They may not be a reliable barometer, but I’d day the number of Trump bumper stickers and yard signs I see as I drive all around Arizona is probably a 20 to 1 ratio in favor of Trump. I drove down one street last week where every house but one had a Trump sign. I’ve seen maybe 3 Hillary stickers total, and I drive around the state a lot.
It might actually be a pretty good barometer of voter enthusiasm, though. If you put on a bumper sticker or plant a sign in your yard in today’s polarized climate, you stand a good chance of getting your car keyed or your house egged. You may also antagonize your neighbors who don’t share your political beliefs. You have to REALLY like your candidate to be willing to risk all that. If you have tepid support for a candidate for which you have to hold your nose to vote, you may well find good reasons not to wait in a long line on election day. Sign & Bumper Stickers = Almost Certain Voter.
This election will largely hinge on voter enthusiasm, I think. Hillary is weak with white males, millennials, the working class, old people, and young minorities. Trump is weak with white females, older minorities, and the college educated. Whichever side has the most motivated electorate in the battleground states has the best chance. The size of the rallies for Trump v. Clinton might not be the best predictor, but it does demonstrate relative voter enthusiasm levels, which are a significant factor.