Top panel: velocity difference between ISM lines and [CII] vs. velocity difference between Lyα and [CII] for the 29 objects for which we were able to reliably estimate z_ISM. Red, green, and blue circles indicate objects with f_esc(Lyα) < 0.05, 0.05 <  f_esc(Lyα) < 0.3, and f_esc(Lyα) > 0.3, respectively. The yellow and cyan regions indicate the regimes of slow and fast outflows in the uniform expanding shell model by Verhamme et al. (2006, 2015): for v_out >  300 km s⁻¹, the Lyα offset is expected to always be small, regardless of the value of the outflow velocity; for v_out <  300 km s⁻¹, the Lyα offset is determined by the neutral hydrogen column density NHI. Bottom panel: histogram of the Δv_Lyα for all 53 galaxies in the sample (red) and for the 29 galaxies (green) for which we were able to determine z_ISM, which are shown in the upper panel.