50mm/sec - yeah, it would have to be one of those bigger ants I saw sometimes outside in the summer, hiding in crevices in the sidewalk, while I was growing up in Eastern Europe.
http://www.asu.edu/clas/sirgtools/ecology-1991.pdf says that Pogonomyrmex rugosus travels at 0.1914 * T - 1.983 meters per minute, for ground temperatures T (in C) between 20 and 40. This is roughly 3.8 m/minute or 60 mm/sec.
The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.
The equation for Messor pergandei is 0.0878 * T - 0.1724 or roughly 40 mm/sec.
So 50 mm/sec seems quite possible.