There are some really illogical and silly arguments on this thread...
Speed doesn't kill - if it did every plane would land with lots of dead bodies on board 
, astronauts wouldn't do very well and as for those involved with land speed records like Bloodhound, I suspect they must surely drop dead every time they think about it!
As has been accurately stated above, inappropriate speed can kill (yet rarely does) - and that could as easily be 25mph in a 30 limit past a school as 90 on a clear motorway... In fact I know of an accident where a pedestrian ran across a double carriageway into a car - the car driver was being cautious doing 25mph in a 50mph zone - had they been going at 50mph there would have been no accident!
So the speed that kills is not a reflection of whether it is above or below the speed limit - but whether it was the wrong speed for that situation. It is also worth noting that when you look at accident stats there are a number of influencing factors causing accidents (tiredness / incompetence / alcohol / drugs / anger / human mistakes / wildlife / road conditions / mechanical condition of the car / the speed at which the car was travelling). Excess speed (all accidents involve speed of some level!) is actually a small % of being the main / sole reason for the accident... just the easiest to measure.
Speed limits were originally set based on road shape / dynamics, and car capabilities - both of which have changed a lot in intervening years... speed limits today are as often set by political forces as for any other reason - e.g. a road near me - long long open straight road, with no turnings / no driveways / no houses / no trees or hedges blocking the view - couldn't be a safer road, yet for some reason it has recently be changed to 50mph from 60mph - no logic, no change in circumstances, just political change... The road hasn't suddenly become less safe - so it must logically be as safe to do 60mph there as it was 6 months ago - yet now it would be illegal - so for those arguing that illegal = unsafe, sorry, that logic fails
As with some of the others on here, I have done considerable amounts of advanced driving / and coaching or tuition - to a high level, as a result, my ability to physically handle a car, and observationally handle the setting is far better than previously - as a result I probably drive faster than most, but equally I drive slower than most - also, the more I learn, the more I realise how much I have to learn - as a result my caution and humility on the road increases - not decreases... the arrogant drivers are usually the bad drivers... one of the big differences between those who have done advanced driving and those who have not is the differentiation in their driving - matching speed to appropriateness...
I see this all the time in the countryside around here, just today I was in a NSL (60mph) single-carriageway road behind another car - and approaching a group of cyclists - I had come up behind the other car as I was driving faster than they were, but I held back seeing the situation ahead... They didn't slow down - but overtook the cyclists on a blind right-hand bend, luckily nothing coming the other way or it could have been a terrible accident... by this time I was doing under 20mph in case they crashed, I stayed at that speed several car lengths behind the cyclists around two bends until the road opened up enough for me to overtake them safely with a good margin... A few minutes later I had caught the other car and seeing a safe opportunity I passed it... So a good example of how I was driving both faster and slower than they were - and as a result much much safer driving. Yet I suspect that the driver of that car, like many on here has swallowed the mantra that speed kills - and probably believes that their 45mph was far safer than my higher speed - totally unaware that they nearly killed a bunch of 6 cyclists...
There is a real danger in just taking on board political slogans and believing them - indeed living by them without understanding the reality is a very risky approach to life - I am fortunate to know some of the best drivers in the country and what distinguishes them all is the intelligence of their drive - yes I would agree that someone just getting in a car and always driving at 90mph in a 70 limit without thought is increasing risk - but that risk comes from the lack of thought, not the speed per se - I have several cars I drive regularly, ranging from a 150bhp diesel to a 400bhp car with a race based engine and race brakes, and from 2 seater sports car to a classic 4x4 - the intelligence comes in adapting to car / weather / setting / etc. - how I drive each is different.
so lets ditch the spurious argument that speed kills - it doesn't and science clearly shows that it doesn't... however bad driving - yes, that kills regularly, so lets focus on bad driving - and lets be aware of what that is: tiredness / emotions / fractious passengers / lack of observation / poor mechanical skills / poor mechanical condition of the vehicle / arrogance / drink & drugs / etc. - these are the things that cause accidents - so the more we can do to reduce those the better... We have very high standards of driving in this country, but can always do better - the three main things which would improve this are:
- good mechanical condition of the car (it is amazing how many people don't check lights / tyres / oil / etc.)
- good observational skills - this is what advanced driving is about, think of it as chess on the roads - understanding and anticipating every other road user - and being ready for their move before they make it - anticipating and protecting yourself and them...
- good personal condition - making sure that as a driver we are relaxed and alert, not tired and grumpy :) - not always easy, but makes a huge difference...
but speed - nope that actually isn't one of the biggest issues - just the easiest to measure and therefore has become a generally inappropriate target