Your $300 billion number just isn't true. If you're looking at that Forbes article, well, it covers 6 years. So it's only $50 billion a year. About 30% of that you're counting twice because that was money from the US military budget they spent on foreign aid. So, a bit under a whopping $35 billion a year, or less than 1% of either tax receipts (which are actually over 4 trillion) or less than 0.5% of the US budget itself (6.8 trillion)

0.5% not 8%. These numbers are very different.

Although this bridge seems disconnected from the highway system, so I'm not even sure the US government even funds it at all.