There has to be a balance between reasonable safety and reasonable utility. Viper's last couple of comments are not realistic in a society where there is value placed on life. In particular, his comment ''and Natural Selection will take care of the remainder anyway" isn't shared by most of society, and probably not by most here either given the number of "oh no, not again" type of comments that appear in another forum section here on a regular basis.
The guiding principle to setting urban speed limits is the mix of traffic. Greater presence of slower and vulnerable road users (pedestrians and bicycles) always point to lower speed limits. That number of 50 kmph is no more sacred here than it is in the Europe that many like to point to as driving Mecca.
With respect to city speed limits, saying that pedestrians shouldn't take leisurely strolls down the road misses the point. Speed limits are never set on the basis of perfect performance by all users because there is no such thing. They are set to allow some measure of recovery from error regardless of who made the error, and crash survivability when the error is not recovered from.
Ideally people should cross at corners, but they don't always. Ideally cars shouldn't speed, but they do. For every pedestrian who gets hit because they are oblivious to oncoming traffic, there is a pedestrian who gets smoked at an intersection crossing as he or she should with the light. The other day a person in a parking lot far far away from the street got smoked by a motorcycle that lost control while making a turn at a city intersection.
I can see a 40 kmph limit as being reasonable in much of downtown Toronto and in densely population residential areas. The reality is, there will be bicycles and pedestrians in large numbers in those locations so the frequency of interaction between different classes of road users is already high, and residential intensification in urban areas will do nothing but increase over the future. We're not about to turn those streets into mini equivalents of our limited access expressways. We will most likely follow the example of Montreal, and several small Ontario towns and villages that have already set town- and village-wide 40 kmph speed limits.