It should be realistic but not conservative. 'Biggest' fulfilled length requirement, better would been 'most interesting'.
In cities in the U.S. you frequently see power lines, phone lines, cable tv, etc. on poles in the air instead of underground as you see in other countries.
In Buffalo NY there are neighborhoods that run that infrastructure through back yards and alleys which gives a very "European" look to the area.
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Also consider eliminating or greatly curtailing parking, certainly no "free' parking -- if you get rid of the parking, traffic goes away too, you then don't have to build roads. Visiting Montreal as a motortourist is a great experience, but you will pay for parking.
Having lighter vehicles on the surface might have the additional advantage that you could bury utilities in such a way that they could be accessed for maintenance and repair without digging. Perhaps with just some sort of sturdy lid over the trench. That would save a ton of expense and inconvenience for maintenance.