I used to catch them when I was kid and living in different parts of the city believe it or not...I now live in an area where they seem to be quite plentiful as when I had bought and just moved into my current home the front and back yard was full of them.I still see them pretty regularly and I suspect they live underneath my porch.
Anyone ever see a brown snake flatten its head like that before, though? And all the browns that I have interacted with have been slender little guys. Body shape and head-flattening behavior make me want to vote for garter.
Remember kids, this species is in the subfamily Natricinae which includes Garters, Dekays and Water Snakes. That's why you're seeing similar traits across the board.