Expensive road upgrades happen when your blocks are too long.
By "block" I mean any straight stretch of road between intersections (or corners). All the buildings facing that single section of road add their residents to the load of that section and the buildings are often packed tightly together (because there are no cross-streets consuming land). You get more buildings in the area, so good for you, but you suffer the need for better roads to allow all those residents to get around. Put in cross streets (make shorter blocks), so when you get the need to upgrade the road, you might be able to just turn a building to face the cross street so not all the buildings are using the same road. When you do finally have to upgrade a section of road, it is significantly cheaper to do a short section, so even if altogether it will cost about the same, it's easy to do short sections one at a time.
You're the Mayor. City planning is the hardest part of your job. Don't cram in as much as you can. Let some light in.
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