The petition is too vague. Speed limit of 20mph (that'd be good on say, Birchfield Road wouldn't it?) on what types of road? What types of road justify higher or lower limits? How's it going to be enforced?
The City Centre is a 20mph zone now and the whole thing is a joke - its universally ignored unless traffic volumes are such that it brings the speed down. West Mids Police won't bother to enforce it either.
If it actually gets to the point of having enough signatures to generate a reply (which I doubt) it'll say: £140m for Cycle England, and that most towns have cycle networks (admittedly of the off-road variety).