Tough to justify the expense of paying for a billboard when the message would only be intended for less than 1% of people driving by.
On the other hand, it would be somewhat comical to see what would happen if there was an anti-JW billboard in NY between the highway exit ramp and the entrance to the Warwick campus. Hundreds (maybe more) of JWs driving by that one every single day-- tour buses, visiting families, Bethelites commuting, etc.
Something non-confrontational, simple, and direct:
"It's time to discover the truth about 'The Truth'
jwfacts.com" (putting the logo in the blue (jw.borg) box style, of course)