The yard sign test is a valid statistic. However many people are voting "against" Trump rather than "for" Biden. Which is why, while they could care less about putting a Biden sign in their yards, they still go out and vote for Biden on E-day.

I wrote The Final Case for Biden https://medium.com/politics-fast-and-slow/the-final-case-for-joe-biden-143b4b70c071

and The Final Case Against Trump https://medium.com/politics-fast-and-slow/the-final-case-against-donald-trump-2cc0b171e153

and saw the same thing. The Trump piece got twice the page views of the Biden piece - again, people "hate" Trump more than they "love" Biden but as Roger Stone and Al Sharpton agree - hate is a very strong motivator for voting.