Evolution of the distribution of the relative angle between the velocity vector of the filament crest and the filament direction (θ_Vφ) in High_B (top) and Low_B (bottom). The black dashed lines mark the random distribution for the relative orientation of two independent vectors in 3D. The distributions of θ_Vφ show a marked tendency of the filaments’ crest velocity to align with the filament over time. At earliest stages, the velocity distribution of μ = 2 shows a preference towards perpendicular configurations (top left). After the onset of the H II region, both distributions shift to resemble the random distribution (last column).