The finances of these metros rarely incorporate road costs; their accounting does not require them to incorporate the future liability for infrastructure maintenance and so everything looks good until they need to redo the roads.
If it's anything like my municipality, all that stuff is balanced in the budget by issuing bonds. My accounting knowledge is weak, but as I understand it, even though a city's credit rating and the prospective interest on its bonds are a feedback mechanism to help moderate the spending, it's still tempting for a city to overbuild without a plan to fund the bonds plus the incurred cost ongoing maintenance without ending up with too much debt.
The other thing people often get wrong is these things are funded by 20 year bonds, but they last for much longer. If you make the incorrect assumption that something must be replaced when the bond is paid off the suburb cannot support itself. However pipes last a lot more than 20 years in most cases. Even if the road surface only lasts 20 years (which is true in some extreme cases but not in general), the right of way is a one time expensive, and the ground work below the surface is generally one and done so the replacement costs are much less than the first time new road costs.
