Iswet_flag

To avoid or suppress confusion about Iswet flag:

The wet condition of OSM ways may affect their attractiveness for the route planning. Depending on riding purpose and personal perception sensitivity*), there is a point where a biker re-evaluates his/her priorities about choosing alternatives.

  *) The main purpose of iswet in bike context is for trekking bikes on mixed surface OSM way network.
  Road bikers may find just a limited use for cobblestone/sett road cases.
  MTB bikers riding soil only trails may find no use for it. 
  Some waterproof hardcore bikers may not feel affected by weather nor surface wet conditions, 
  so iswet flags is definitely useless for them. 

Cobblestone, setts, stones generally and grass can be slippery.
Sand, dust and fine gravel can get addhesive to equipment.
Soil and dust can get sticky, muddy, slippery, "splashy" and "pondy".

Iswet flag attempts to address such temporary conditions of natural origin by different set of road preferences, compared to dry conditions. Persistent wet condition is out of iswet scope. There are ways of their proper OSM tagging, like surface=mud, or ford=yes.

Iswet flag is intended for such situations, when a biker feels surface wetness in the area crossed his/her wet decision point. Most bikers are familiar with typical scenario "I usually take a shortcut along this soily but fine track, but if it is muddy*), I rather take a mainroad detour." If a biker never feels his "wet condition decision point" is reached, iswet flag is not for him/her. Others may find a use for it.

*) What is muddy is biker dependent.

Iswet decreases preferences ( by various amount ) for such surfaces, that according to OSM tagging can be weather affected. Like those getting slippery ( cobblestones / setts / grass) , muddy ( ground, earth, dust) or unpleasant ( sand, fine gravel ). OTOH, it mildly increases preference of paved ways( not the case for cobblestones/setts ) to stay on the more convenient ( or safer in cobblestone/sett cases ) side.