Geographic proximity is acknowledged to be a key factor in research collaborations. Specifically, it can work as a possible substitute for institutional proximity. The present study investigates the relevance of the "proximity" effect for different types of national research collaborations. We apply a bibliometric approach based on the Italian 2010-2017 scientific production indexed in the Web of Science. On such dataset, we apply statistical tools for analyzing if and to what extent geographical distance between co-authors in the byline of a publication varies across collaboration types, scientific disciplines, and along time. Results can inform policies aimed at effectively stimulating cross-sector collaborations, and also bear direct practical implications for research performance assessments.