watch-dogs
I often like driving police cars in games. Usually I can find one at a police station or simply spawn one in. Neither of these seems to be an option in Watch underscore Dogs.
How can I reliably get a police car, preferably without messing with police chases?
You need to find a square plate with a check mark on it, it's shown on the minimap as well. It could be either on a wall or the ground.
When you find it, you have to walk to it and press E if you're using keyboard or Y if you're using a gamepad. Additionally, you can check-in from distance by pressing middle mouse button or Up on a gamepad.
Doing this will "check in" and show you some information about the spot.
No, unfortunately you can neither repair nor modify your vehicle. There is basically no need to repair vehicles, because you can simply "respawn" any vehicle you could have destroyed with the "Car on Demand" app. I have completed about half of the game and have not come across anything to modify a vehicle, but feel free to correct me if something comes up later.
As Lyrical already mentioned in a comment, you unlock new cars for the app by simply entering them. If it is the first time sitting in the type of vehicle, the message "new vehicle unlocked" will appear in the top right corner and then you can buy access to it in the app. Another way to get cars (often rarer ones) is to hack civilians' phones. To make this easier, you can unlock a skill in the "Hacking" branch of the skill tree that marks civilians with especially valuable hacking opportunities (including cars, songs, crimes and rich bank accounts) with a blue square, so you can already see them from afar. The chance of finding vehicles that way is still rare though. Hacking people's phones is one of the few things you can do while waiting for a multiplayer session to start, so that is a good time to go searching for people with cars.
A good way to unlock sports cars, is to play Fixer Contracts that require to race through the city with a given car (which was already a sports car in the first mission I played). You can unlock new fixer contracts by hacking ctOS towers, which reveals new missions and points of interest in the area.
If you want a specific car from these missions, here is a list by segmentnext.com with fixer contracts and the vehicles you will be driving there.
Best Answer
I confirm the information in @Timot_lambo answer, but I feel like a screenshot of the location on the map is required, because youtube videos come and go, but screenshots stay forever.
There is also certain fixer mission that leads you to a police station with 3 police cars parked. Unfortunately, in free roam the parking lot seems to have random cars. I'm not sure if you can start the mission, take the police car and quit mission while keeping the car, because once you finish the fixer mission, you can't re-do it (at least I don't know how?).
Credit goes to @Timot_lambo for the video he provided