How does a weigh bridge measure the weight of a pull, friction coefficient etc?
Try it, next time you are on a weigh bridge tie yourself to something off the bridge and pull.
You don't need to know the coefficient of friction you just need to know approximately how much wieght (of pull) is needed to start to lift your landy, with the brakes on, up over a brick (or a slab, or a bit of wood, or a small dead animal, or anything).
If the direction of pull is above your centre of gravity (ie above the bonnet) it will be less, if you are on pea gravel, on an iced over lake, in a soap factory on a rainy day it will be less too.