The nation’s infrastructure is aging rapidly. Highways and bridges, particularly in older urban areas, are badly in need of repair, leading to safety concerns and traffic delays. Congress passed a $305 billion transportation package in December, but that is considered only enough to maintain the status quo. President Obama has proposed raising federal taxes on oil, but the idea lacks any Republican support. Most of this year’s presidential candidates have said they want to spend more on infrastructure, but few have specified how they would pay for their plans. States, which together spend more on infrastructure than the federal government, are resorting to creative budgeting to fill funding gaps. Meanwhile, the lead-poisoning crisis in the Flint, Mich., water system has drawn attention to funding shortfalls for drinking-water infrastructure.