https://github.com/ros-planning/navigation2/blob/main/nav2_regulated_pure_pursuit_controller/src/regulated_pure_pursuit_controller.cpp#L375

The number of points to lookahead terminates with the carrot. Since the PP algorithm creates an arc that drives to the carrot, that's a natural termination point. However the "max time to collision" then is only able to collision check up to the carrot, and not further.

This seems to be a natural termination point since that's where the arc is leading to. However, it might be useful to decouple these ideas so that you can collision check all the way to the "max time to collision" if set to something like 2 seconds but the carrot is only 15 cm away. Even if you blow past the carrot, the arc is still the trajectory of travel and it might be good to allow collision checking after that fact.