[emathias]

Grids aren't necessarily more efficient, it really depends on the use. They probably do make Manhattan more efficient, and maybe Chicago, but it's argue that is only because Manhattan is long and thin, so there just aren't many 45 degree trips. Similarly, in Chicago, the density of the city is pushed up narrowly along the Lake, which is also where diagonals are also more common. I don't think it's a coincidence that some of the most popular neighborhoods away from the Lake are bisected by diagonal streets.

In an unbroken grid, and trip that requires you to go an equal distance North or South and East or West is, by the rules of geometry, 40% longer than perfectly point-to-point trip would be, whereas the worst trips in Paris rarely add more than 25%. In cars, this can be worse if measuring in time rather than distance, but the addition of grand avenues and boulevards linking the most important points reduces that while leaving shorter routes for the pedestrians and Metro. And once you realize that to optimize for car traffic density goes down, it becomes apparent that grids really increase the perception of efficiency more than they actually increase absolute efficiency.