Optimal food supply is essential for any animal, and the behavioural mechanisms underlying the way animals manage to regulate their food intake have been well studied at an individual level. Little is known however about how collective groups, where individuals share food, function as regards food collection. Social insects are especially interesting as only a small fraction of the individuals collect food for the entire colony. Among social insects ants and termites are particular because, along with humans, they are the only animals known to display bidirectional flows along the paths they share during collective moment. In their natural environment ant trails can be physically constrained to a very narrow path, and there can be so many individuals on a trail that maintaining smooth collective motion might become challenging. As foraging is costly (in terms of energy andmortality risk) and essential to colony survival and success, we expect foraging efficiency to be maximized. We investigated how colonies of Argentine ants (Linepithema humile) regulate their traffic on foraging trails. We used different colony sizes to find wether a same ratio of foragers to the total numbers of workers was maintained across a range of colony sizes. We also manipulated trail width by forcing the ants to cross a bridge to access a food source. By using different colony sizes and bridge widths, we were able to study flow, speed and the number of contacts between ants across a wide range of density. We found that ants regulated accurately the number of foragers according to colony size. Ants were also able to maintain high flows of foragers even at surprisingly high densities especially when compared to human traffic (pedestrians and vehicles) where flow decreases beyond a threshold density.