So, it turns out they did call in an emergency and the police did shutdown the bridge. The police had no direct way of reaching the workers and weren't going to wander out there with an imminent impact. So, evidently, no there was not enough time.
'So,' I said that in my initial comment. The police may not have had a way to reach them, but the horn would have. There was a lot more time than none, who's to say if it would have helped. I think the correct answer is as given by others, that actually the loss of power would have disabled the horn too.
