I had a conversation once with a herpetologist that told me the following:
Due to loss of habitat, snakes have been "balling" together over the winter and interbreeding. Even a harmless snake may develop poisonous characteristics.

I had the chance to observe this first hand one day. Just like you, I was driving down the road when I saw a 6' long ribbon lying across the road.
I stopped, got out, and the snake "coiled" up on the shoulder of the road.

From all appearances it was a Black snake, which is harmless. However, after coiling up, It started shaking its tail as if it had rattles. A long stick placed by its head caused it to strike. This thing had 1 1/2" fangs.

Crossbreed..........

Even non poisonous snakes might give you tetanus if they bite.