Something to alert you that the driver training has maxed for that particular area.
For example, in Physical, when driver has 20 in stamina and you get the weight to 46/51Kg the text could change colour when the "-1 Weight" text appears, so you know to change training. Either that or could the bar change colour when on the final skill in that area - anything to draw the attention that you now no longer need to train the driver in that area/switch training.
Would stop "over training" that Im sure a few of us have all done at some point...