Staying in the middle of the track is not a necessary requirement in the reward function. The reason I include it is to speed up the learning time in the beginning. You can remove it once you learn a reasonable policy and see it the agent can find the optimal apex path. I will do a test tonight.
Just like in human world : You first learn how to drive before you learn how to drift the car.
Just like in human world : You first learn how to drive before you learn how to drift the car.