This post has been very interesting and has resulted in some additional study on my part. I found I was distracted by more obvious problems and needed the have the blinders removed.
My focus on Top Down Cracking has been on subgrade soil reaction because the vast majority of our local pavement failures actually point to subgrade failures. I have observed a local tendency, featuring local governments, to blame the asphalt layer for most failures because then the contractor can be pressured to redo projects which do not work out. I am finding that our municipal project geotechnical exploration, design and final specifications often result in a minimal amount of subgrade preparation. Of course, more subgrade preparation implies more initial project cost, which is undesirable.
I am also distressed by the idea of 'Standard Specifications' which prove to be very resistant to change, or even serious discussion of change. As our area grows, the new construction gets into areas which have differing soils, climatic issues and other changes from 'the old part of town'. You know the drill, "this has worked for x number of years and how dare you suggest a problem". But, as construction continues in previously undeveloped (read arid, non-irrigated) areas, failures continue to occur with more frequency.
I have gone this far in order to say that local experience is STILL struggling through the basics. My issue of true Top Down Cracking is probably hidden within more elementary problems of design, specification and construction. In the course of my recent research, I 'googled' this. Hope it helps.
National Cooperative Highway Research Program - Completed Project Project 1-42 Top-Down Fatigue Cracking of Hot-Mix Asphalt Layers - Phase I