Braking Distances - Which car determined the 'standard'?

[XplusYplusZ]

It's commonly said that todays cars easily out-perform the highway code stopping distances. So I was just wondering if anybody here knows how the braking distances in the Highway Code were established - which car(s)? I'm assuming they were 60s / 70s without power assistance and before ABS..

[XplusYplusZ]

Vanhosen, a good point. So how did they come to a .67g constant?