Flame spread along horizontal surfaces in the presence of a forced convective airflow depends primarily upon the transfer of radiant energy to downstream surfaces from a flame front that is tilted in the direction of flow. The soot in the flame serves as the primary source of radiation but the soot that emerges from the flame and mixes with the forced airflow also serves to attenuate the radiation thereby decreasing the radiant flux at the downstream surfaces. There is, then, a complex interaction between the quantity of soot produced and the radiative heat transferred to the downstream surfaces. It has also been shown that when flames transition from over-ventilated to under-ventilated conditions, the soot yield increases while both the total combustion efficiency and the convective fraction of the combustion efficiency begin to decrease. This transition occurs when the flame is unable to entrain sufficient air to burn either at the stoichiometric fuel-air ratio or on the over-ventilated side of stoichiometric. When the total and convective combustion efficiencies do not decrease at the same rate, the radiative fraction tends to increase at equivalence ratios in the range of 1 to 1.3. This results in an increase in radiative heat transfer to the downstream surfaces that causes the flame spread rate to accelerate. A flame spread model was developed to quantify these processes. Predictions from the model are in excellent agreement with experimental data for flame spread along surfaces of styrene-butadiene rubber conveyor belts in a largescale fire gallery where the transition to under-ventilated combustion occurs when the flames begin to impact the roof of the gallery.