# Question about drift velocity in a circuit?

I was trying to find out why the drift velocity is proportional to the electric field applied in a circuit. I came across this website which provided a derivation. However I am stuck on one point.

In the derivation, it is stated that

Average random velocity of free electrons, u=0. If ‘v’ is the velocity just before the start of next collision the n,

$$Drift \, \, Velocity \, \, v_d = \dfrac{u + v}{2} = \dfrac{0 + v}{2} = \dfrac{v}{2}$$

My question is: why is it the case (or if it is not exactly true, why can we assume) that the electron looses ALL of its kinetic energy in a single collision and therefore the starting speed (just after a collision) is zero?