White roofs have the double effect of significantly reducing the air conditioning load within the building. This reduction in power consumption will probably reduce global warming by avoiding CO2 emissions as much if not more than the direct reflection effect... The peak power demand days in California are during the summer because of all the air conditioning.
One study found that there was between a 15% and 60% reduction in cooling power use just by applying a white roofing compound.
One problem with this is that high albeido (white-ish) pavement doesn't stay that way for very long because concrete ages and gets dirty.
You can read more about this here.