I've been a BSL user my whole life, and I've signed for longer than I've been talking (signing since about 1 year old, and talking since about 5. Still haven't shut up in either language
), so the amount of "thought processing" I've got to do in order to use BSL is probably equal to the thought processing that hearing people have to go through in order to speak.
That would need some kind of brain wave analysis to prove one way or the other, but think that talking only requires moving the mouth and vocal cords and no other muscles. Signing is going to affect your driving in a number of ways just connected with using an arm:
- taking a hand off the steering wheel - you now have to steer with one hand (yes, we can all do that), you have to transition from two hands to one (may require a momentary change of concentration.
- you move your hand to the left away from the wheel. Next time you do this, check whether your gaze remains directed forwards, or follows your hand as you move it into the free space to your left.
- your vision may follow your hand to the left. Even if it doesn't initially, it's likely to follow your hand to some extent while you are talking with it.
- as your hand, and potentially your vision moves to the left, you now have to change your steering focus slightly to maintain a straight course.
Try the following. While driving in a straight line, look to the left (not necessarily at right angles, perhaps just at the centre of the windscreen below the RVM). Do this for 2 seconds, then look back at the road and see if you're still on the course you thought you were.
Now take your left hand off the wheel and put it in the free space in the centre of the car as well. Again, hold for two seconds, then check your course on the road.
Now do the same but make some signs for 2 seconds, go back and check your course relative to the white lines on the road.
If you continue to drive in an absolutely straight line without thinking about it, you're safe. Any deviation, and we've shown that even your basic control of the car's direction is affected. This doesn't even acknowledge any loss of mental focus that may be accompanying the distraction - loss of peripheral vision to the right, loss of attention on events in front, etc.