No, not so much the number of yard signs in front of houses or on property (though there is that, too). Rather, in this case, the ease with which a supporter can actually go get a yard sign.
That's often not as simple as you might think. While campaigns may aggressively seek out some prime spots for signage, what seems like a diminishing number of campaigns actually make it easy for an interested supporter to simply get one.
So check this out from the Steve Novick (U.S. Senate in Oregon) campaign: Detailed instructions on where exactly to get signs, and lists and maps of 25 places around the state (spottier in the east, but then most human organization is) where they can be had. Many pointed to supporters, who in turn will help build the organization.