The population density of NYC will never change. It is to big a place. its too intense. it may vary slightly but the impact will not be great enough to reduce road and rail traffic to the point where the roads require less maintenance. Besides, The cost of living here is already extremely high and has been rising steadily and beyond the scope of what many can practically afford. NYC has already priced many of its former residents out of town. Case and point, Manhattan, over the last decade or two has pretty much gentrified most of it's low and middle income neighborhoods driving many of its former residents out of town. They could no longer afford to live in the gentrified versions of their neighborhoods and guess what ? The void was immediately filled by a wealthier set who can afford to live there. Whats worst, is if anything it has resulted in putting more strain on the roads and mass transit because now, many of those same people who once lived and worked in the city and could walk or take the subway to get to work now have to take to the roads, i.e. cars, buses, etc. to reach the same places they once reached on foot or by bike. And it even had a further detriment in that when these people were forced out (many of varying ethnic groups) it forced a lot of the fascinating cultural and ethnic components that made have NYC a unique and interesting place to visit. Now, we can go a step further and do what was done in Manhattan to Brooklyn, Queens and The Bronx, namely pricing middle class and lower income people out of their neighborhoods and out of the city but all in all it would be a major loss , potentially turning some of those areas into a remnant of downtrodden places like Detroit or Camden NJ but on a larger scale and thus greatly reducing the robust GDP that the city generates and from which the entire state benefits. Or, the other and more likely effect would large scale gentrification forcing lower and middle income workers out of the city. But where do they go ? The outlying suburbs i.e. Westchester, Rockland, Nassau, etc. are already very crowded, extremely expensive and way over the price that many who would be forced to leave the city could practically afford. Their only other choice would be to go further out and their principal mode of transport would then be the roadways and bridges, essentially putting more people on the roads and requiring more road repairs.
I think we have to face the facts. Like it or hate it NYC is one of the biggest and most popular cities in the world. It has among the highest GDP of any city in the world. By the time you raise its already extremely high cost of living to the point where the population within all five boroughs starts to diminish sufficiently the rest of the state, and most likely the whole country will be in dire straits. We have already seen this on a smaller scale in which the cost to live in NYC has risen dramatically yet rather than seeing a decrease in population (i.e. a mass exodus) the city has grown and continues to grow. NYC real estate is not only very expensive, it is extremely desirable and it is in extremely high demand. This hold true not just for Manhattan but for the other boroughs as well. NYC has some of the largest ethnic populations and cultural centers in the country. Its just too big a place, maybe too big for its own good, but, at this juncture in time shrinking it down is not an option. As it stands for now the cost to maintain NYC is high and, like it or not, fair or unfair, some subsidies are going to be needed.
I am not trying to say that various counties in NY should put their needs on a back burner to NYC. Certainly if Buffalo has bridges or tunnels that need fixing that should be a priority for Buffalo and they shouldn't have to put a hold on fixing their bridges while paying to fix some bridge in Brooklyn that they will probably never see or use. And, as a resident of NY State I have no problems if a certain percentage of my money goes to help Buffalo fix its bridges. It may seem insignificant... why should I care less about some bridge in Buffalo ? But I see it as a cumulative effect.One single bridge ,may not matter much to me so, why should i care less ? But, multiply that bridge by numerous other bridges needing repair throughout the state i.e Albany, Schenectady, North Tonawanda, Grand island, Rochester, Troy, Buffalo, Yonkers, White Plains, Brooklyn, Manhattan, Staten Island, The Bronx, etc. and it adds up. As New Yorker's we should all be working together to keep our state functioning at its best, regardless of what part of it we live in.
That is why I consider shared funding and subsidies to be a reasonably policy in helping to keep the state running across the board. After all, I may not live in Buffalo but it is a part of my state. I could find myself living there tomorrow or a year from now.