Crawford makes an interesting point on this:
The main reason that the cops pulling up on Rice so quickly looks so bad is because we assume they know where he is. But if they were focused on the swingset, rather than the gazebo further ahead, they may have concluded that the person they were looking for had left (or that the call was bogus) when they didn't see him at the swings, and so sped up after they passed the swings. In that scenario, they could have been surprised upon suddenly seeing a person matching the description as they passed the gazebo, and so put on the brakes. The focus on the swingset would also explain why they decided to enter the park from 99th St. rather than try to survey the park from the parking lot adjacent to the gazebo--as you can see below, there is a line of trees between that parking lot and the swingset (the diagram on the left is from the Ohio Highway Patrol report):
As you can see from the above, the decision to enter the park was made well before the police knew where Rice was, and likely because it offered the only clear view of the swingset. We don't know how fast the car was driving as it passed the swingset, which is where the police would have expected Rice to be based on the dispatcher's description. The Highway Patrol report estimates they were traveling at 19 mph before braking by the gazebo, where they might not have expected to see Rice.
That said, all that is just speculation. I still think it matters a great deal whether they intentionally pulled up right next to Rice or stopped when they saw him unexpectedly. Unfortunately, as the reports point out, the two cops involved aren't talking. (I assume this is why the prosecutor complains about the police union being uncooperative with his investigation at the webpage where he posted the reports.)
Weirdly what you're describing is that they kept driving till they saw a black human and immediately shot him.