Streesblog has an article out today about 20 mile per hour (~32 km/h) speed limit zones in New York City and London. London has installed over 400 of these 20mph zones since 2001 which have contributed to 27 fewer pedestrian deaths and serious injuries and reduced serious traffic injuries by 46 percent. It's quite a compelling read: How London is Saving Lives with 20MPH Zones.
There's even talk of expanding 20mph zones city-wide. Yet in Brisbane we have speed limits twice that all across our network and even through the CBD. The knee-jerk reaction is that it's not economically feasible to slow speeds. What are the costs of slowing traffic? What are the costs of not slowing traffic?