The role of Coulomb collisions in the solar wind, their number of occurrence for protons, and the correlation between collisional effects and the stream structure and large-scale interplanetary magnetic field are investigated. Helios plasma data, displayed in Carrington rotation plots, clearly show that the relative number of collisions (ratio of expansion over collision time scale) maximizes near the heliospheric current sheet, which is revealed as the main collisional domain of the solar wind. Statistical results indicate that for about 20% of the time the low-speed solar wind plasma is influenced strongly by Coulomb collisions.