  Cloud collision have been proposed as a way to link the small-scale star
formation process with the observed global relation between the surface star
formation rate and gas surface density. We suggest that this model can be
improved further by allowing the productivity of such collisions to depend on
the relative velocity of the two clouds. Our adjustment implements a simple
step function that results in the most successful collisions being at the
observed velocities for triggered star formation. By applying this to a high
resolution simulation of a barred galaxy, we successfully reproduce the
observational result that the star formation efficiency (SFE) in the bar is
lower than that in the spiral arms. This is not possible when we use an
efficiency dependent on the internal turbulence properties of the clouds. Our
results suggest that high velocity collisions driven by the gravitational pull
of the clouds are responsible for the low bar SFE.
