Discription of Events: A mailman was delivering mail in a pickup truck on shoulderless, rural Webb Road heading south into Derby from Wichita. Traffic slows because they have to go into opposing lane to pass. Im next to go around the mailman, and as I slow, I see there is no opposing traffic, so I start to go around the mailman. The silver truck behind me decides they cant wait and pass both the mailman and I. I have to slam on my brakes because there is no room for all three of us widthwise. The silver truck has to drive with left wheels in the grass in order to go around. I had a baby in the car so I was really upset by this kind of impatient and stupid behavior.