ...
TORT TIABIUTY 183 stringent RRR standards. A highway agency involved in a tort case under such circumstances should use a defense similar to tle one it offers for a design exception, and, again, costs of construction alone are not a suffrcient criterion.
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From page 196... ...
184 DESICNINGSAFERROADS . RRR projects routinely correct or upgrade fe¿tures such as paYement edge drops, signing, guardrails, and median barriers, often the targets of tort claims.
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From page 198... ...
7 Findings and Recommended Design Practices for Resurfacitg, Restoration, and Rehabilitation Projects Summarized in this final chapter are findings on tlle selection and design of resurfacing, restoration, and rehabiliøtion (RRR) projects; the cost and safety trade-offs involved in improving geometric features on these projects; and the influence of design standards.
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From page 199... ...
FINDINGS AND RECOMMENDATIONS 187 whereas the effect on safety appears less certain. Inadequate information about the safety payoff of improvements to existing highways underlies much of fhe confusion and difference of opinion about the appropriate level of safety-motivated improvements on RRR projects.
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From page 200... ...
188 DESIGNING SAFER ROADS o Engineers who administer state traffic and safety programs seldom partícipate in tlæ design of RRR proiects. They are usually the agency staff members most knowledgeable about accident daø and special safety measures, but they have other duties and assignmenfs.
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From page 201... ...
FTNDINGS AI,I D RECO M MEN DATI oNS TABLE 7-l Organization of Study Recommendations Safet y Cons cious D es ign Proces s r89 [. Assessment of Site Conditions Affectine Safetv2.
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From page 202... ...
190 DESIGNINGSAFERROADS 2. Design practices for key highway fe¿tu¡es-recommendations that specify the existing highway conditions that wdnant a geomeEic improvement ouright or a serious evaluation of a geometric improvement, as well as design practices that should be followed routinely for key features.
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From page 203... ...
FINDINGS AAID RECOMMENDATIONS 191 Assess Current Conditions Recommendation l: At the begínning of RRR project design, highway designers should assess existing physical and operatíonal conditions fficting safery: o Analyze accident and tavel dats tn identify specific safety problems that might te corrected and to determine if the site has been unduly hazardous compared with the systemwide performance of similar highways. o conduct a tlørough site inspection using penonnel trained to identify features that pose safety hazards under common operating conditions and recognize opportunities for safety improvements.
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From page 204... ...
I92 DESIGNINGSAFERROADS These improvements can provide significant reductions in the frequency and severity of accidents. The safety benefrts of these improvements, coupled wittr ttreir low costs, are such ttnt they can be higtily cost-effective on RRR projects.
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From page 205... ...
F I N DTNGS A1'I D REC O M M EN DATT ON S t93 Review the Design Recommendation 4: Trffic and safety engineers should rourinely review safery and design reports, as well as proposed RRR designs before final approval. Direct participation by traffrc and safety engineers in the design of RRR projects contributes to a more safety-conscious design process.
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From page 206... ...
194 DFSIGNINC SAFER ROÁ,DS Lane and shoulder widths on two-lane rural highways meet these conditions, and minimum values ¿¡re recommended. Cross-section features a¡e particularly importrnt because they can affect highway safety and cost over the length of a highway.
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From page 207... ...
FINDINGS AIVD RECOMMENDATIONS 195 The FFIWA and søæ highway agencies can use fhese recommended minimum lane and shoulder width values ûo set minimum RRR design standa¡ds. These recommended values are similar to the minimum lane ánd shoulder width values proposed by the FIIWA in 1978 but include several modificarions to improve safety cost€ffectiveness: o The average daily üaffic (ADÐ ranges are adjusted so that a larger number of roads with high ADT and fewer roads with low ADT would be improved.
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From page 208... ...
196 DESIGNING SAFER ROADS their incremenøl effects are available. Distinctions based on vehicle speeds and fuck percentages are common in the RRR standards currently in use in many states.
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From page 209... ...
FINDINGS AN D RECOMMENDATTONS r91 improvements. Review of current state RRR practices revealed that lane and shoulder widening is relatively routine but alignment improvements are uncommon.
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From page 210... ...
198 DESIGNING SAFER ROADS Vertical Curvature and Stopping Sight Distance Recommendation 8: Highway agencies should evaluste the reconstruction of hill crests when (a) the hi| crest hides from view rnajor hazards strch as intersections, sharp horízontal curves, or norrow bridges; (b)
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From page 211... ...
FIN DING S AN D RECO MMENDATION S Design Year Volume (ADT) Usable Bridge Width (Í)
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From page 212... ...
200 DEsIcNINcSAFERRoADS . Cost of replacing the existing bridge with a wider bridge designed to AASHTO standa¡ds for new bridges, o Cost of widening the existing bridge (if widening is practical)
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From page 213... ...
FINDINãSAI,IDRECOMMENDATIONS 201 between different roadside features that influence the safety benefits of a particular improvement. Instead of proposing nationwide standards, Recommendation l0 requires highway agencies fo deveþ and apply fheir own procedures for identifying and selecting sideslope and clear zone width improvements on RRR projects.
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From page 214... ...
202 DESIGNINGSAFERROADS c Selectively pave shoulders at points where out-of-lane vehicle encroachments and pavement edge-drop problems are lilæly to develop (e.g., at hori' zontal curves)
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From page 215... ...
FINDINGS AND RECOMMENDATIONS 203 intersections. Nevertheless, accidents tend to be concentrated at intersections-more than one-half of all accidents in urban a¡eas and about one-third in ru¡al areas occur at intersections 13)
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From page 216... ...
2M DESIGNINGSAFERRoADS the roadside -- be restored to generally match new construction requirements. Resurfacing projec6 provide highway agencies the oppornrnity Ûo correct deûcient cross slopes at little or no additional cosL Although the safety effects have not been measured, restoring crOSs slOpes to matgh neril construction standards is a good practice that highway agencies should routinely follow when resurfacing.
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From page 217... ...
FINDINGS AI,I D RECO MMENDATI ONS 205 Speed Recommenduion 15: When evalwting geometric improvements where vehicle speed is a key factor, highway agencies should estimøte running speeds in a manner appropriate for the feature under consideration. Review of highway agencies' practices revealed that most agencies select a single "design" speed for a RRR project based on highway type, terrain, or the posted speed limirs.
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From page 218... ...
206 DESIGNING SAFER ROADS be made, designers must choose the specific design values to be used, which can range from minimum RRR standards to new construction søndards. Many highway agencies choose design values based on new constnrction standards, reasoning that once an improvement has to be made it is sensible to use new construction søndards in order to reduce the need for future improvements.
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From page 219... ...
FI N DINGS AI{ D REC O M MENDATI ON S 207 PLANMNG AND PROGRAMMING RRR PROJECTS Highway agencies select RRR projecs primæily on the basis of pavement repair needs and seldom consider safety needs until preliminary design begins. Given curent budget levels and existing highway conditions, pavement repair needs will continue to be the dominant factor in the selection and scheduling of RRR projects.
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From page 220... ...
208 DESIGNING SAFER ROADS capacity and pavement preservation issues. Thus, most highway agencies have not deærmined where geometric improvements to existing highways would have the greatest safery payoffs and where such improvements would be the most cost-effective.
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From page 221... ...
FINDINGS AI,IDRECOMMENDATIONS 209 about the safety effects ofdesign opportunities and choices, designers with the training and methods fo apply this knowledge, and, finall¡ enough resources devoted to design to permit a thorough design process on each project. Despiæ more than one-half century of modern road building, knowledge of the safety consequences of highway design decisions is limiæd.
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From page 222... ...
2r0 DESIGNING SAFER ROADS The primary objective of the recommended compendium is to provide designers with the best available safety data and simple application methodologies. Its contents might include the following: o Background information on lhe use of accident data and accident models to estimate the safety effects of highway design improvements; ¡ Easy-to-apply procedures for estimating the safety effects of improvements to specific design features, including a description of the information needed to apply the procedures; and o How to use thc e"stimates of safery effects.
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From page 223... ...
FINDINGSAI'IDRECOMMENDATIONS 2TT been usefully applied in subsequent design work. However, as a rule, such efforts lack the statistical controls necessary to develop relationships that can be reliably transferred to other locations or generalized for nationwide application.
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From page 224... ...
2I2 DESIGNINGSAFERROADS include reviews of completed RRR projects or conferences that provide forums for designers to sha¡e experiences. Both types of activities can be effective, and can offer highway agencies a range of options to frt time and budget constraints.
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From page 225... ...
Appendix A Summary Comparison of Nonfreeway Geomeffic Design Standards and Guidelines Table A-1 contains a description of three sets of geometric design søndards and guidelines for nonfreeway highways. The AASHTO RRR guidelines are from the American Association of State Highway and Transportation Offrcials' Geometric Design Guide for Resurfacing, Restoration, and Rehabilitation (RRR)
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From page 226... ...
TABLE A-1 Summary Comparison of Nonfreeway Geometric Design Standards and Guidelines 1978 FHWA RRR AASHTO RRR Guidelines Proposed Standards AASHTO, Policy for New Construction Tiaffic data ADT DHV percent trucks, and ADT DHY percent trucks, accident ADI DHY directional distribution,(current) turning movements at locations, and descriptions, traffic composition (percent trucks)
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From page 227... ...
TABLE A-1 continued 1978 FHWA RRR AASHTO RRR Guidelines Proposed Standards AASHTO Policy forNew Construction Superelevations Use new construction rates unless Rates for new construction apply. Rarøl function ofDS, terrain, climate; constraints do not permit.
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From page 228... ...
TABLE A-1 continued 1978 FHWA RRR AASHTO RRR Guidelines Proposed Standards AASHTO Policy forNew Constn¡ction Rural multilane Minimum lane and shoulder widths DS < 50 and trueks l0 ft (2 ft) < l0percent DS > 50 or üucks 10 ft (2 ft)
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From page 229... ...
TABLE A-1 continued AASHTO RRR Guidelines 1978 FHWA RRR Proposed Standards AASHTO Policy for New Construction Horizontal curvature, maximum grade, and minimum stopping sight distance Bridges Minimum width (existing bridges) Improvement should be considered at high-accident iocations.
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From page 230... ...
TABLE A-1 continued AASHTO RRR Guidelines 1978 FHWA RRR Proposed Standards AASHTO Policy for New Co:nstruction Clear zone Rural Urban Salety appurtenances From edge ofpavement, 30 ft desirable, but there must be many exceptions. Emphasis on removing frxed objects identiñed as hazardous by accident analyses.
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From page 231... ...
2r9 REF'ERENCES t. Geatnetríc Desígn Guídcs fot Resurfacing, Restoratíon, a¡ú Rehabitítation (RRR)
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From page 232... ...
Appendix B Case Study State and Local RRR Programs The øbles in this appendix contain a description of resurfacing, restoration, and rehabilitation (RRR) ûnance and expenditures; programming methods; and design standards and practices in the ståte highway agencies and local governments chosen as case studies.
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From page 233... ...
TABLE B-1 Characteristjcs of the Case Study States Federal-Aid System Program Administration Special CertificationRRR AcceptanceStandards forNon-Interstate Approved Primary DesignState 1984 Mileage (thousands) Percent State Àdministered Fiscal'{ear 1984 Federal-Aid Apportionment (rnillions)
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From page 234... ...
TABLE B-2 Percent of Federal-Aid Highway Miles Under State Administration, by Federal-Aid Systern, Case Study States Federal-Aid Highway System Interstate and Primary Secondary ïbtal FederalUrban Aid System Arizona California Florida Illinois t f:^L:-^-Mtu¡uË,alt Mississippi Missouri New Hampshire New Jersey New York Ohio South Dakota Tèxas Virginia Washington r00 98 94 99 100 r00 98 97 95 r00 99 100 100 r00 38 8 9 t4 1aLL 30 100 100 5 66 73 t2 99 99 l5 6 5 26 26 aJ L4 29 38 10 l6 2Q 4 4L 53 J 62 37 55 49 JU 5t 94 88 26 60 67 50 93 93 42 United States 63234998 Souncn: Highway Statistics /984 FHWA, Tiable HM-14.
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From page 235... ...
TABLE B-3 Distribution of Non-Interstate Rderal-Aid Project Expenditures by Project Category in Case Study States Percent olTotal Non-Interstate Federal-Aid Expenditures State Reconstruction Resurfacing and Bridge Safety Im- Intersection and New construction Minor widening work provements Improvements other Commentso Arizona California Flo¡ida 58.0 37.'7 71.8 38.0 32.2 13.2 29.0 33.9 I 1.3 13.0 8.9 4.0 7.0l 1.8 14.9 4.2 0.6 FY 1984 programmed; federal-aid primary and federal-aid secondary only. FY 1985 programmed; excludes local federal-aid secondary and federalaid urban; based on projects more than $250000.
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From page 237... ...
TABLE B-4 Distribution of State DOT Expenditures for Fully State-Funded Projects, Selected Case Study States State Reconstruction and Resurfacing and Seal Coats and New Const¡uction Minor Widening Thin Overlays(7r)
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From page 238... ...
TABLE B-5 RRR Project Programming Procedures in the Case Study States State Programming Procedures Arizona Central office selects projects using its pavement management system, an automated p¡ocedure for determining the least-cost schedule ol pavement repairs that will maintain a specifred systemwide minimum performance standard. Inputs to the process include annual measurements ofpavement deflection, cracking, and roughness.
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From page 239... ...
TABLE B-5 continued State Programming Procedures Missouri Central office assigns a mileage allocation to each district. Districts select their "worst miles" of pavement up to their allocations.
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From page 240... ...
228 TABLE B-5 continued State Programming Procedures South Dakota Tèxas Virginia Washington Central office selects all RRR projects through an annual process that updates the state's 5-year construction program' Project priorities are initialty established on the basis ofpavement design and condition (from visual surveys, roughness measurements, and deflection measurements) , drainage adequacy, and traffic characteristics and adjusted after freld reviews.Projects are scheduled in order ofpriority to match allocations ofavailable funds among different functional classes and between resurlacing and other types ofconst¡uction work.
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From page 241... ...
TABLE 8-6 Comparison of Federal and State Funding for Resurfacing, Case Study States Federal-Aid Projects' Share of Total State Highway Agency Non-Interstate Resurfacing Federal-Aid Projects as Pe¡cent of Expenditureso Federal-Aid Projects as Percent of Miles Resurfaced Percent ofStateMaintained NonInterstate FederalAid Mileage Resurfaced in Fiscal Yea¡ Federal-Aid Projects Percent ofTotal StateMaintained NonInterstate Mileage Resurfaced in Fiscal Year, State and Federal FundingState Fiscal Year Arizona California¿' Florida Illinois Michigan Mississippi Missourió.' New Hampshireô. New Jerseyå New York Ohio South Dakotaá lbxasá,.
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From page 242... ...
TABLE Bt Funding for Special Safety Improvement Projects, Case Study States Ariz. Califl Fla.
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From page 243... ...
TABLE B-8 Desþn Practices for Federal-Aid RRR projecrs, case study srates Anz. calif.
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From page 244... ...
TABLE B-9 Minimum Geometric Requirements for Rural Highways in Case Study States V/ith Special RRR Standards Condition California Ftorida Illinois Michigan Highway Design iSpèed(mph) i.qO Notspecifled ARS:4Omph whereposted 35mphposted ì sPeed : 40 mPh i I : 45 Not speciñed ARS : 45 mph Where posted 40 mph posted sPeed : 45 mPh 50 Not specified ARS : 50 mph Whe¡e posted 45 mph posted speed : 50 mPh Lane width (It)
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From page 245... ...
Mississippi New Jersey¿ South Dakotaá 'Ièxas Washington ARS: 36 mph ARS: 4I mph Resurfacing and Restoration Posted speed = 40 mph Rehahilitation Posted speed : 35 mph Resurfacing and Restoration Posted speed : 45 mph Rehabilitqtion Posted speed : 40 mph Resurfacing and Restoraliotl Posted speed : 50 mph Rehabilitation Posted speed : 45 mph Not speciñed Not specified Not specified Undivided multilane highways in rolling terrain Twolane highways except in flat terrain with ADT = 1,500 Undivided multilane highways in rolling terrain Two-lane highways except in flat terrain with ADT = 1,500 All highways Not speciñed (should be logical with respect to terrain and type of highway) Not speciñed (should be logical with respect to terrain and type of highway)
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From page 246... ...
TABLEB-9 continued Condition California Michigan Shoulder Iridth (ft) 6- All highways Althighways.
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From page 247... ...
Mississippi New Jerseya Souttt All highways All highways for resurfacing and restoration projects All highways All highways All highways DS : 50 mph DS : 35 mph and not a highaccident locationd DS : 55 mph DS = 40 mph and not a highaccident locationd DS : 60 mph DS : 45 mph and not a highaccident locationd DS = 65 mph DS = 50 mph and not a highaccident locationd Not specified (adequate for DS or signs should be provided) Not speciñed (adequate for DS or signs should be provided)
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From page 248... ...
TABLE B-9 continued Condition California Florida Illinois Michigan Bridge Clear width(fi) 20 Not permitted Not Permitted Not permitted Not Permitted Two-Lane Highwavs ADT = 250 Two-Lane Highways ADT : 400 DS : 50, ADT : 1,000 Two-Lane Highways ADT: 1000 DS=a0 ADT : 3,000 Minor Rehabílitation Where approach : t6 ft Major ñ ^I^ ^L: l:.
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From page 249... ...
Mississippi New Jerseya South Dakotaå Tèxas Washington Mi:nar Rehabilitation Permitted only ifno accident problern exists and roadway is taÞered before approach Major Rehabílitation Not permitted Minor Rehqbilitation Where approach : 22 rt Mqio, .Rehabilitation ADT: 750or DHV : 200 Minor R¿hsþilÌtøíon Where app¡oach = z4 ft Major Rehabllítation DHV: 400 Not permitted Noi permitted All Colleetors DHv = 200 Not permitted AII Collectors DHV:400 All highways TWo-Lane Highways ADT : 400 Iwo-La:ne Highways ADT : 400 Two-Lane Highways ADT: 750 ADT: 250 (exceÞt not permitted when rehabilitation is done on bridge) ADT - 1000 (except not permitted when ¡ehabilitation is done on bridge)
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From page 250... ...
TABLE B-10 Comparison of Roadside Tieatment Requirements in Special RRR Standards for Rural Highways, Case Study States IllinoisCondition California Florida Michigan Clear zone Not sPecifred (from edge of (removal of lane) obstacles should be considered)
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From page 251... ...
Mississippi New Jersey South Dakota¿ Tèxas Washington ARS> 40 Same as AASHTO Principal and Rural Multilane Not specified, Headwalls: 4 ft new construction Minot Atterials Highways but must be from standardsb Minor reconstruction: 16 ft considered in shoulder 25 ft ifADT RuralTwo-Lane a roadside Other: 14 ft < 1,000, 30 ft Highways hazard review If moved: 30 ft otherwise 7 ft, ADT < 750 report ARS < = 40 Resurfacing: 20 ft 16 ft, ADT > ADT> 750 Collectors : 750 Headwalls: 2 ft Minor reconstruction: from 20 ft if ADT < 500 shoulder 25 ft otherwise Other: l0 ft Resurfacing: 10 ft If moved: 20 ft (ADT < 250)
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From page 252... ...
240 TABLE B-10 continued Condilion California Florida Illinois Michigan Atherroadside None Spgcimentreesand Renoveorupgrade Tlee¡emovalifprovisians unique histori- guardrail frequent accident cal/environmen- Signorlight ortargetposition talfeatules,ifa supportswithin ofhorizont4l hazard, do not clear zoRe should curve harotobe bebreakaway Obstructingsight removed if - Remow 4 in. o¡ distance at inteÊprotected greaterdiameter section troes or protect \úslurlteer trees in Remove4 in.
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From page 253... ...
Mississippi NewJersey South Dakotaø Texas Washington Specimen trees and unique hisforicaV environl1tental features, ifa hazard, do not have to be removed if protected Upgrade safety appurtenanceS (desirable) All safety appurtenances should conform to state gEidelines Roadside obstacles eliminated or shielded by longitu" dinal bariers Upgrade or rcmovÊ guardrail Evaluate existing barriers and end trôatme!
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From page 254... ...
TABLE B-11 Federal-Aid Highway Projects, Case Study Cities City Population Federal-Aid TypesofProjects Category Perlormed With Received FederalAid Columbus, Ohio San Antoniq TÞxas Taiiahassee, Fioricia O¡lando, Florida Phoenix, Arizona Cherry Hill, New Jersey Tioy, New York Kansas City, Missouri Columbia, Missouri Moline, Illinois 62,000 46,000 EAU All types, including resurlacing, reconstruction, intersections, and bridges Resurfacing, reconstruction, and intersections r r^: - l,, -^-., -f^^: -^ - - Ilv!
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From page 255... ...
TABLE B-12 continued County Federal-Aid Category Population Received Types ol Proj ects Perlormed With Federal Aid Schenectady County, New York St. Louis County, Missouri Union County, South Dakota Ingham County, Michigan Los Angeles County, Calilornia Shasta Count¡ California FAU, FAS EAU EAS, FAU, HES, RR Crossing FAU FAS, EAU 60,000 (unincorporated)
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From page 256... ...
TABLE B-14 Allocation of Rderal Aid to Local Governments, Case Study States State Highway Agency Practices Ariz. Calit Fla.
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From page 257... ...
244 TABLE B-ll continued City Population Federal-Aid TypesofProjects Category Perlormed With Received FederalAid Various cities in Mississippi 10,000120,000 EAU Tiaditionally intersections, but shifting toward RRR; lavored because olsmall annual allotment of FAU funds. (In Mississippi it :- ^^ -- ^- -- ^^+i^^ Ê^-ls uuu¡r¡ru¡l pl 4vlruç lvr one consulting engineer to manage under contract several citieJ or counties' road programs.
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From page 258... ...
TABLEB-ll continued City Population Federal-Aid TypesofProjectsCategory Performed WithReceived FederalAid Decatur, Illinois Rapid City, South Dakota Sioux Falls, South Dakota Lansing, Michigan Detroit, Michigan Madison, Wisconsin Knoxville, Tennessee 47,000 EAU EAU, FA Bridge FAU EAU, HES, EA Bridge FAU, HES, FA Bridge FAU, HES FAU, FA Bridge 94,000 Exclusively reconsûuction (favors large projects to reduce number of federal-aid projects and associated federal grant procedures) Almost exclusively reconstruction (one overlay project in last 5 years)
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From page 259... ...
TABLE B-15 Design Standards Used for Study States Local Federal-Aid RRR Projects, Case Has State Developed Special Federal-Aid RRR Standards? Urban System Secondary System Standards Applied to Local Federal RRR Projects State Arizona California Florida Illinois Michigan Mississippi Missouri New Hampshire No NewJersey Yes New York No Ohio No South Dakota Yes Tèxas Virginia Washington AASHTO new construction State's RRR standardso State's RRR standards Either state's RRR standards or AASHTO new construction Special local RRR standardsó State's RRR standards.
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From page 260... ...
Appendix C Summary of Detailed Safety Relationships As a par[ of this study, detailed relationships were developed that describe the tikely effects of the following design features on highway accidents: (a) lane and shoulder conditions, (å)
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From page 261... ...
249 The safety effect of lane and shoulder width and shoulder type can be estimated as follows (1)
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From page 262... ...
250 The rate of bridge-related accidenß on two-lane highways can be estimated as follows (2)
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From page 263... ...
251 The accuracy of Equation 3 may be diminished for curves sharper than about 15 degrees, the approximate limit recorded in the data base from which the model was calibrated. simila¡ to the most likely relationship for accidents at narrow bridges @quation 2)
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From page 264... ...
252 ttre severity of the sight restriction and the nature of the hidden hazard. Procedures for estimating tr" and for selecting Fo, are detailed in Appendix E
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From page 265... ...
253 Roadside Recovery Distance (Jt) Percert Reduction In Acciderts 13 25 35 44 5 10 15 20 Roadside encroachment models arc often used to examine the safety effects of speciflc roadside features.
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From page 266... ...
254 reduction factors to historical accident data collecæd at the location being improved. Should actual accident data be unavailable, however, the analyst must use models, calibrated from other accident-data sources, that directly estimate either accidents or acciden[ ntes.
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From page 267... ...
255 among the combined improvements (ó)
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From page 268... ...
Appendix D Relationship Between Accidents and Horizontal Curvature Although a number of resea¡chers have suggested a relationship between accident rates at horizontal curves and the radius or degree of cuwature, the validiry of these relationships is often unknown because of questionable experimental design and the imprecise definition of both curve-related accidents and vehicle exposure (/)
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From page 269... ...
257 at each end of a horizontal curve. Care was taken to select sites with uniform lane and shoulder conditions and O avoid influences of major bridges and intersectionS, curbs, and Other nearby hOrizontal curves.
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From page 270... ...
258 TABLE D-2 Average Accident Rate (PMVM) on Nominally 0.61-mi Site as a Function of Degree of Curvature Degree of Curve Average Average Accident Rate ADT(PMVM)
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From page 271... ...
259 TABLE D-4 Average Accident Rate (PMVM) on Nominally 0.61-mi Site as a Function of Degree of Curvature and Volume Volume (VPD)
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From page 273... ...
TABLED-6 Average Accident Rate (PMVM)
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From page 274... ...
TABLE D-5 Average Accident Rate (PMVM) on Nominally 061-mi Site as a Function of Degree and Length of Curvature Length (mi)
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From page 275... ...
263 noted earlier, this relationship should not be applied to speciûc project design situations without careful consideration of a variety of site-specific factors. When it is used, the coeffrcient, 0.902, should be replaced where possible by an accident rate on straight segments representative of local conditions for the highway under consideration.
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From page 276... ...
2& TABLE D-7 Percentage Reduction in Accidents Due to Horizontal Curve Flattening Original Degree of Curve New Degree ofCurve Central Angle (deg.)
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From page 277... ...
Appendix E Relationship Between Accidents and Sight Distance at Crest Vertical Curves Neuman and Glennon have developed a hypothetical model for estimating the effects of restricted sight distances at crest vertical curves on accident rates û,2)
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From page 278... ...
266 For a highway segment containing an isolated vertical curve, the accident model can be expressed as N = AR¡ (L)
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From page 279... ...
267 TABLE El Constants Used for Determining Length of Restricted Sight Distance (L,) by Equation 2 Highway Operating Speed on Vertical Curve (mph)
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From page 280... ...
Severity ofSight Restriction (mph) 268 TABLE E-3 Accident Rate Factors (F,l Degree of Hazard in Sight-Restricted Area' Minor Significant 0 (0.3)
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From page 281... ...
269 not applicable when the actual operating speed is less than the design speed for either the unimproved or improved condition. In such a circumstance, further improvements to the design speed are not expected to result in substantial added benefits to safety.
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From page 282... ...
Appendix F Relationship Between Accidents and Specific Roadside Features Described in this appendix are ttre development and calibration of a roadside encroachment model, one of two types of models used to quantify the safety effects of the highway roadside envi¡onment. The second type, regression modeling, was used in work by Zegeer et al.
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From page 283... ...
27r . Hazardous object is sufficiently close to the travel lanes that conEol is not regained before encounter or collision between vehicle and object, and .
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From page 284... ...
272 validated and it was concluded that, given the inappropriateness of the encroachment data, no available model could be recommended for use in analyzing the safety effecß of roadside haza¡ds on two-lane highways. As a result, an independent calibration was undertaken based primarily on accident rather than encroachment data.
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From page 285... ...
Pr(E¡p) = l0F6-0 The denominafor of Eçation 5 is changed fo 5,280 when Ex(E)
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From page 286... ...
274 function only of the angle of departure, ttre dimensions of the object, and the width of the colliding vehicle. For this investigation, each utility pole was assumed to have a square cross section with 8-in.
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From page 287... ...
275 available (fable F-1, accidents per collision)
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From page 288... ...
TABLE F-l Most Likely Consequences of Encounters with Roadside Hazards Type of Hazard Accidents per Collision CasualtyAccidents per Accident /8/Zegeer and Parker Extrapolated /4,/ 0.90 0.45 0.50 0.50 0.50 0.30 0.20 0.40 0.20 0.35 0.35 0.45 0.30 0.35 0.rs 0.15 4,25 0.35 0.45 0.60 0.
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From page 289... ...
277 TABLE F-2 Length Of and Offset To Utility Pole Near-Side Encroachments Far-Side Encroachments Zone (Figure F-1) Segment No.
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From page 290... ...
278 dividing each mean accident frequency in Table F-3 by the corresponding pole density, replaced the expected number, Ex(A) , in Equation 12.
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From page 291... ...
TABLE F-4 Calibration of Roadside Encroachment Model Predicted Mean Lateral Accident Frequencyó Distanceo (Utility Pole Accidents(fÐ per Mile per Year) Correlation Coefrcient ln(Z)
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From page 292... ...
280 TABLE F-5 Summary of Calibrated Models Laæral Travel Distribution Model Excursion Rate (Equation 4) Lateral Travel Distribution (Equations 7-9)
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From page 293... ...
28t É UJ e.a fL l¡lJ5 u¡ fL antu¿ =To ocÉroz t¡J 0 LATERAL DISTANCE (fI) FIGURE F-2 Comparison of lateral travel disribution models on the basis of the frequency of roadside encroachments.
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From page 294... ...
282 LATERAL DISTANCE (fI) FIGURE F-3 Comparison of lateral travel dist¡ibution models on the basis of the rate of roadside encroachments.
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From page 295... ...
TABLE Fl Comparison of Predicted and Actual Roadside Accidents All Accident Severities Casualty Accidents Clear Zone Policyo Predicted ROR Actual SVRORAccident AccidentperMVM perMVM Predicted ROR Actual SVRORAccident AccidentperMVM perMVM Nonclear zone l.l7 4:1 clearzone 0.84 6:l clearzone 0.65 0.68 0.40 0.25 0.54 0.33 0.18 0.32 0. l8 0.10 Norrs: Actual data derived irom Graham and Harwood ûl)
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From page 296... ...
284 attt!
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From page 297... ...
285 (9) yields particularly extreme estimations.
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From page 298... ...
Appendix G Physical and Operational Features Affecting Safety at Intersections Although simple quantitative relationships to predict the effects of specific intersection improvements are generally not available, nonetheless, a substan'ial body of information exis's that designers use in remedying deficiencies ai intersections. This appendix contains a summary of the effects of physical and operational features on safety at intersections.
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From page 299... ...
287 directly the result of some aspect related to number of lanes, such as the number of potential conflict points, or whether it is indicative of indi¡ect effects, such as larger trafûc volumes. In any event, selection of ttre number of through lanes on intersection approaches is determined predominantly by capacity rather than by safety considerations.
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From page 300... ...
288 conflicting movements, and approach speed- The primary impact of auxiliary lanes is on collisions between vehicles on the same approach, particularly the rear-end type. They are typica[y more effective in reducing hazard when accommodating left-tuming vehicles than right-turning ones, and a separate left+urn phase at signalized inærsections is often necessary for full benefits to be realized.
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From page 301... ...
289 standpoint, right turns are more critical ttran left turns, and the degree of hazañ is related o both vehicle size and traffic volumes. FIXED LIGHTING At night, flxed lighting provides advance warning of the presence of at-grade intersections and allows the approaching driver to view objects outside the freld of headlight illumination.
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From page 302... ...
290 the vehicle on the right. V/hen sight distance is restricted or as traffic volumes increase, safety demands more positive control of traffic.
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From page 303... ...
29r unparking maneuvers disrupt through-raffic movements. The degree of hazard is intensifled at locations of concentrated pedestrian activity.
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From page 304... ...
Appendix H Highway Accidents on the Federal-Aid System The characteristics and frequency of highway accidents point not only to the role that highway design plays in safety generally, but also to the potential for accident reduction through incremental improvement to design elements. Discussed in this appendix are accident classifications, the characteristics of accidents on federal-aid systems, and accident ¡ates.
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From page 305... ...
293 For the analysis of a speciûc highway feature, researchers often focus on accident types believed to be most influenced by the feature in question. For example, skid-resistånce studies focus on wet-weather accidents, and roadside studies concentrâte on single-vehicle, run-off-road accidents.
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From page 306... ...
294 TABLE H-2 Characteristics of Fatal Accidents on Non-Interstate Federal-Aid Systems Single-Vehicle Accidents Multivehicle Accidents System Percent Percent of Fixed Total Object Most Common Objects Percent Pedestrians 4844 46 58 3t Percent Percent Percent Head-on Rear-End of Total Collision and Angle Rural Primary 45 Secondary 60 Urban Primary 50 Arterials 55 Collectors 66 Tiees Guard¡ail Utility Poles Tiees Ditches utility Poles Utility Poles Tiees Guardrail Utility Poles Curbs Tiees Tiees Utility Poles Curbs 552T 56363747 Source: 1985 Fatal Accident Reporting System Data (as summarized by L Griffin)
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From page 307... ...
295 TABLE H-3 Fatal Accidents and Fatal Accident Rates on Federal-Aid Highways [Fatal Accidents per 100 Million Vehicle Miles (MVM)
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From page 308... ...
Appendix I Initial Cost to Flatten Highway Curves The development of models for estimating the cost of flattening highway curves is described in this appendix. using the engineering approach, the cost models are based on estimates of hypothetical quantities of typical pay items and their related unit construction costs.
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From page 309... ...
297 TABLE I-1 Pay ltems and Unit Construction Costs Item Unit Cost ($) Simpliñcation lor Vertical Curve Cost Model Other Costs Earthwork Cost/CY : 10-0.00025 CY for CY < 14000 Cost/CY : a9,000/CY+ 3 for CY > 14000Surfacing Cost/T: 30.5-0.00027T S27lT Base (CSTC)
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From page 310... ...
298 where C = construction cost in 1983 dollars,Dr = original curvature in degrees, D2 = new curvature in degrees, .I = central angle in degrees, a = 4.0944 - 0.405 (00'1ola, andå = -0.0758 (O¡o'e<8. For the wide range of realistic conditions investigated, the maximum error in the approximation of Equation I was found to be + 10 percent.
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From page 311... ...
299 TABLE I-3 Construction Quantities for Lengthening Crest Vertical Curves Item Quantity Remarks Earthwork Equation 2 Cross section in Figure I-l' all excavation; grade and ground elevations the same at beginning and end of both cufves Surfacing T : 0.7 Lz 4-in pavement, 2-in' paved shoulder Base T : 1.8 Lz 8-in. pavement, 10-in.
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From page 312... ...
300 where C is the construction cost in 1983 dolla¡s and UC is the unit cost of excavation as given in Table I-1. Acquisition of additional right-of-way is an anticipated consequence of horizontal curveprojeets and, deponding on initial right-of-way width, may be required on vertical curve projects.
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From page 313... ...
Appendix J Relationship Between Cost per Accident Eliminated and Benefit-Cost Ratio Approaches IllusUated in this appendix is the relationship between cost-effectiveness analyses for a hypothetical horizontal curve improvement using cost per accident eliminated and rhe benefit-cost ratio approach. Assumed conditions for the example are r 1,000 ADT, ¡ 2Q-deg¡ee central angle, o 35 mph design speed before improvement, .
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From page 314... ...
302 . Estimate øccidents eliminated-4.10/year, based on assumed conditions before and after improvement and the accident relationship for horizontal curves presented in Appendix E; o Estimate added cost -- $111,000, based on assumed conditions and the cost relationship presented in Appendix I; o Annualize the added cosl -- $8,950lyear, bæed on the assumed discount rate and project life; and c Calculate cost per accident eliminated-$8g,500, calculated as 8,950/0.10.
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From page 315... ...
Appendix K Effects of Lane and Shoulder \Midths on Travel Time Narrow lanes and shoulders on two-lane roads cause motorists to drive closer to vehicles in the opposing lane. They must compensate for driving closer to opposing rafûc by slowing down and allowing larger headways between nèttictes in the same lane.
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From page 316... ...
3M . VHTIVMT is hours per vehicle mile-the inverse of speed; .
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From page 317... ...
305 The "free flow" speed (i.e., the speed at low trafûc volumes) of 58 mph is based on an assumed design speed of 60 mph or gleater and a speed limit of 55 mph.
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From page 319... ...
307 information given in Table 8-6 of the manual can be used to develop more precise estimates of SFD for a given vehicle mix.
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From page 320... ...
Appendix L Alternative Lane and Shoulder Width Standards Used in System-Level Analyses Described in this appendix are the four sets of lane and shoulder width standards examined in the national system-level analyses presented in Chapter 5. These standards apply to two-lane rural highways only.
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From page 321... ...
309 t2 t2 t2 t2 t2 t2 t2 t2 t2 t2 t2 T2 ll t2 t2 50 60 70 TABLE L-l AASHTO New Construction Standards for Lane and Shoulder widrh ø Width of Road Feature (ft) by Projected Design Traffic Volume Design Speed ADT ADT4OO DHV DHV DHV Under400 andOver 100-200 200-400 Over400 Arterials: Lanes Arterials: Shoulders All speeds Collectors: Lanes l0 l0 1l 1t 1l ll Collectors: Shoulders (Graded)
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From page 322... ...
310 3. Shoulder widths for highways in mountainous terrain were reduced by 1 fr, 4.
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From page 323... ...
311 REFERENCES t. A Policy an Geometríc Desígn of Hþhways and Streets.
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From page 324... ...
Study Committee Biographical lnformation Pernn G Korrxow, Co-Chairman, is a consulting engineer and Counselor to the President, American Trucking Associations.
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From page 325... ...
313 tory Aberdeen Proving Grounds in Maryland. Before joining the faculty of MIT, he was Chief Scientist, Office of the Secreøry of ttre U'S.
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From page 326... ...
3t4 more that 70 technical publications cover such diverse subjecs as physics, matl¡ematics, traffic engineering, Eansportation energy, human factors, trauma analysis, and trafûc safety. His main professional interests focus on faffic safety research.
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From page 327... ...
315 Ezn¡ Heurn is an engineer and Professor, Depætment of Civil Engineering, University of Toronto. He received his bachelor's and master's degrees from Technion University in Israel and his Ph.D.
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From page 328... ...
316 Angeles District; Disrict Traffic Engineer; Assistant Tlaffic Engineer, Division of Highways; Computer Systems Engineer; Chief, Office of Local Assistrnce, Caltrans; Chief, Office of State Planning; Chief, Division of Equipmenq and Chief, Division of Value Engineering.
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From page 329... ...
3t't Bnra¡¡ o'Nern is President of the Insurance Institute for Highway Safety (IIHS) and its associated organization, the Híghway Loss Data Institute iUt-Ot¡, two independenr organizations dedicated to reducing the lisses-deaths, injuries, and property damago-resulting from motor vehicle crashes.
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From page 330... ...
318 and has served as Director of the Project Deveþment Bureau and ttre safety and rraffic Division for the New York søte Deparrnent of rransportation.
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From page 331... ...
319 served in the following positions at the Idaho Department of Transportation: Planning Engineer, Deputy State Highway Engineer, State Highway Adminisrator, and Chief of Engineering Services. He was Vice Chairman of the AASIilO Standing Committee on Highway Traffic Safety, and formerly Chairman of the AASHTO Subcommittee for Traffrc Engineering and the AASFIIO delegation to the National Committee on Uniform Trafûc Control Devices.
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Key Terms
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