The problem is that the most up-to-date speed limits were introduced on the basis of calculations done in 1960, some 50 years ago. Back then they had to account for cars from the 40s in calculating sensible speed limits. Sure there are a lot of plonkers on the roads, but let's bring things into perspective. When talking about "old" bangers from the '90s, you are still talking about cars which are 50 years more technologically advanced than when the current speed limits were introduced. Most of the accidents I see day-to-day on the road are nothing to do with the speed limit of a road, but are to do with junctions, not paying attention and not keeping a safe distance. All these will continue to cause accidents whatever the speed limit.
I don't want to be bringing up numbers, but there are plenty of examples (eg. Poland with huge speed limits or Germany without them at all) which prove that in the modern era, it is all about the driving standards as opposed to the legal speed limit.
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One important thing that really gets to me is the argument that "bad" drivers are the reason speed limits should not be increased. Surely, the implication should be the eradication of "bad" drivers and the subsequent re-in-station as "good" drivers. Harder driving tests, more scrupulous driving education standards & programmes, re-testing, you name it.
If you have a leaking pipe somewhere in your house and you wish to install a power shower, is the answer to not install one as the leak will get bigger and the pipe might burst?