How does the incident with the cars on Soldier’s Road serve as an example of situational irony?

Soldier's Bridge can only handle one vehicle at a time, and black people driving wagons often have to back down the bridge when a white person starts down it from the opposite side. Even though a Model T truck starts down the bridge, Uncle Hammer speeds up his Packard, forcing the Model T to back up. The Wallaces, all touch their hats as the car approaches, thinking the car belongs to Mr. Granger. This scene is filled with irony, as they see the Logan family inside, knowing they just pulled over for blacks, who should have pulled over for them. The appearance of the Packard was deceiving, and Uncle Hammer took full advantage of his position. He was awesome.