poisonous?

Last night, my roommate and I were walking along the backside of our dorm, which faces the FGCU lake, and came across a coiled up snake at the bottom of the stairs. It was a skinny snake and as it uncoiled and slithered away, we realized just how long it was. For being so skinny, the snake was probably at least 3 feet long. My roommate was freaking out because she automatically said it was poisonous because it had yellow on it... and she's an environmental major. However, I am pretty sure that just because a snake is red, yellow, and black, doesn't mean it is poisonous, but matters on the order of the colors. I would have liked to have know for sure by looking at the color order, but it was too dark to tell and it slithered away.