I saw this happen in broad daylight. A woman started crossing in a crosswalk the instant the light turned green for the cars, thus blocking them. There were only cars in the right lane and a car approached the left lane and continued (since he had a green light) and the woman stepped out in front of him and got hit. The driver stopped and the lady was taken away in an ambulance. In this case I'd blame the woman since she crossed when the crosswalk said "don't walk" and there is no way the driver could have seen her.
A few months later I was driving home late at night and the same thing happened. A person wearing dark clothing waited until the signal said "don't walk" and I waited for the idiot to cross. Another car came through in the other lane and the guy missed getting creamed by only a few inches. Again, the light had been green for some time and the idiot waited until just after the cross light turned red like he had a death wish. Too many times I've seen stupid pedestrians crossing when it's not safe to do so.
Also, as a driver I too would be distracted by sirens, trying to find the source of them. No driver can look ahead all the time, i.e. checking the mirrors. If someone claims that they're always looking ahead they're either lying or a really bad driver.