During the past four years, U.S. infrastructure has improved from a grade of D to D+, according to a report by the American Society of Civil Engineers (ASCE). Despite the slight progress, the report concludes that the country's infrastructure needs $3.6 trillion in investment by 2020 to bring it to an acceptable level; $2 trillion in spending is projected.

Every four years, ASCE releases its Report Card for America's Infrastructure, which uses a committee of more than 30 engineers to assign letter grades based on capacity, condition, funding, future need, operation and maintenance, public safety, resilience and innovation.

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