Maybe this:
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Global "random"
Every tick: set random to rand(20)
Compare values: tickcount%5 = random
-> Create "Car" at ((rand(3)%2) * 100, rand(6)*10)
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I guess this is a simple way of randomizing time... if you change rand(20) to something higher, it will be less likely to happen
(rand(3)%2) * 100 will create the car either at left or right
rand(6)*10 is to create the car randomly at 7 different Y positions with 10 pixels between them