Well-used corridors become dirty with time

code/game/objects/items/weapons/mop.dm

@@ -23,6 +23,7 @@
 obj/item/weapon/mop/proc/clean(turf/simulated/A as turf)
 	reagents.reaction(A,1,10)
 	A.clean_blood()
+	A.dirt = 0
 	for(var/obj/effect/O in A)
 		if( istype(O,/obj/effect/rune) || istype(O,/obj/effect/decal/cleanable) || istype(O,/obj/effect/overlay) )
 			del(O)

code/game/turfs/simulated.dm

@@ -8,7 +8,7 @@
 	nitrogen = MOLES_N2STANDARD
 	var/to_be_destroyed = 0 //Used for fire, if a melting temperature was reached, it will be destroyed
 	var/max_fire_temperature_sustained = 0 //The max temperature of the fire which it was subjected to
-
+	var/dirt = 0
 /turf/simulated/New()
 	..()
 	levelupdate()
@@ -27,6 +27,11 @@
 	if (istype(A,/mob/living/carbon))
 		var/mob/living/carbon/M = A
 		if(M.lying)	return
+		dirt++
+		if (dirt > 40)
+			dirt = 0
+			if (!locate(/obj/effect/decal/cleanable/dirt, src))
+				new/obj/effect/decal/cleanable/dirt(src)
 		if(istype(M, /mob/living/carbon/human))
 			var/mob/living/carbon/human/H = M
 			if(istype(H.shoes, /obj/item/clothing/shoes/clown_shoes))