The problem isn’t just the width of the road. It is the number of people who are outside. R’ Moshe and a few other Rabbonim did some research, and determined that approximately 1/5th of the population of a city can be expected to be outdoors. That would mean that Brooklyn needs a population of 3 million to count as a reshus harabim. Now, it doesn’t have to at any given time – it has to occur only once a year. R’ Moshe said that even if the population is slightly below 3 million, it’s still a reshus harabim, because during the summer, thousands of people go to Coney Island beach, and they’re all outdoors.
Sefardim have it even tougher. R’ Moshe held that people in cars don’t count towards the necessary 600,000. R’ Ovadia Yosef shlita holds that they do count, so for sefardim, you definitely have the 600,000.
I heard all this from a rebbe of mine, who had discussed the issue with R’ Moshe many years ago. He is also a prominent Rav in Brooklyn.