Reduction in accident frequency and severity*, lower air pollution**, quieter and safer places... 20mph is a great idea! But lets have all residential areas covered, especially around schools, lets also lower speeds on other roads - and lets make sure that 20mph is enforced properly! I hope Bristol City Council goes well beyond tinkering with this green idea.
*At 20mph a pedestrian you knock over stands a 90% chance of surviving. At 40mph they stand a 90% chance of dying. http://www.greenflag.com/help/saferdriving_speed.html
**Most energy is used in accelerating and braking, which happens more frequently in cities . Driving in any particular gear consumes more energy per mile the faster you travel. Driving steadily at 20 mph is more efficient and thus cheaper and cleaner. http://drivingefficiency.org/driving_gently.pdf