  For quantum integrable systems the currents averaged with respect to a
generalized Gibbs ensemble are revisited. An exact formula is known, which we
call "collision rate ansatz". While there is considerable work to confirm this
ansatz in various models, our approach uses the symmetry of the current-charge
susceptibility matrix, which holds in great generality. Besides some technical
assumptions, the main input is the availability of a self-conserved current,
i.e. some current which is itself conserved. The collision rate ansatz is then
derived. The argument is carried out in detail for the Lieb-Liniger model and
the Heisenberg XXZ chain. The Fermi-Hubbard model is not covered, since no
self-conserved current seems to exist. It is also explained how from the
existence of a boost operator a self-conserved current can be deduced.
