Central Square Conundrum

What’s up with this? Two days ago this sign went up on
Inman St. in the middle of Central Square, Cambridge, in front of Hollywood
Express and across
from the Post Office.

What exactly is a "hollow sidewalk"? And why can’t cars stop in front
of it? It’s not like they are stopping on TOP of the sidewalk! What exactly
are they worried about? The road collapsing?  The SIDEWALK collapsing?

This is the kind of conundrum that will disturb my piece of mind until
I get some answers!