Then we have the cleaning trucks that spray water but they also have brushes fitted to them so they can clean the kerbside as well.
They have those cleaning trucks too here in Warsaw, but the ones i'm talking about don't do the kerbside, they just spray the whole road with massive amounts of water (well I assume water).
Dust must be removed.
They're not that dusty though, it's Warsaw centrum I'm talking about here. Most of the roads have been resurfaced in the last 4 years.
It is to keep them from drying out
It's modern tarmac they're spraying, i could be wrong but i don't believe it can 'dry out'