Did you ever wonder where the shape for the standard sign denoting a US highway came from? Turns out it came from the great seal of the US department of treasury, which was adopted around 1789. It was adopted as a road sign in 1925.
Well, it looks like the treasury seal takes its lead from the great seal of the United States which was approved in 1782, 7 years earlier.
Here's a website describing the history of the road signage.