Democrats argue that their stimulus proposal must include a massive infrastructure component not only because it will help the economy (which it won't), but also because the government has been chronically underinvesting in infrastructure in recent decades.
The Congressional Budget Office puts the lie to that claim. This chart is of all public capital spending on infrastructure, from 1956 to 2006:
It's obvious that both the federal share and state and local share are increasing at a healthy rate (just like most other government spending).
This shows all public spending for infrastructure capital, as well as operation and maintenance:
The picture is the same. Spending on both functions has consistently increased.
This last graph shows only federal spending:
No matter how you look at it, there's been no significant dropoff in infrastructure spending. Government at all levels is spending more on infrastructure -- roads, bridges, transit, water, etc. -- than at any time in the post-war era. And that includes the period when we created the National Highway System.
Democrats may want to spend more -- they always do. But they can't argue that there's been a drop in spending.
These slides are from a CBO powerpoint presentation given in September, 2008.