The external surface sets the internal load
A car park with loose gravel at its edges, a damaged section that holds water or an unswept approach delivers a continuous supply of soil to every entrance on the site. Increasing internal cleaning frequency addresses the symptom at the most expensive point. Sweeping the approach and dealing with the source is cheaper and improves the result everywhere, which is why the external zone deserves scoping properly.
Pedestrian routes matter more than the parking bays themselves. The paths people actually take from vehicle to door, including any informal shortcut across a garden bed or an unsealed edge, are where the soil is picked up. Mapping those real routes rather than the designed ones is a short exercise that usually identifies one or two specific problems responsible for most of the load.
