score:1
Where I live, they're already up in people's yards. Then again I live in New Hampshire, where the first presidential primary happens in January 2020. They'll mostly go away after that and be replaced with signs for whoever ends up winning in each party.
A few people have never taken down the Trump signs from their yards.
If you're filming major highways, you aren't likely to see many such signs though. Those won't start showing up on billboards in any significant quantity until the end of 2019 and into 2020, depending on the state. Virtually none of these will remain up long past election day.
Overall I'd say that if you don't want such signs in your video, there probably isn't all that much you can do. You're going to see one in the US eventually regardless of when you come. As a general rule I'd say the time to see the least amount of election signs is from a few months after a presidential election to the following year. In the second year, signs will start coming up for Congressional races...
You might be able to learn how to remove them from your video with a video editing tool, the way they removed the coffee cup from Game of Thrones.
From your requirements I'd say the best time overall to avoid election signs of any sort, and have good driving conditions all over the country, would be spring through autumn of 2020.