Add Differing Ticker Times - scheme

Question

Hi can you help me add a way to have different timer lengths for each light. I'm doing a world state/big bang program that creates a traffic light that cycles between green, yellow and red while it goes immediately to green if you hit the Spacebar. I have everything running but I couldn't adjust the ticker to be different its always a constant time.
(require 2htdp/image)
(require 2htdp/universe)
;; =================
;; Constants:
(define WIDTH 600)
(define HEIGHT 350)
(define MTS (empty-scene WIDTH HEIGHT))
(define RAD 50)
(define WIDTH_2 (/ WIDTH 2))
(define CTR-Y (/ HEIGHT 2))
;; =================
;; Functions:
;; trafficLightNext Tests
(check-expect (trafficLightNext "red") "green")
(check-expect (trafficLightNext "yellow") "red")
(check-expect (trafficLightNext "green") "yellow")
;; Traffic Light -> Boolean
;; Find the current-state of a Traffic Light (red, yellow or green)
(define (isRed? current-state)
(string=? "red" current-state))
(define (isYellow? current-state)
(string=? "yellow" current-state))
(define (isGreen? current-state)
(string=? "green" current-state))
;; Traffic Light -> Traffic Light
;; Finds the next state for the Traffic Light
(define (trafficLightNext current-state)
(cond
[(isRed? current-state) "green"]
[(isYellow? current-state) "red"]
[(isGreen? current-state) "yellow"]))
;; Render Tests
(check-expect (bulb "red" "red") (circle RAD "solid" "red"))
(check-expect (bulb "green" "green") (circle RAD "solid" "green"))
(check-expect (bulb "yellow" "red") (circle RAD "outline" "red"))
(define (light=? current-state color)
(string=? current-state color))
;; Traffic Light -> Image
;; Renders the the light
(define (bulb on c)
(if (light=? on c) (circle RAD "solid" c) (circle RAD "outline" c)))
;; Traffic Light -> Image
;; Takes a Traffic Light places the image on the scene
(define (trafficLightRender current-state)
(place-image
(bulb current-state "red")
WIDTH_2
52
(place-image
(bulb current-state "yellow")
WIDTH_2
CTR-Y
(place-image
(bulb current-state "green")
WIDTH_2
298
MTS))))
;; TrafficLight -> TrafficLight
;; Traffic Light changes every second
(define (traffic-light-simulation initial-state)
(big-bang initial-state (on-tick trafficLightNext 1) (to-draw trafficLightRender) (on-key ambulance)))
;; Key -> TrafficLight
;; Changes light to green everytime key is touched
(define (ambulance initial-state key)
(cond [(key=? key " ") "green"]
(else initial-state)))
(check-expect (ambulance "yellow" " ") "green")
(check-expect (ambulance "red" " ") "green")
(check-expect (ambulance "yellow" "d") "yellow")

Answer 1

As this looks like a school assignment, I won't give you a full solution, only leads.
It would have been easier if the rate-expr of the on-tick clause of big-bang were a function taking the current state as input, but this is not the case, so you need a (functional) way around.
One possibility is to make your world state a bit more complicated: instead of being only the current light, it could be the light plus a countdown value. At each tick, you do not immediately change the light, but instead you decrement (subtract 1 from) the counter of the state. When the countdown hits 0, you change the light and reinitialize the countdown to a value that depends on the new light.
The main changes are in the trafficLightNext function and tests, but the rest of the program must also be modified, as the state is different.