I agree with Fox, the main deterrent for me is the punishment. I was caught speeding back in March and given 3 points and a £60 fine and that plus the slight insurance premium increase annoyed me enough to act as a deterrent, and as a result I slow down to nearer 50 when the M3 becomes the A316 now
In my head the main deterrent is the punishment, but I speed anyway, so it can't be effective. I guess then the main deterrent is getting caught - I slow when I know I will. I had 9 pts on a 200hp+ RWD car from age 20, so I know all about annoying insurance premiums. However, the fact remains that I can earn more money to pay for those insurance premiums. I'd say the bigger threat was losing my license, but I didn't significantly slow down when I knew that one more incident would see me hit 12 pts and lose my license under totting up rules.
I think what would see me slow down is the abolition of speed limits. I have a rebellious nature, and do not like being restricted when it cannot be proven that it is necessary. I have never seen or heard a scientific or logical justification for 70 on the M40 when it is clear, and so I do not respect that limit. I will overtake police cars at a GPS 75 because I know that no officer can be bothered to stop me for that. I drive at a GPS 55 through 50 SPECS limits because I know I can get away with that; I carefully calculate the indicated speed that I can drive at - in the Mondeo I measured that I could drive at an indicated 58.5, a point where the needle did not touch the white mark indicating 60. I speed up and follow police cars that are on call, to see how fast they are going. I've followed them for a while at over 120 no more than some car lengths behind. In hindsight it's lunacy, but I feel compelled to push the boundaries. It's not arrogance - I don't do it because I think I'm better. I do it because I think the limits are rubbish.
But I also believe that the law is there for a reason. 100+ on a busy motorway is asking for a pileup. 100+ on an empty motorway at 3am when most people's bodies (and consequently reaction and judgement times) are feeling more tired than they would be during the day in most case, and things could still get nasty. It's more difficult to judge a car's distance and closing speed at night when you look in your mirrors - you're looking at blurred lights in the distance versus a clearly visible large metal object during the day, it only takes one perhaps less-experienced teenage girl for instance driving home from uni to pull out a bit too close in front of the person doing 100+ for that person to panic, perhaps swerve a bit violently, and oh dear..
I think the law is there because it's there. Look at the history of speed limits in this country - they aren't set using detailed studies or fact, they are set mostly arbitrarily. Additionally, they are arbitrarily set based on the performance of vehicles a very long time ago. The 70 limit was intended to be temporary due to fog (70 in fog was considered fine for the 50s relics that were still milling around), and was introduced in 1965 - a time when the top speed of a vehicle was usually about 70! Sure, people's reaction times are still the same today as they were then (though I wouldn't be surprised if they were slightly faster - we have better coffee nowadays), but the performance and safety of motor vehicles has increased hugely. The braking distance of the average family car in 1965 is much longer than in 2010, without mentioning now standard driving aids such as ABS, and soon to be standard ESC, that all massively increase the reality with which the average person can manoeuvre a vehicle at high speed in an emergency. I'd imagine that if, in 1965, someone was driving along in their Riley RME at 70, just 5 MPH below it's absolute maximum speed, and had to brake and swerve in an emergency, it would be game over. The car would probably explode and roll raining death and destruction upon all those near. Today, many cars are not long out of 2nd gear at 70, and are able to brake and corner at that speed without serious risk of randomly losing control.
When you combine the huge amount of grip a modern road car has with the far more powerful brakes, firmer suspension, thick roll bars, ABS, ESC, TPM etc., together with the fact there's little scientific basis to the existing limits, I think the argument for 70 on a motorway is beginning to fade away. I would be happy with no limits and far more onus being placed on the driver (with "advisory" speeds on all roads), with lower limits enforced in poor weather conditions. I would happily see a mandatory 50 or whatever on the motorway in very poor weather in return for an autobahn-esque unlimited speed in good conditions.
I want limits based on evidence and fact, not the latest statistical skullduggery mashed together by the speed nazis.