I'm not sure people will interpret pointing to the ground as meaning "I'm going straight."

I haven't thought of a good hand signal for "I'm going straight." I need this for when I'm on the highway, and I'm going by an exit without taking it. I'd use the left hand in a tomahawk gesture, but there's no guarantee that a driver will know what that means.

I take the red path rather than the blue, but I always watch the cars behind me with my mirror.