The graph shows how far a bus is from its depot during the first $15$ minutes of its trip along a straight road. What is the bus's average speed over these $15$ minutes?

Fig. 1: Graph of a bus's distance from its depot against time: 0 to 3 km in the first 6 minutes, flat at 3 km from 6 to 10 minutes, then up to 7 km at 15 minutes.