We've had this discussion before on here.
http://www.20splentyforus.org.uk/
We had comments from a guy in our local paper regarding fuel economy improving etc. I sent him an email about it & he sent me back links to a couple of government sites.
http://www.transportpolicy.org.uk/Future/20mph/20mph.htm
Even though they have yet to introduce the 20 limit, at peak times my local roads are horrendous & I found that I could actually implement some of the driving techniques.
We have stretches of road 3 or 400 yards long with as many as 3 sets of pedestrian crossings, if you stop on amber you can wait up to 30 seconds for a green - it doesn't sound long but just time it... it's an eternity when you are sat at a red light looking at an empty crossing
If you accellerate slowly you can see pedestrians approaching the crossing, if you are lucky you can attract their attention, motioning them to cross without pressing the button - so no waiting at the lights.
Hellcat "Remove the speed limits and people will drive within their ability, the capability of the car and the prevailing conditions and location. It would most likely be a little under 30 for residential streets, around 30-35 along the larger streets, around 40ish along inner-town dual carriageways getting faster as you head to less populated areas. Then you could use the signposts to warn of dangers to which people would adjust their speed."
Only some will, many will see it as a free-for-all & use any bit of straight road as a drag strip. We have a couple of local roads with a 30 limit - but look safe to drive at 60. Trouble is, kids cross those roads and they have little concept of estimating speed. A few fatalities over the years have seen the limits reduced from 40 to 30.
While many of us 'know' we are decent drivers, you only have to look on youtube to see that there are just too many morons on the roads even with limits in place.