This is the issue with using evidence like this.Damn, I had hoped things would go differently here in Arizona based entirely on the bumper stickers I've seen.
1 Trump bumper sticker
1 Hillary bumper sticker
like 3 dozen Bernie bumper stickers
I think people are afraid to publicly show their support for Trump. And they're not hugely excited about Hillary either.
There is no causal relationship between people willing to put bumper stickers on their car and how people vote. It may mean some subset is louder and prouder about their candidate. They wan't everyone to know who they are voting for.
Most cars have no bumper sticker. Doesn't mean they won't be voting.