"Although it's hard to prove conclusively, it looks like safety concerns played a part in determining where the highways were crossed. Several paths seem to converge at points where on-ramps and off-ramps are separated from the main flow of traffic. Crossing at these points allows the pedestrian to have breaks of median before having to make the next step. It looks as if some people have been willing to sacrifice a certain degree of time in order to cross a little more safely at one of these points."
Thanks to Streetsblog