What is the significance of the turtle? Who does the turtle represent?

The world seems to be passing families like the Joads by. The advent of agribusiness and mechanization are literally leaving sharecroppers in the dust. The turtle is a metaphor for these families. He is trying to make his way crawling across a fast paced highway. Motorists simply don't see the turtle anymore, more do they really care if the turtle makes it or gets squished. The Joads too feel like this turtle. The turtle is heading tin the same directions as the Joads just as slow and irrelevant, to the modern world, as the Joads are.