800. Piling sandbags in the back of a truck would increase friction between the wheels and the ground, but wouldn't it also increase the truck's inertia, making it harder to stop on an icy road?
Adding sandbags to the back of a pickup truck increases the truck's traction and adds to the truck's mass. Fortunately, the truck's traction increases more dramatically than its mass and it becomes easier to start and stop the truck, rather than the reverse. That's because even a modest amount of sand can double the force pressing the rear wheels against the road and thus double the frictional forces the wheels can experience. That same amount of sand won't double the total mass of the truck.