The flow number simple performance test for asphalt mixtures was one of the final three tests selected for further evaluation from the 24 tests initially examined under the NCHRP 9-19 project. Currently, no standard triaxial testing conditions for the magnitude of the deviator and confining stresses have been specified. The objective of this research was to develop recommendations for the selection of the deviator and confining stresses that best simulate the stress conditions encountered in the pavement under traffic loads. Extensive mechanistic analyses of three different asphalt pavement structures subjected to moving traffic loads at various speeds and under braking and nonbraking conditions were conducted with the 3D-Move model. Prediction equations for estimating the anticipated deviator and confining stresses as a function of pavement temperature, vehicle speed, and asphalt mixture's stiffness have been developed. An increase of 40% was observed for the deviator stress when braking conditions were incorporated. A preliminary validation of the recommended magnitudes for the deviator and confining stresses on a field mixture from WesTrack showed consistent results between the flow number test results and field performance.