The American Society of Civil Engineers recently awarded the US a minimally passing grade of D+ for its crumbling infrastructure, and identified $3.6 trillion of unfunded requirements.

This is because US public infrastructure (highways, bridges, airports and so on) is paid for by user taxes and tolls that politicians are loath to raise, or by direct government grants that are equally unpopular. Thus it is continually under-depreciated, under-maintained, and under-financed.