What is the meaning of this traffic sign? ("no HM")

Factual Questions
1

I live in Oakland, CA. I have noticed these signs in Berkeley only, not in Oakland or anywhere else. Pic

What does this mean? And am I an idiot for not knowing (is it common knowledge)? Are these signs common anywhere else?

2

Your link is broken.

I would guess, “No heavy machinery”

3

No Hazardous Materials.

Here is a picture of one.

See Manual of Traffic Signs.

4

Similar signs in the Houston area say “HC”. Why the difference. Hazardous loads are banned on several of the freeways here and this is the sign used here.