Research at the University of Michigan, funded by the NHTSA (National Highway Transportation Safety Administration), examined numerous formats and types of content used for display in road hazard and warning signs.  Subjects reported that their preference overall was for signs that quickly conveyed a message in brief, meaningful TEXT.  They preferred text instead of lines, arrows, drawn hazards, and other signage that required cognitive interpretation to deduce their meaning while safe response distances rapidly closed at highway speeds.  This extends also to simple steady or blinking lights, which in some instances might be so small, weak, repetitive, or uninteresting, as to gain little attention.