  We present a systematic study of collisions of homogeneous, plane--fronted,
impulsive light--like signals which do not interact after head--on collision.
For the head--on collision of two such signals, six real parameters are
involved, three from each of the incoming signals. We find two necessary
conditions to be satisfied by these six parameters for the signals to be
non--interacting after collision. We then solve the collision problem in
general when these necessary conditions hold. After collision the two signals
focus each other at Weyl curvature singularities on each others signal front.
Our family of solutions contains some known collision solutions as special
cases.
