The graphic above, made by Mikael at Copenhagenize (you can follow him on Twitter here), shows clearly why speeds must be kept low wherever cars and pedestrians get close to each other (notice that the ratios of deaths, injuries, and uninjured are color-coded).
20 MPH/30 KPH seems to be a good compromise, with the number of deaths being very small and the number of uninjured victims being more significant, unlike with higher speeds where things are reversed. Though of course a low speed limit should be the last thing we count on to save lives; cities should be designed for walking, and the infrastructure should be designed to make things safe for everybody, including pedestrians.
But sadly, the way speed limits are set right now is totally backwards. Rather than start from a point of valuing human life and trying to protect it, most cities observe how fast drivers are going and then create speed limits based on the 85th percentile rule (more about it here).