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52 chapter four CONCLUSIONS AND NEEDS FOR FURTHER RESEARCH CONCLUSIONS This synthesis report collects information on practices of U.S. states in load posting of highway bridges and culverts. Information in this report is collected from federal regu- lation and publications, state statutes and administrative codes, state department of transportation (DOT) publica- tions, publications of AASHTO and TRB, and from a sur- vey distributed to state DOTs. Forty-three states responded to the survey. This synthesis report examines the prevalence of load posting among U.S. bridges and culverts; tabulates the dis- tribution of load posted structures among owners, route sys- tems, structure ages and conditions; lists the legal loads and permittable overweight loads; and cites the methods of load rating. Implementation of posting through signs for weight limits and fines for violations of weight limits is included in the report. Posting for load is an outcome. Decisions to post for load are made in a context of legal limits on vehicle weights and engineering methods for evaluation of safe load capacity. Legal limits on axle weight, tandem-axle weight, and gross vehicle weight are set in federal regulation, state law, and local law. States exempt some vehicles from load limits. Exemptions are based on vehicle use, the commodity carried, or vehicle owner. States allow overweight vehicles, by per- mit, to exceed legal loads. This synthesis collects 286 provi- sions in state law for legal loads, identifies 122 exemptions to state legal loads, and lists 418 examples of overweight permit loads. Structures are posted for load when their safe load capacity is not adequate for legal loads or for routine overweight permit loads. Safe load capacity is determined, for most structures, by computational load rating. This synthesis report presents methods and bases for load rating, and tabulates the use of load rating methods by states. The synthesis report presents statesâ policies on selection of weight limits for load posted structures. Most survey states use operating-level load ratings to set weight limits for load posted structures. Load ratings are computed for present-day conditions of structures; conditions that can include deterioration in com- ponents of structures and additions to dead load on structures. For deterioration in components, most states use field mea- surements to establish remaining sections. States also use AASHTOâs condition factor to account for general deteriora- tion in structures. Load ratings for structures are evaluated using rating vehi- cles; numerical specification of counts, spacings, and weights of axle groups. Various rating vehicles impose load effects similar to legal loads or to overweight permit loads. States use rating vehicles defined by AASHTO, and also use state- specific rating vehicles. Load postings are implemented with weight limit signs and enforced with fines. Most states use U.S.DOT standard weight limit signs at load posted structures. The median fine for violation of weight limits is $0.20 per pound of excess weight. States have the authority to post state-owned structures for load. Under federal regulation, states have the respon- sibility to ensure the inspection, load rating, and load post- ing of most bridges and culverts on public roads within state boundaries. States are not responsible for structures owned by the federal government. A few states can load post struc- tures owned by local governments. More often, state govern- ments advise local governments on required load posting of local government structures. From the perspective of bridge owners, and especially state DOTs as custodians of state-owned structures, load posting is one aspect of the management of mobility. DOTs maintain mobility in highway networks, evaluate structures for over- weight loads, and identify routes for overweight permit vehicles. State DOTs evaluate structures for legal loads, and restrict loads on structures that do not have adequate capacity. State DOTs identify the capacities of structures over a wide range, and permit or restrict loads on structures appropriately. Ten percent of U.S. bridges and culverts are posted for load. More than 80% of load posted structures are owned by local governments. Seventy-six percent of bridges and cul- verts posted for load have average daily traffic of fewer than 400 vehicles. Ninety-two percent of posted structures have average daily truck traffic of less than 100 trucks. In contrast, less than 1% of structures on interstate highways are posted for load.
53 63 and 65). Questions here include: How do bridge owners manage the load ratings and load postings for these struc- tures? What urgency do bridge owners attach to load rating of these structures? Should techniques be developed to assist load raters in rating these structures? ⢠Effectiveness of Weight Limit Signs in Restricting Use of Structures Report on the rate of misinterpretation of weight limit signs by road users, on the rate of road usersâ incognizance of weight limit signs, and on sources of misinterpretation and incognizance. ⢠Effectiveness of Communication of Weight Restrictions Report on communications, other than weight limit signs, that provide road users with information on the presence of load posted structures and the availability of other routes. ⢠Effectiveness of Maintenance of Weight Limit Signs Report on the absentee rate for weight limit signs at load posted structures. Report on the number of missing signs in relation to the number of load posted structures, on the num- ber of signs replaced annually, and on the average duration of absence of signs at structure sites. ⢠Effectiveness of Enforcement Report on the frequency and magnitude of illicit crossings of load posted structures. Report on the relation of illicit crossings to the level of enforcement by state police and to the value of overweight fines imposed on violators. ⢠Practices of Local Governments in Load Posting Report on local government practices for load posting focusing on a sample of municipal and county governments. ⢠Transience of Load Posting Report on the annual rate of structures joining and leaving the population of load posted structures. Report on the aver- age time duration of structures in load posted status. GAPS IN KNOWLEDGE AND NEEDS FOR FURTHER RESEARCH This synthesis report presents information on the practices in load posting of bridges and culverts, but does not pro- vide information on the effectiveness of these practices. It presents information on practices of state governments; however, local governments own most of the load posted structures. There are several areas that need further research as described here. ⢠Effectiveness of Decisions in Load Posting Report on the number of structures that have inaccurate posting status or incorrectly implemented weight limits. Status issues can include lack of weight restriction where restriction is needed, incorrect load limit implemented, or load limit implemented but not correctly documented. Inac- curate weight limits can be quantified in magnitude and in direction. ⢠Effectiveness of Quality Control of Load Rating in Load Posting Report on the quality control procedures of load ratings and the effectiveness of quality control procedures when posting is needed. ⢠Effectiveness of Implementation of Load Postings Report on the number of structures that remain without posted weight limits, although bridge owners are aware that posting is required. Report on the reasons for failure to post for load and on impediments to coordination between state and local governments for load posting of structures. ⢠Effectiveness of Load Rating in Load Posting Report on the number and magnitude of errors in load rating and on the number of errors in load rating that alter decisions in load posting. ⢠Hazard at Un-Rated Structures More than 95,000 National Bridge Inspection Standards structures lack load rating analysis (NBI Code 5 for NBI Items
