Road safety - how not to reason from data

My apologies for going quiet over the summer.  I've been in China, and they're not too big on blogs there (blogger is blocked).

Just a short post here, on something fairly obvious.  Here's an extract from post number 51 by Cambridge News' Cycling Blog on speed limits.

In Great Britian in 2011, 7 people were killed on a road with a 20 mph limit. 636 killed in a 30 mph limit. 289 people were seriously injured in a 20 mph limit, 13,168 in a 30 mph limit... Yes, these are large numbers. But it is the proportions that matter here... So that is proof then. Lower speed limits means fewer people killed.

[In case you're wondering, there's no irony in the last sentence.  At all.]  I assume you're all thinking the same as me by now.

1. There are a lot more 30mph roads (by length) than 20mph ones.
2. 30mph roads are often busier than 20mph roads, so there's more things to collide with each other.

After a cursory glance for official proof of the first point I came up empty-handed, but I imagine you'll believe me.

There are a lot of good reasons to think that 20mph roads are safer than 30mph ones, at least if the speed limits are observed, just from simple physics.  A car travelling at 30mph has 2.25 times as much kinetic energy as one travelling at 20mph.  As a driver reacts before braking, the car travels 50% further; as it starts to brake, this will happen more slowly the faster the car is going to start with.  I am personally in favour of 20mph limits - they're a lot more pleasant to cycle on, because I can actually keep up with the traffic.  They certainly feel safer, because drivers are less likely to try and overtake me inappropriately.  But what's listed above isn't proof of anything.