(A) Three functions are often used to describe vehicular, fluid or pedestrian flows. The Greenshields function (Greenshields et al., 1935) was the first speed-density relation function used to analyze data recorded for vehicular traffic in the field. This function assumes a linear relationship between v and k leading to a parabolic shape relation between q and k. There are two parameters to be determined: the free speed vf corresponding to the speed of an individual without contact and the jam density kj, which is a threshold over which q becomes null. In the Pipes-Munjal function (Pipes, 1953), used for both pedestrians and car traffic, the relationship v - k is given by a power-law. This function requires a third parameter α to describe the function. The third function is the Underwood function (Underwood, 1960), which often describes well pedestrians or fluids traffic, where v decays exponentially rather than linearly. These functions made no a priori assumptions as to the behavior of the individuals and speed-density relationships are only obtained by fitting function to observed traffic data. (B) Experimental set-up. During an experiment, an ant colony (400 to 25,600 workers, 35 colonies in total) had access to a source of food (1M sucrose solution) placed at the end of a bridge (width: 5, 10 or 20 mm). The colonies were starved for five days before each experiment. The sucrose solution was spread over a large surface so that all ants had access to the food. The traffic on the bridge was recorded by a video camera for one hour. Inbound and outbound ants were counted over 1·sec intervals. Counting began as soon as the first ant crossed the middle of the bridge. A total of 170 experiments were performed.