The carbon footprint of motorized transport modes per unit length traveled encompasses the unit share of the vehicle lifetime emissions, that of the transport infrastructure, and those of the motor energy, considered both from “well to tank” and from “tank to wheel”. In the active modes of transport, i.e., walking and cycling, the counterpart of motor energy is human energy, which is associated with two kinds of carbon flows: the carbon footprint of food intake, – which we call the Food to Body component – and the carbon dioxide emissions of respiration – say the Body to Foot component. In this article, we provide a model in simple mathematical form to assess those carbon flows per unit length. It involves the modal speed in (i) the Metabolic Equivalent of the Task (MET) which gives rise to the energy and carbon flows, and (ii) the ratio of time spent to length travelled. The two influences of speed onto a modal carbon footprint combine in the net MET per unit length, with some compensation. The carbon footprint of food intake varies widely depending on the food diet of individuals. In a numerical study, the Food to Foot carbon emission of walking, cycling, e-scooter riding, and driving a car are estimated and compared to the rest of modal carbon footprint. Under conditions typical of France in the 2010s based on the average food diet and low carbon intensity of electricity, the inclusion of F2F in modal footprints changes the ranking of the modes according to the carbon footprint per unit length.