The 5 sisters and all the other bings are the waste from the shale mines in the area.
They are mostly Burnt Shale (Blaze), which usually compacts really well.
As far as safety goes, shale lies in layers and can form sharp edges, possibly shred tyres???
There is also issues with the remaining oil in the shale and the storage of heat, which can start fires. (common in the 90's)
Further to this Im sure the 5 sisters is a landmark and there is a nature reserve / SSSI in the area. Vehicle use is prohibited on all the bings around west lothian and lanarkshire and will usually involve the police showing up.
Most of the bings are being cleared for development or to use the shale as a fill material in construction projects.