Found an exploit with the suburban road, as long as two are next to each other the bump each other up. Add an upgrade on that and you can max out the money in about 5 minutes.
I got 00:02:36 using this exploit. Start with 3 corners to generate income, then use 2 more corners and a suburban road on the other side, then as you acquire the money replace roads with suburban roads, starting with the ones next to the existing suburban roads.
