Grid layouts don't optimize for heavy traffic [1] because drivers choose shorter paths, making it worse for all [2]. In real life you don't have grid layouts without large roads, one-way streets, overpasses and other ways to improve flow.
A tree layout avoids this by directing through traffic to higher-capacity roads without intersections [3]. The downside is that individual commutes get longer. It's just one possible solution, but an interesting one because it improves traffic (removes the Prisioner's dillema) by closing roads (denying options to the game actors).
[1] http://en.wikipedia.org/wiki/Grid_plan#Late_19th_century_to_...
