Bi-dimensional histograms of the total number of cloud counts (top row) and cumulative molecular gas mass in clouds (bottom row) within bins of 0.1 kpc and 0.25 km s⁻¹, considering the xy-plane distance and the velocity offset to the closest spiral arm. Different cloud subsamples are considered: the distance-reliable sample (left), the subset of this sample that contains an ATLASGAL source (middle), and the subsetwith a high-mass-star-formation (HMSF) signpost (right). The vertical green line indicates the ΔV = 10 km s⁻¹ that we use to define a spiral arm, and the horizontal green line indicates the median spiral width of 580 pc.