This paper describes the development of a novel method of measuring the rate of crack propagation in asphalt mixtures using image analysis techniques. The techniques developed incorporate methods to observe the crack more clearly as well as a semiautomatic analysis of the images taken during 3 point bend tests on beams of asphalt. The results show that modified binders increase the resistance to crack propagation and that the path of least resistance to cracks appears to be the interface between the coarse aggregate and the matrix In addition a model is presented for both polymer modified and unmodified asphalts for calculating the effect of crack propagation on analytical design techniques. The model developed is based around the 3 point flexure of beams of asphalt and monitoring the degradation of load (stiffness) with time.