Highways were routed to minimize land acquisition costs.
When the highways were designed sixty to seventy years ago, blacks typically occupied only a small part of any city.
In 1940, the Bronx was 98.3% white according to Wikipedia.
https://en.wikipedia.org/wiki/Demographics_of_the_Bronx
Bridges were built 30, 40, 50, even 60 and 70 years ago. Buses have gotten taller, and longer (accordion buses, for example), just in the last 20 years.