Slug

Estate agents use creative language, I know. But this sign puzzled me for at least a block on my walk to work this morning.

Here's the property:

Here's a closer view of the sign:

SLUG?

I know ROW.  In fact, I've posted a picture before of the Right Of Way behind this property.  But SLUG?

Real estate agents: helping us ward of dementia by setting puzzles to keep our minds active.  I'll put the answer I came up with in the comments, so that you can think about this a little yourself.