For your situation, I would have put "FL please" or "near MIAMI".
I figure, if you're on the southbound highway (I-95), why put "South" - kinda obvious? (It would be funny to walk a way down I-95 South and hold a sign saying "Maine" or "North"!)
Maybe being at the on-ramp is limiting you to just people pulling out of that stop, rather than people travelling from further north and heading further south? Especially in a business district, you'd largely be getting just work-time commuters, I'd think - people only going 15 miles down to home in the suburbs(?).