Helvetica and the New York City Subway system

“For years, the signs in the New York City subway system were a bewildering hodge-podge of lettering styles, sizes, shapes, materials, colors, and messages. The original mosaics (dating from as early as 1904), displaying a variety of serif and sans serif letters and decorative elements, were supplemented by signs in terracotta and cut stone. "

Towards the end of the 1960s, Helvetica was chosen by the designers Massimo Vignelli and Bob Noorda to create the new signage for the New York Metro and the Graphic Standards Manual, one of the most famous visual identity manuals in the history of graphic design. Helvetica was originally created in Switzerland. It was a neutral typeface from a neutral country and gained runaway popularity starting in the 1960s for its modern grace. Helvetica became the winning typeface to display the New York City subway system due to its legibility as well as its visual accessibility.

See also: MIT Press book description:
https://www.amazon.com/Helvetica-York-City-Subway-System/dp/026201548X

and this blog post by Sarah Cantavalle:
https://www.pixartprinting.co.uk/blog/history-font-helvetica/