Have enough room but still needs more for high density

Land value and happiness are separate concepts.

• Land value aka desirability encourage wealthier sims to move in.
• Happiness encourages density increases.

Residential happiness comes from shopping! Turn on the commercial dataview and make sure that shoppers can go from residential zones to commercial zones with available goods. (Goods resupply automatically at 6 am and 6 pm. Goods have nothing to do with freight).

Residentials without money or without nearby goods will instead go to parks to get happiness. This happiness is just as effective at growing density as shopping (ie. your buildings will grow). However, there are some downsides. Parks cost you money, while commercial buildings pay tax. Parks take up more space to get the equivalent capacity of commercial buildings. Parks also increase land value - so you may accidentally bring in wealthier Sims than you really want (wealthy buildings hold fewer people).

If you need more jobs, you might need denser industrial zones. Industrial happiness comes from making shipments of freight. Turn on the industrial view. If there is a large pile of shipments at any building, that industrial is in trouble. Make more commercial zones or trade buildings to receive those shipments. Once received, those shipments will automatically vanish at some point allowing more to be received.

The service area is always the entire city. Travel time determined whether you'll reach the destination before a building burns down, patient dies, or criminal gets away.

Early on, with little traffic, one single service placement can usually cover the entire city. Adding more vehicles lets you dispatch to more places at once. Once traffic increases, the effective service area is reduced, no matter how many vehicles you add (except for helicopters). You can wind up in a situation where a vehicle can't reach the building next door because it can't even exit the vehicle bay. Good placement, road layout, and public transportation will keep your services most effective.

You'll notice that the game lets you plop the vehicle bays apart from the building. Don't lay them all next to each other so they exit on the same road in the same direction. Place them on opposite sides of the road and across intersections. That way, traffic in one direction will only clog up some of your vehicles instead of all of them.