In "The Grapes of Wrath" the Joad family is the symbol of all the workers negatively affected by The Depression. Their travels along Route 66 are a prime example of the unfair treatment given to families as they were trying to survive in the 1940's.
One of the first scenes in "The Grapes of Wrath", Tom Joad stumbles upon one of the examples Steinbeck uses in this novel to portray rich owners suppressing workers in the Depression. As Tom Joad interacts with the truck driver he challenges the truck driver by asking him if he would let a sign put up by a rich white guy tell him what to do. By doing this, Steinbeck is trying to show two things.
First, is to illustrate Tom Joad as a cunning person. By challenging the drivers self management he gives the driver no choice but to give Tom Joad a ride.
By painting the driver as a mean man, which the driver does not believe he is, Tom Joad puts him in a moral dilemma. He knows he is a kind man, and does not want to let a sign tell him what to do, so he gives Tom Joad a ride.
Also, this scene shows the potential of Tom Joad and other Joads to be leaders in revolution against owners. By tricking the driver, Tom Joad starts a mini-revolution against the owners of the trucking company. Chances are the driver will now see the how he is being suppressed by the owners. Down the road the driver will think about how he is letting his morals be trounced upon. He could also get other drivers to thinking about their own situation. This scene shows the potential of a revolution against the owners of the truck company,