Though much research has been conducted regarding traction of tires in soft granular terrain, little empirical data exist on the motion of soil particles beneath a tire. A novel experimentation and analysis technique has been developed to enable investigation of terramechanics fundamentals in great detail. This technique, the Shear Interface Imaging Analysis Tool, provides visualization and analysis capability of soil motion at and below the wheel-soil interface. The method places a wheel (or other traction device) in granular soil up against a transparent sidewall. While driving or towing the tire, images are taken of the sub-surface soil, and are processed with optical flow software. Analysis of the resulting displacement field identifies clusters of soil motion and shear interfaces. Complexities in soil flow patterns greatly affect soil structure below the wheel and the resulting tractive capability. The Shear Interface Imaging Analysis Tool visualizes and helps analyze these complexities in richer detail than possible before, and allows for a deeper understanding of the physics behind wheel-terrain interaction. Results are presented for rigid wheels at various slip conditions, and various wheel configurations such as diameter, grouser spacing and compliance.