In another thread, Rigpig wrote:
The 'Speedo' page on the main SS site offers a compelling argument suggesting that constant speedo checks detract from the time one has to observe the road ahead. Little wonder that this hook has been grasped by so many. Yet I've always thought of it as being a bit of a red herring - an argument based on specious thinking.
Human beings do not, as handy pointed out 'time slice'. We don't view the world as a series of individual images running like a cine film. When we glance down at our speedo (as apart of the normal driving process as opposed to tuning the radio which is entirrely different), we don't throw away a number of frames from the scene ahead and when we look back to the road we don't need to rebuild the scene afresh. It's all part of a smoothly running multi-tasking process which humans are very good at. In fact, it's like the diffrence between AVI and MPEG video encoding if you like.
To suggest that up to 20% of ones time is 'lost' staring at the speedo in the vicinity of a camera is, I therefore propose, misleading in-extremis.
I agree strongly with your main point about the human brain carrying out parallel multitasking. Yes, we are continuously aware of various deveoping circumstances when we drive - we can take account of at least 5 and maybe ten developing hazards.
But the human eye cannot multitask. It's looking in one place at one time and one place only. We have peripheral vision, and certainly gross changes in peripheral attention draw one's attention.
Driving safely depends entirely on the high resolution centre part of one's vision. It's only the high res centre part of our visual field that can pick up the frequent subtle clues to danger ahead. Just think of the shadow behind a parked vehicle or the glint through a hedgerow that we readily accept as a warning of danger.
If we're not looking in the right place we don't get the warning. If we're looking at the speedo, we're not looking in the right place. And when you apply a small drop in attention to the road ahead across a huge population, it will certainly lead to unhealthy coincidences between lost attention and developing hazards.
I do believe that the whole 'speedo' argument is important and significant, and you're right that it's picked upon because it's easily understood and communicated. But it's a very far from our most important arguments. I see it as a bit of a 'loss leader' - we may not 'profit' from it greatly, but it sure gets folk in through the door!