Probably just scare tactics.
Anyone that has ever lived or visited South Florida will attest to the fact that S. Florida has the world's best traffic jams because they're moving 75mph! I95, 595, Turnpike, and I75 are comparable to Daytona during qualifications! Remember when they used to put the cruisers on the embankments of I95 and 595 with mannequins in them to deter speeding?
I honestly don't think any terrorist act will ever be attemped in South Florida simply because where else are they going to train? Nowhere else in the U.S. can a terrorist fit in without being suspected of any foul play.