But do it for actual directions, would mean calling a directions engine, a little over 2 billion times, being as the times would be different for each direction. Clearly that is out of the question for most people, 'we' certainly cant really do that against Googles data; Google I guess could, but its still a lot of work.
So I would hazard a guess that such a source doesnt exist. GPS devices etc will calculate it live (or a small cache) against a network map.
Why do want it anyway? What is your application? Can you use something like the Google Maps API (ie are you showing a map?) If so you could use the current table to get a estimate, and then only run a actual directions request for the small sample, and store that.