You can dissipate some amount of energy per second when breaking. The energy is proportional to the velocity squared, so the breaking distance is proportional to the velocity squared. The density is inversely proportional to the spacing (here the breaking distance). The flow is density times velocity, which is ~ 1/V^2 * V ~ 1/V.
Therefore, the density decreases linearly with speed.
That means that the slower you go, the more parcels per seconds you will get, because the breaking distance increases more quickly than the velocity!
Of course this is a simplification, Because this assumes the trucks are points.
The best case of this model would be an infinite density of non-moving trucks!
In reality trucks have some length, which means that they can not be infinitely close, so 30kmph is probably close to the optimum.
That means that the slower you go, the more parcels per seconds you will get, because the breaking distance increases more quickly than the velocity!
Of course this is a simplification, Because this assumes the trucks are points. The best case of this model would be an infinite density of non-moving trucks! In reality trucks have some length, which means that they can not be infinitely close, so 30kmph is probably close to the optimum.