The "Thinking Distance" is based on a response time of 0.68 seconds. The response time includes processes such as recognising a situation, deciding the appropriate action, putting that action into effect (e.g. object appear in road ahead, object to be avoided, move foot from accelerator to brake and depress pedal). However, there is not a simple calculation for the response time that covers every driver in every situation under all conditions. The figure should be high enough to err on the side of caution and allow for drivers with lesser capabilities since they are allowed to use the roads unhindered.
Many studies over many years in many countries have tested response times. TRL car simulator tests have shown a typical "expected event" response to take around 0.85 seconds; unexpected events, of course, lead to longer response times. US Highway Design test protocols uses an ultra cautious standard response time of 2.5 seconds and Australia plumps for their average time of 1.5 seconds.
If the UK was to adopt 1.4 seconds as a reasonable response time this would see Thinking Distance double, and if brake technology has reduced typical Braking Distance to 70% of the old value (which I doubt is true for a significant number of road legal cars with more than one occupant) how would Stopping Distances compare?
Old ... 30mph = 9m + 14m = 23m
New... 30mph = 18m + 20m = 38m
Old ... 60mph = 18m + 55m = 73m
New... 60mph = 36m + 38m = 74m
Old ... 70mph = 21m + 75m = 96m
New... 70mph = 42m + 52m = 94m
I suspect not the results expected by advocates of Stopping Distance reform.