I went to the museum, and not suprisingly, it was posted 30.06.
The sign was not legal, the letters were obviously less than an inch high.
I am also wondering, who owns this property? IS it the city of Dallas? If so, that'd be another reason for the sign to be illegal.
Here's a picture:
(The fact that they don't even want me taking pictures in there made me even angrier.)
EDIT: It's owned by a non-profit, not the government, as per:
http://www.jfk.org/General_FAQs.htm
The Museum is a non-profit organization and receives no governmental funding for operating expenses.