Why this is a Folkestone-specific problem
Cheriton is the UK terminal for the Channel Tunnel, and the M20/A20 approach into it carries a constant flow of HGVs and coaches heading to and from the crossing. Properties along and near that corridor pick up diesel particulate and general road film on top of the usual weathering that every Kent driveway deals with, a grime type most towns in this ring simply don't have to think about.
Why traffic film is different from normal dirt
Traffic film is a mix of diesel exhaust residue, brake dust, tyre particles and grit that binds to a surface differently to algae or moss. It tends to sit as a thin, greasy layer rather than growing on the surface, which means a standard wash-and-scrub approach often just smears it rather than lifting it.
What actually shifts it
Hot water pressure washing is noticeably more effective on diesel and oil-based traffic film than cold water, because the heat helps break the oil molecules down before the pressure does the lifting work. For driveways with heavier build-up, a degreasing pre-treatment applied before the wash gives the water something to work with rather than trying to blast through the film cold.
Where we won't just blast and hope
Block paving and resin-bound driveways need the pressure kept moderate even with traffic film present, since excessive pressure can still strip jointing sand or damage a resin surface regardless of what's sitting on top of it. We match the pressure to the surface first, then decide whether a degreaser pass is worth adding.
Sources: SoftWash UK, "How to Remove Oil Stains from Driveways"; Pureseal Services UK, "How to Remove Oil Stains From Driveways".