  We have examined the ratio between the integrated luminosity of massive young
stellar objects detected by the Red MSX Source (RMS) survey and the mass of
molecular clouds in the Galactic Ring Survey region, as a function of
Galactocentric radius. The results indicate that 60--80% of the observed
increases in the star-formation rate density associated with spiral-arm
features are due to source crowding within the arms. Of the remainder, most of
the increase in the inner Sagittarius arm is due to an enhancement in the
simple star-formation efficiency, i.e. in the number of RMS sources per unit
molecular gas mass. In the inner Perseus arm, the residual increase is due to a
higher than average mean source luminosity, which implies a top-heavy IMF, and
this is entirely due to the presence, in the GRS region, of the W49
star-forming complex, which appears to be exceptional in its nature. The
results also suggest that there is little or no increase in the star-formation
efficiency on kiloparsec scales in the Scutum tangent region which includes
W43. We discuss the possible role played by the spiral arms in influencing the
star-formation efficiency and conclude that the most likely mechanisms are
related to orbit crowding within the arms.
