When are you most aware of your motion in a moving vehicle: when it is moving steadily in a straight line or when it is accelerating? if you were in a car that moved with absolutely constant velocity (no bumps at all), would you be aware of the motion?
(a) accelerating. you would not be aware of the motion if you did not look outside the car.
(b) accelerating. you would be aware of the motion because humans can sense speed.
(c) moving steadily in a straight line. you would be aware of the motion because you can feel velocity even in a closed car.
(d) moving steadily in a straight line. you would be aware of the motion because you can feel the speed.