Railworks America

Would anyone know where I can find an ambient traffic/highway noise for the sim?

What I'm talking about is in the beginning of this video:

https://www.youtube.com/watch?v=jD7FfLI6rHU

I've been wanting to know how to add or create sounds for my custom routes but have yet to find a discussion on that.

For your question, the youtube video which you posted, it may be on that current route which looks to be The Racetrack: Aurora - Chicago Route Add-On it may have those sounds. or... it was added as a custom ambient sound.