Transportation logos are an interesting breed of logo: most states follow the [State] Department of Transportation formula. The US Department of Transportation began using the new logo in 1980, which is a circle with inscribed swooshes, which I am assuming stand for the different modes of transportation USDOT administers – and it is usually the abbreviated format because the full title is soooo long.
The following logos, for the majority, use the abbreviated format. And almost all DOT logos use italics or stripes to denote speed. Because, deep down, you want your roads to be flat, fast and devoid of state troopers.