In addition to being a funny trait in a sloth, Flash choosing to be fast on the road also ties nicely into the movie's theme
of not judging a book by its cover.
A lot of Zootopia is about challenging stereotypes through animal metaphors. Says the film's co-director Byron Howard (in an interview here):
[What] I really take from [the film] is the theme of that we define ourselves. The world may have an opinion of us, but it is up to you to define who you are. The world does not define you, you do, and that's the theme that I went away with. Yes, because both of the characters, both of the lead characters Nick and Judy lived their whole lives with people and animals telling them who they should be. And struggling against those limits.
And we see throughout the movie that:
- A small, "helpless" bunny turns out to be a tough cop that solves the case;
- A "duplicitous, conniving" fox turns out to be her loyal and helpful friend;
- A feared and powerful mob boss turns out to be a tiny shrew;
- A cheetah on the force is actually timid and sheepish, and their lion mayor is somewhat ineffective as a leader;
- Many of the predators in the film were actually innocent while the true villain turns out to be a fluffy, seemingly frail sheep.
So it's simply fitting to end the movie on this note, with the sloth turning out to be a bit of a speed demon. :)
[I'm not sure the assumption that Flash can be physically fast is correct, though; they only show him driving in a straight line, so I'm not sure you need that much physical speed to accelerate a car or to eventually apply the brakes.]