go过程中的堆栈溢出（递归太深）错误

Question

运行代码时，有时会出现以下错误：

stack overflow (recursion too deep)
  error while observer running ASK
  called by procedure GO
  called by Button 'go'

这只是间歇性发生，我不确定是什么原因造成的。

to go 
ask cars [
speed-up
fd speed
if([pcolor] of patch-ahead 1 = yellow)
[stay]

 ]


 tick
 end

 to speed-up
  let car-ahead one-of turtles-on patch-ahead 3
 ifelse car-ahead != nobody
  [ slow-down-car car-ahead ]
  [ speed-up-car ]
  if speed < speed-min [ set speed speed-min ]
  if speed > speed-limit [ set speed speed-limit ]
  fd speed
  separate-cars

 end
 to separate-cars ;; turtle procedure
    if any? other turtles-here [
    fd 3
   separate-cars
  ]
 end

to stay
  set speed 0
ask cars
[


   ifelse  count-up = 0 [speed-up
 fd speed
   set color green

 ] [  set count-up  count-up - 1
   stay ]
  ]
 end

to slow-down-car [ car-ahead ] ;; turtle procedure
;; slow down so you are driving more slowly than the car ahead of you
set speed [ speed ] of car-ahead - deceleration
 end

to speed-up-car ;; turtle procedure
  set speed speed + acceleration
end