About frigging time
The NSL was set in 1965 when the average car on sale had a max speed of 72mph, woeful passive safety features (seatbelts/airbags/crunch zones) and even more woeful active safety features (breaks/traction) compared to even the lowliest piece of crap on sale in the second hand market today.
As others have pointed out, you can drive safely at 100mph and dangerously at 10mph. Yes, all things being equal, more speed equals higher chance of a serious accident but other factors are much more important and this is all about risk vs. convenience - if you want everyone to be perfectly safe you'd have to ban driving altogether. There's a sweet spot somewhere, and I think it's fair to say we're well below it at the moment.
In the UK today, even on an average family car you spend most of the time looking at the speedo instead of looking at the road to make sure you stay beneath the absurdly low speed limits and that's dangerous in itself. 30mph should only apply anywhere were pedestrians are often by the road; on a clear day a motorway with 3 lanes and barriers on both sides is perfectly safe at 90 or even 100mph.