what i've found is that it has to do with shearing forces on the very rough roadway (and treads on street tires) and the adhesion of rubber as it heats up.
The fact that physics theorys are based on perfect situations doesn't completely screw them up in the real world, though. What we did in class was to take a wood block about 6x8in on one side (area of 48 sq/ins), and only 1x8in on the other (8 sq/ins). We hooked a scale up to the block, and pulled it along the table on the large side, saw that it took X newtons to pull. Then put it on the thin side, and it would still take X newtons to pull.
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