No, not bending moment. The equation D = W*L^3 / 3*E*I is the deflection of a cantilever beam, one end fixed and the other free, at the point where the load (or impact force in this case) is applied. I see that Rolschwarz has just explained this.
Surely, this is also the stopping distance for the vehicle and is why we can equate the two equations !!! By using this method, we dont have a problem with time, because time doesnt come into the equation, we have used `distance` instead. We cant calculate the impact force directly from the KE equations, because we dont know what the stopping distance is. But, because the vehicle has hit a pole, which will deflect by an amount proportional to the impact force we can use the above equation to calculate this deflection or stopping distance.
The bending moment on the pole is, the impact force x the distance up the pole to the point of impact.
All the best,