The pedestrian may have been crossing legally but at a time where the driver could not stop. Say I am driving legal limit of 30 mph down the road. The typical stopping distance is 75ft. What happens if the pedestrian enters the crosswalk when I am less than 75ft away? The pedestrian may have the right of way but the vehicle still could not stop in time.
You wouldn't actually need to come to a complete stop, you would just need to get on the side of the lane and slow down so they can clear your car. If they were walking at a leisurely rate of 2 miles per hour, you could get by with not slowing down a bit about 90 feet away. If they were going the average speed of 3.1, you could stay at the speed limit as little as 58 feet away.
Under about 30 feet, you should be able to stay close to the middle of the road and avoid them, even if you don't accelerate, assuming a 9 foot wide lane.
That leaves a 28 foot stretch where you actually need to be doing any braking or accelerating at all.
But here's the most important bit: even under these circumstances you're acting too late. At every cross walk, *you* should be looking both ways for pedestrians liable to cross. You start reacting when you see a crosswalk, not when you see a pedestrian in your path.