Number of DWI arrests in a city?

You are correct the state police mostly stick to the highways but if they are off the highway (say on a meal break) and see a car driving erratically they can and do pull them over for a possible DUI stop but those would be small in number compared to the dui stops/arrests they make on the highway. If you search time period includes a holiday (like new years eve) you may want to try and obtain the information because those are the times when enforcement is stepped up and multi agency DUI task forces are sometimes utilized and state police may be used on the local streets.