  Spiral arms are some of the most spectacular features in disc galaxies, and
also present in our own Milky Way. It has been argued that star formation
should proceed more efficiently in spiral arms as a result of gas compression.
Yet, observational studies have so far yielded contradictory results. Here we
examine arm/interarm surface density contrasts at ~100 pc resolution in 28
spiral galaxies from the PHANGS survey. We find that the arm/interarm contrast
in stellar mass surface density (Sigma_*) is very modest, typically a few tens
of percent. This is much smaller than the contrasts measured for molecular gas
(Sigma_mol) or star formation rate (Sigma_SFR) surface density, which typically
reach a factor of ~2-3. Yet, Sigma_mol and Sigma_SFR contrasts show a
significant correlation with the enhancement in Sigma_*, suggesting that the
small stellar contrast largely dictates the stronger accumulation of gas and
star formation. All these contrasts increase for grand-design spirals compared
to multi-armed and flocculent systems (and for galaxies with high stellar
mass). The median star formation efficiency (SFE) of the molecular gas is 16%
higher in spiral arms than in interarm regions, with a large scatter, and the
contrast increases significantly (median SFE contrast 2.34) for regions of
particularly enhanced stellar contrast (Sigma_* contrast >1.97). The
molecular-to-atomic gas ratio (Sigma_mol/Sigma_atom) is higher in spiral arms,
pointing to a transformation of atomic to molecular gas. In conclusion, the
boost in the star formation efficiency of molecular gas in spiral arms is
generally modest or absent, except for locations with exceptionally large
stellar contrasts. (abridged)
