So the options at the time were riding on the sidewalk or in the street, and Effective Cycling became popular because it contained some genuinely good advice for cycling on a street with no infrastructure. Unfortunately Forester felt that this was the best and only way to safely cycle, which is why he immediately became popular with state and local DOTs who did not want to build bike lanes.
It is not an exaggeration, he set national cycling infrastructure policy in the US that stood for decades.
https://en.wikipedia.org/wiki/John_Forester_(cyclist)#1978_C...