So if you don't want Bristol to become a playground for the scamra van sign the petition and pass this on to as many Bristol friends as you can
Ok, some points:
1) I used to live in Clifton and often cycled around Bristol when I was a teenager.
2) I now live in Portsmouth where we've got 20mph limits on most residential roads and have had for a while now.
3) I go back to Bristol once or twice a year to my mother's, so I've still got a passing familiarity with the place.
Now down here the 20mph limits pretty much rely on "the cop in your head", ie there's no enforcement of them that I've ever seen (it would be impossible to catch more than a few speeders) and some people tend to ignore them completely, but a lot of people do at least pay lip-service to them.
In Bristol I can think of a lot of similar residential roads where a 20mph limit would have the same effect and results and, frankly, much of the traffic isn't doing more than 20mph anyway when it's busy, so putting signs up isn't likely to do much.
So is it worth it? I doubt it and think it would be better to provide better public transport/ cycleways/ junction layouts etc, but trying to stop Councils from going ahead with this sort of thing is very difficult indeed once they've made their minds up!