A message may be displayed only while at the scene of a crash or a disabled vehicle. The message that may be displayed is “SLOW DOWN”, displayed as two lines in all upper-case letters. Any tow truck-mounted portable variable messaging sign shall only display static text that: Is colored yellow or amber, on an opaque black background; Is one consistent letter height of at least 10 inches; and Contains words within which the letter spacing is between 25 and 40 percent of the letter height. The vertical spacing between the lines of text shall be between 50 and 75 percent of the letter height. The message shall not include animation, flashing, dissolving, exploding, scrolling, or other dynamic display elements.