Agnew's Snowplow Problem

breed [plows plow]
plows-own [ distance-traveled ]
patches-own [ snow ]

to setup
  clear-all
  ask patches [
    set snow starting-snow
  ]
  snow-color
  set-default-shape plows "default"
  create-plows number [
    set color red
    setxy 0 0
    set heading (random 4) * 90
  ]
  reset-ticks
end 

to go
  snow-fall
  snow-color
  ask plows [
    move
  ]
  tick
end 

to snow-fall
  ask patches [
    set snow snow + 1
  ]
end 

to snow-color
  ask patches [
    set pcolor scale-color gray snow 0 (contrast * plow-rate)
  ]
end 

to move
  set snow (snow - plow-rate)
  if snow < 1 [
    set snow 0
    set pcolor black
    if (pxcor mod street-spacing = 0) and (pycor mod street-spacing = 0) and (random 100 < turning-frequency) [
      rt (random 2) * 90
      lt (random 2) * 90
    ]
    fd 1
    set distance-traveled distance-traveled + 1
  ]
end