Parameters of the Galactic density-wave spiral structure: Line-of-sight velocities of 156 star-forming regions

•The line-of-sight velocities of HII regions in the solar vicinity are studied.•The minor effects caused by the Lin–Shu density waves are taken into account.•Several spiral patterns with different number of arms m may coexist in the Galaxy.•The Sun lies between the main trailing spirals in Sagittarius and Perseus.•The single-armed m=1 spiral mode dominates the pattern.