Type IV pili (T4P) are protein filaments used by certain Gram-negative bacteria for a mode of surface motility called twitching. We have developed experimental and data analysis techniques to quantify the expansion of twitching P. aeruginosa colonies at an agar-glass interface as a function of agar concentration C. The advancing front consists of finger-like protrusions of bacteria, with cells in the expanding colony arranged in a lattice-like pattern. We find that the average finger width increases linearly with C, whereas the average finger speed is independent of C. This latter result, coupled with the observed transition from monolayer to multilayer coverage within fingers, suggests a critical number of T4P are required to break through the agar-glass interface. In addition, we have calculated line profiles across fingers, quantifying their average speed and bacterial orientation, and performed high spatial and temporal resolution studies of the fingers.