  The radio luminosity function (RLF) of radio galaxies and radio-loud quasars
is often modelled as a broken power-law. The break luminosity is close to the
dividing line between the two Fanaroff-Riley (FR) morphological classes for the
large-scale radio structure of these objects. We use an analytical model for
the luminosity and size evolution of FRII-type objects together with a simple
prescription for FRI-type sources to construct the RLF. We postulate that all
sources start out with an FRII-type morphology. Weaker jets subsequently
disrupt within the quasi-constant density cores of their host galaxies and
develop turbulent lobes of type FRI. With this model we recover the slopes of
the power laws and the break luminosity of the RLF determined from
observations. The rate at which AGN with jets of jet power $Q$ appear in the
universe is found to be proportional to $Q^{-1.6}$. The model also roughly
predicts the distribution of the radio lobe sizes for FRII-type objects, if the
radio luminosity of the turbulent jets drops significantly at the point of
disruption. We show that our model is consistent with recent ideas of two
distinct accretion modes in jet-producing AGN, if radiative efficiency of the
accretion process is correlated with jet power.
