I’m not certain, but I believe the 40s refers to the numbered streets in midtown Manhattan. Five lanes deep means traffic was clogging every available lane on the avenue. “Throbbing” refers to the hustle and bustle or high energy and excitement of NYC nightlife. I’m not sure about “dark” — perhaps there was a power failure (“blackout”). Depending on the technology of the time, a blackout could have caused traffic lights to malfunction, thus aggravating the nighttime traffic jam.