How do I make sure that i don't spawn too many objects in Y?

I am making a game with 4 different lanes where obstacles will spawn in all 4 lanes at random intervals. One thing i want to avoid is that obstacles will spawn in such a way that they are impossible to avoid as shown in the picture. Someone told me i should probably use offset Y but he wasn't sure how and i am pretty new with Construct 2. Is there someone that would be so kind as to explain this to me?

Edit: for some reason the imgur link isn't working with the img tags. But imagine having 4 lanes next to each other where 4 obstacles in a row would be impossible to pass. Also the lanes are horizontal, going from the left of the screen to the right.