The United States has approximately 608,000 bridges on public roads subject to the National Bridge Inspection Standards mandated by Congress. About 48% of these bridges are owned by state governments and 50% by local governments. State governments generally own the larger and more heavily traveled bridges, such as those on the Interstate Highway system. Only 1.5% of highway bridges are owned by the federal government, primarily those on federally owned land. About 9% of all bridges carry Interstate Highways, and another 25% serve arterial highways other than Interstates. Interstate and other major arterial bridges carry almost 80% of average daily traffic. The highest traffic loads are on Interstate Highway bridges in urban areas; these account for only 5% of all bridges, but carried 37% of average daily traffic in 2013. Federal law requires states to periodically inspect public road bridges and to report these findings to the Federal Highway Administration (FHWA). This information permits FHWA to characterize the existing condition of a bridge compared with one newly built and to identify those that are structurally deficient or functionally obsolete. A bridge is considered structurally deficient "if significant load-carrying elements are found to be in poor or worse condition due to deterioration and/or damage, or if the adequacy of the waterway opening provided by the bridge is determined to be extremely insufficient to the point of causing intolerable traffic interruptions." A functionally obsolete bridge, on the other hand, is one whose geometric characteristics—deck geometry (such as the number and width of lanes), roadway approach alignment, and over/underclearances—do not meet current design standards or traffic demands. A bridge can be both structurally deficient and functionally obsolete, but structural deficiencies take precedence in classification. As a result, a bridge that is both structurally deficient and functionally obsolete is classified in the FHWA's National Bridge Inventory as structurally deficient. A bridge classified as structurally deficient or functionally obsolete is not necessarily unsafe, but may require the posting of a vehicle weight or height restriction. The proportion of bridges classified as structurally deficient has declined 57% since 1990, and fell almost every year between 1990 and 2013 (see Figure 1 ). In 2013, approximately 64,000 bridges, or 10% of the total number of bridges, were classified as structurally deficient, as compared to 138,000 in 1990. The number of functionally obsolete bridges fell from 100,000 to 84,000 over the same period, a drop of 16%. The share of bridges classified as functionally obsolete in 2013 was 14%. In total, then, almost a quarter of U.S. bridges were deficient in that year. Bridges on the most heavily traveled roads, such as Interstates and other arterials, are generally in better condition than bridges on more lightly traveled routes. For example, 4.4% of urban Interstate Highway bridges were considered structurally deficient in 2013, about half of the 9.4% structural deficiency rate of urban bridges on local roads. Likewise, 3.8% of rural Interstate Highway bridges were structurally deficient in 2013, about a quarter of the 16.5% structural deficiency rate of bridges on rural roads handling local traffic. As the bridges on local roads are usually owned by local governments, locally owned bridges had more than twice the structural deficiency rate of state-owned bridges in 2013. Some 14.1% of locally owned bridges were categorized as structurally deficient in 2013, versus 6.7% of state-owned bridges. For bridge deficiency and obsolescence rates by state see Appendix A . Every two years, FHWA assesses the condition and performance of the nation's highways and bridges, documents current spending by all levels of government, and estimates future spending needs to maintain or improve current conditions and performance. As with any attempt to forecast future conditions, a host of simplifying assumptions, omissions, and data problems influences these estimates. Among other things, they rely on forecasts of travel demand and assume that the most economically productive projects (i.e., projects with the highest benefits relative to costs) will be implemented first. Despite such uncertainties and assumptions, these estimates provide a way to assess the level of current spending compared with what would be needed in the future under different scenarios. The 2013 needs assessment, the most recent available, shows that in 2010 $100.2 billion was spent on capital improvements to the nation's highways and bridges. Of that amount, $82.2 billion was spent on roadways and $18.0 billion was spent on bridges. The vast majority of the expenditure on bridges, $17.1 billion, went to rehabilitate or replace existing bridges, with the remainder devoted to construction of new bridges. The $17.1 billion spent in 2010 was an increase of 35% over the $12.7 billion spent in 2008. The funding increase was largely due to the American Recovery and Reinvestment Act of 2009 ( P.L. 111-5 ; ARRA). Because of the modeling involved, FHWA's future needs estimates for bridges are limited to fixing deficiencies in existing bridges only when the benefits outweigh the costs. The future needs estimate can therefore be measured against the $17.1 billion expenditure in 2010. The U.S. Department of Transportation (DOT) estimates that fixing all existing bridge deficiencies would cost $106.4 billion (in 2010 dollars). Of course, fixing all deficient bridges overnight is not feasible. FHWA, therefore, estimates how this investment backlog will change at various levels of spending over the 2011-2030 period, taking into account the deterioration of existing bridges over that period. The results of this analysis, seen in Figure 2 , are strongly influenced by the ARRA funding. ARRA appropriated general funds for highways to add to the funding from the highway trust fund that had already been authorized. According to FHWA, to keep the backlog at the 2010 level through 2030 would require $8.9 billion annually (in 2010 dollars), less than the level of spending in 2010. To eliminate the backlog by 2030 would require an investment of $20.2 billion annually, implying roughly a 1.6% annual increase in inflation-adjusted spending. Spending between $8.9 billion and $20.2 billion per year, FHWA estimated, would improve the conditions of the nation's bridges but would not entirely eliminate the investment backlog. At the level of spending in 2010, $17.1 billion per year, the total dollar cost of correcting all remaining deficiencies would decline by 93% by 2030. Federal assistance for the maintenance, rehabilitation, and construction of highway bridges comes principally through the federal-aid highway program administered by FHWA. FHWA, however, does not make the determination as to which bridges should benefit from federal funding. Almost all funding under the federal-aid highway program is distributed to state departments of transportation, which determine, for the most part, where and on what the money is spent. States must comply with detailed federal planning guidelines as part of the decision-making process, but otherwise are free to spend their federal highway funds in any way consistent with federal laws and regulations. Bridge projects are developed at the state level, and state departments of transportation let the contracts, oversee the construction process, and provide for the inspection of bridges. The 2012 surface transportation reauthorization, the Moving Ahead for Progress in the 21 st Century Act (MAP-21, P.L. 112-141 ), further strengthened the states' ability to determine spending on bridges by eliminating the Highway Bridge Program (HBP), which provided money to the states specifically for bridge construction and rehabilitation. Bridge improvements remain eligible for funding under two programs created by MAP-21 that distribute funds to the states under formulas specified in the law, the National Highway Performance Program (NHPP) and the Surface Transportation Program (STP). Under both programs, the states determine how much of their federal funding goes for bridges as opposed to other uses, primarily highway construction and improvement. These funds may also be used for the seismic retrofitting of bridges to reduce earthquake failure risk. FHWA is involved in the project decision-making process in two significant ways. First, MAP-21 (§1111) requires FHWA, in consultation with the states and federal agencies, to classify public road bridges according to "serviceability, safety, and essentiality for public use ... [and] based on that classification, assign each a risk-based priority for systematic preventative maintenance, replacement or rehabilitation." However, none of the MAP-21 programs appear to require the new classification and risk-based priority metric be used to determine program eligibility. In addition to developing this metric, FHWA imposes certain performance measures that states must meet to avoid funding penalties pursuant to MAP-21. For example, if more than 10% of the deck area of a state's bridges on the National Highway System is structurally deficient, the state is subject to a penalty requiring it to dedicate an amount of its NHPP funds equal to 50% FY2009 HBP spending to bridge projects. While the HBP existed, bridge program apportionments—the money states were entitled to receive each year under the HBP—were trending upward. Spending (obligations) on bridge projects, however, tended to be substantially lower than the apportioned amounts. The transfer by the states of HBP funding to other highway programs, while permitted by law, was controversial following the collapse of the I-35W Bridge in Minnesota in 2007. At the time, critics saw the widening gap between annual apportionments and spending as evidence of state transfer of resources to nonbridge uses. However, bridge spending was an eligible expense under all the core formula programs, not just HBP, and total federal grants obligated for bridge work exceeded HBP apportionments every year between FY2007 and FY2012 ( Table 1 ). Under MAP-21, which eliminated the HBP, the states have even more freedom to decide how much of their federal surface transportation grants to spend on bridges. During FY2013, states obligated roughly 8% more federal-aid highway funds for bridge work than in FY2012.