Mechanical forces are important factors in the development, coordination and collective motion of cells. Based on a continuum-scale model, we consider the influence of substrate friction on cell motility in confluent living tissue. We test our model on the experimental data of endothelial and cancer cells. In contrast to the commonly used drag friction, we find that solid friction best captures the cell speed distribution. From our model, we quantify a number of measurable physical tissue parameters, such as the ratio between the viscosity and substrate friction.