the fuck is "road snot"?

Expanding on the other comment, it's the mixture of snow, slush, salt, and loads of sand/grit that gets churned up by car tires into a brown and sticky slime. It sticks to boots, gets caked on cars behind the wheels, and gets tracked shockingly far into buildings. It's sort of like a "slushee" consistency but made out of nasty brown goo instead of corn syrup.

Probably the slush and slurry from melting snow, salt, and road debris. It's often black once the debris reaches a certain density.

    :-O