Requeim for a Civil Defense Logo

FEMA et al have released a brand new logo to replace the old Civil Defense logo initially adopted in 1939.

Quoting the NY Times:

The CD insignia, which the association called “a relic from the cold war,” was eulogized by Richard Grefé, the executive director of the American Institute of Graphic Arts.

“The old mark fits in the same category of simplicity and impact occupied by the London Underground map,” Mr. Grefé said.

I must be missing a historical allusion. Is a map of the London Underground a simple and impactful object?

Please, if I’m missing what this means, inform me…