People get accidents when there is a bee in the car, when changing radio channels, when talking with the kids in the back. When people get distracted, they do not notice other things. That is basically the definition of distracted.
When I drove in Argentina, I saw a sign called 'vado'. My Spanish was not good enough to know what it was, but I knew if they put a sign where I was (in the middle of nowhere), there is a reason for it, so I better slow down. It was a bridge out of order and I had to wade though the water. Not an issue, but it would have been if I had not reduced speed.
I could see a HUD doing something similar. When a red triangle pops up I know I need to pay attention. I need to slow down and have time to asses the situation.
Now if this happens each time I go a bit over the speed-limit, it will not have any effect. If it is because I get to close to the car in front of me, then it can be useful.
And the reason cellphones are singled out is because they are use way more then you have a bee in your car or you change the channel. Not per person, but in general. That means the duration of being distracted is much, much longer, resulting in a measurable increase.