From Charles Denny Chief Engineer at the Philadelphia Streets Department:
For all new designs of pavement markings we will be following the below standard.
At non signalized locations the stop bar will be located 4 feet behind the crosswalk.
At signalized locations the stop bar is to be placed 10 feet behind the crosswalk.
This will provide two safety benefits, one to pedestrians and the other to bikes. The benefit to pedestrians is that the greater distance from the stop bar lessens the chance of drivers of larger vehicles not seeing pedestrians because of the height of their front ends. For bikes it provides a space for them to move to the front of motor vehicles and get a head start when making left or right turns at intersections.
It's not quite a bike box, in fact it can be argued that legally bicycles are supposed to stop at the bar. But without MUTCD approval it is difficult from a liability standpoint to install bike boxes (the MUTCD is coming up for an update).