Here is a simple way to scale the difficulty. Of course, there are many others, but I'm just sharing a simple idea to spark your interest.
You have a variable you use to control how much time the player has until they reach the light, right? Let's call that variable X.
Well, you can scale the difficulty from one round to the next by making sure the time slightly decreases each round. The first round, it can be X... the second round, X * 0.99... the third round, X * 0.98... the fourth round, X * 0.97... in other words, each round the time limit will be equal to X * (100 - Y), where Y is the number of the current round. This is of course a very subtle slope, which would take a long time for the player to notice; I'm just giving this as an example of how you can build a scale.
Happy building!