According to the flight crew, just after they began to taxi, they observed a bus passing in front of the airplane requiring the captain to suddenly apply the brakes to avoid colliding with the bus. The captain immediately called the flight attendants (FA), who were conducting their safety demonstration, to check on their condition and the condition of the passengers. The forward FA indicated that he had been thrown backwards into the flight deck door but was alright to continue with the flight. While enroute to Denver, the forward FA informed the flight crew that he was a little sore, but he and the rest of the FAs were fine to continue their return flight to Detroit. However, after arriving in Detroit, the forward flight attendant sought medical treatment and was diagnosed with a broken rib. According to the operator, the Delta employee bus was in the process of training new drivers. There are four roadways that cross the taxi lanes between the two terminals. At the time of the accident, the only policy was to stop at the stop sign and visually confirm it was safe to cross the taxi lanes. A review of video from the bus showed that it stopped at the stop sign and then proceeded to cross the taxi lanes. The video shows that from the perspective of the bus driver, it is difficult to see the area in which the airplane was parked from the roadway and the 45-degree pushback made it difficult to observe the airplane moving towards the roadway. As a result of this event, Delta implemented a restriction on vehicle crossing the taxi lanes when there are airplanes in the vicinity of the roadway.