But officer I wasn't not going that fast

I was looking for good scenery but traffic and the Iowan landscape made pictures difficult. When Buffy started impatiently tailgating an enormous truck I shook loose from my fear and noticed some writing. She got more than close enough for me to make out the message so I got the camera out and snapped this picture.

It's a little hazy. I didn't have time to get a really good shot. By the time I was able to get the focus and framing right Buffy was flooring it and passing semi.

As I understand the message it means that driving 55 mph is too slow to ensure that the produce will be fresh when it finally arrives. So the driver has to go faster than 55.

It's more evidence of negative concord in PdE. That slash-circle is the visual equivalent of a negative morpheme. In some English dialects it would have to be silent in order to avoid further semantic content: It isn't fresh at 55. But in other dialects we can imagine the negative working for emphasis.

FOREMAN: Them potatoes better be fresh. And you better not get no speeding ticket on the way here!
DRIVER: It ain't gonna be fresh at no 55 miles an hour!

I've seen instances of this visual negative concord before. Remember the Red Rocker? His shirts are still out there on eBay.