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. I

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.