Infrastructure Investment and Economic Returns: What Research Shows About Public Capital

Infrastructure investment became a prominent policy focus following the passage of the Infrastructure Investment and Jobs Act in 2021, which allocated approximately 1.2 trillion dollars over five years for transportation, broadband, water systems, energy, and related investments. The bipartisan appeal of infrastructure reflects its concreteness, its connection to economic development, and a broad consensus that American infrastructure has been underinvested relative to need. Research on the economic returns to public infrastructure investment, its effects on productivity, employment, and inequality, provides context for evaluating specific investment priorities and expected outcomes. Public capital in the form of transportation networks, water systems, energy grids, and communication infrastructure has been studied by economists for decades as a contributor to private sector productivity. Research using macroeconomic data finds consistent positive associations between public capital stock and aggregate productivity, with estimates of the output elasticity of public capital typically in the range of 0.1 to 0.2, meaning a ten percent increase in public capital is associated with a one to two percent increase in output. These estimates are imprecise and vary across methodologies, but the positive direction is consistent across studies. Road and highway infrastructure is the most studied category of public capital. Research on highway construction, particularly the Interstate Highway System, finds significant positive effects on the economic development of regions connected to the network. Studies using historical variation in highway construction, including alignment decisions driven by political rather than economic factors, find that highway access increased population, employment, and wages in connected areas. However, research also documents displacement effects: transportation infrastructure that improved access to some areas may have drawn economic activity away from others. The returns to infrastructure investment depend on how it is financed, maintained, and targeted. Research on the fiscal multiplier of infrastructure spending, meaning the ratio of GDP increase to government expenditure increase, finds values that depend significantly on economic conditions. Infrastructure investment during recessions, when idle resources can be mobilized, produces higher multipliers than investment during periods of full employment. Research on state highway investment finds that maintenance spending, which preserves existing infrastructure value, produces higher economic returns per dollar than new construction that adds capacity to low-demand corridors. Broadband infrastructure investment has emerged as a priority given the central role of internet connectivity in economic activity, education, and healthcare. Research on broadband access and economic outcomes finds positive associations between broadband availability and employment growth, business formation, and income in communities that gain access. Studies comparing communities above and below broadband availability thresholds, using the boundaries of service areas as natural experiments, find consistent positive effects on economic and social outcomes. The COVID-19 pandemic accelerated both the demand for broadband and the recognition of access gaps, particularly in rural areas and tribal communities. Water and wastewater infrastructure investment is less politically prominent than transportation but has significant health and economic returns. Research on lead pipe replacement and water system improvements finds substantial benefits in reduced lead exposure and its associated cognitive and health effects. Studies of the economic consequences of contaminated water supplies, including long-term effects of lead exposure on earnings and educational attainment, suggest returns to safe water infrastructure that substantially exceed the cost of remediation. The Flint water crisis produced research on the health and developmental consequences of lead contamination that reinforced the case for proactive investment in safe water systems. Environmental infrastructure including flood control systems, stormwater management, and climate resilience investments has received growing research attention as climate change increases the frequency and severity of flooding, drought, and extreme heat events. Research on the economic costs of extreme weather events finds that they are substantial and growing, and that investments in resilience infrastructure produce returns through avoided damage. Studies of specific flood control investments find benefit-cost ratios well above one, particularly in vulnerable communities. The geographic distribution of infrastructure investment has significant implications for equity. Research on the historical pattern of highway investment finds that highways were often routed through or near low-income communities and communities of color, creating displacement, community division, and concentrated pollution from traffic. More recent research on the equity implications of infrastructure investment argues for prioritizing investments in communities that have been underinvested and that face the most significant infrastructure deficits, rather than directing resources to already well-connected areas where political constituencies are stronger. Project delivery capacity is a practical constraint on infrastructure investment. Research on infrastructure project timelines and costs in the United States finds that projects frequently experience cost overruns and schedule delays relative to initial projections, and that American infrastructure costs substantially more per unit than comparable infrastructure in peer nations. These differences reflect regulatory complexity, permitting processes, labor practices, and institutional capacity for managing large projects. Improving delivery efficiency is as important as increasing investment levels for realizing the potential returns that research suggests infrastructure can provide.