Platoon dispersion is a key element in traffic simulation models designed to measure arrivals at traffic signals. One of the most popular models is contained in the TRANSYT program. In this program, dispersion is defined by the platoon dispersion factor, which is given for three types of conditions in relation to external friction. This study demonstrates that platoon dispersion depends not only on external friction but also on internal friction between vehicles in the platoon. As volumes and densities increase, platoon dispersion increases up to a maximum, which is attained at half the capacity. As volumes and densities increase further, dispersion decreases and reaches a minimum value at volumes around maximal capacity. Experimental analysis on eight arterial sites in the city of Montreal (Quebec, Canada) has confirmed this relationship between dispersion and traffic volumes. Mathematical models (having a parabolic shape) relating platoon dispersion to internal and external friction were developed for three different categories of arterial streets representing low, moderate, and heavy friction levels. These models could be introduced into signal coordination programs and could contribute to a better simulation of arriving platoons at intersections as volumes change in relation to time.