Y'all, being a contraction of you all, would seemingly imply that it is indeed plural. And, according to Dictionary.com, it is an address to two or more people. "All y'all" denotes that an entire group is being included.
As far as ya'll is concerned, that's just bad spelling. Or, as the online urban dictionary calls it, it's "how idiots spell y'all". The speedway needs to change their sign.
