I found 1.4s as a time that is used in crash investigation. I've read anywhere from 1 to 2 seconds also.
Flicking a switch is slightly different to driving a car - you have to move your foot off of the throttle and on to the brake for a start... and that's not a straight forward movement. Also - sitting and waiting for a light and knowing its coming, and flicking a switch under controlled conditions isnt anywhere near the same as on the road. If you drove like that the whole time you would be a nervous wreck. I think people tend to relax slightly in the car - which is probably better for fatigue in the long run. You're also taking in a lot more visual information than looking for a light - there are all sorts of things to concentrate on.
However - interestingly - the car that crashed in to me was on hand controls! Not sure how that effects things.