Most likely travelling in an ox-drawn wagon, the eunuch and his party would have been moving at a walking pace.1 This made it possible for Philip to run up alongside the chariot. Since it was the custom at the time for people to read aloud2 (in order to aid remembering),3 Philip would have heard the man reading as he walked by the side of his chariot.
30 So Philip ran to him and heard him reading Isaiah the prophet and asked, “Do you understand what you are reading?”