Nomad I am not required to drive the maximum allowable speed for your convenience.
Pianotuna here in Illinois I see truckers that travel at 55 or less with a 70 mph speed limit. Some of these truckers have a beacon like a construction vehicle others have a battery powered flashing light that is magnetic.
These extra flashing lights are probably technically illegal, but I've never seen one of these slow trucks pulled over.
I googled battery powered flashing magnetic light. A light from Walmart looks like the one I've seen stuck on the back door of a container.