Katryn's uncle recently became a police officer, and showed off his car to Katryn.
He even let her turn the lights and siren on.
I think in her mind it was just another car, only she got to sit in the front seat, instead of the back seat. A special privilege for sure.
Really though, the adults probably thought it was cooler than she did. She was very clear that she did not want to sit in the back, after peering back there and seeing the little caged seat for the bad guys.