Vintage Mechanical Safety Signal

Made of steel and about the size of a modern auto license plate it looks to be a rear mounted warning signal of some type. An illuminated glass red lens is at top center. There are two flags that raise on either side by pulling down on the spring loaded levers. It may have been connected to a foot petal? It looks like a licence tag could have been mounted below the light. Majority is painted black. SAFETY FIRST and I.C.U. SIGNAL PAT. MAY 28, 1918 are printed in white. Wonder what its called ? What was it used on?