How it was done:
For the specified number of journeys, a random selection of start and end points are chosen randomly within the specified radius (cogniscent of the non-linearity of GPS units versus metres).
For each of these journeys, 3 distances are noted. The direct line-of-sight distance, the distance to walk it as per google maps and the distance to drive it.
Using this, a ratio figure is constructed to reflect the extra distance a car must travel (due to one-way streets etc) compared to a simple walk.
This ratio thus reveals an aspect of each city, with respect to how efficient its street organisation is.