There must be sufficient tread on the tires and the running surface must be sufficiently rough to make the coefficient of friction as high as practical.
(the running surface; must be; sufficiently rough to make the coefficient of friction as high as practical) (the running surface; to make; the coefficient of friction as high as practical)
the running surface the running surface
must be to make
sufficiently rough to make the coefficient of friction as high as practical
the coefficient of friction as high as practical