WSP does use impala's for their older unmarked vehicles. My friend (who is a WSP trooper ) drives one. However, they are currently doing a slow switch of all their unmarked "ADAT" vehicles to the new 2017 dodge charger. Other than that they use the chevy caprice for marked units. That would be a cool addition. I have heard rumor within the WSP that a marked 2017 dodge charger could be in the works.
Caprice and ford interceptor utility are standard issue vehicles
Tahoes within the wsp are special use vehicles
dodge chargers are currently only for unmarked use.
impalas are also CVE and unmarked use only.