Despite all other factors involved in the accident, it seems fundamentally hard to make this kind of control obvious with a touch screen. With a physical throttle control, like used in aviation [1], it's more obvious when you're pushing one side or both, as you have both vision and tactile senses to confirm position.

[1] https://tse2.mm.bing.net/th?id=OIP.y8DdQsqhqkM_iA1f8k-qcAHaE...