It is a bit of an odd one - we do not allow higher touch voltages on other outdoor things unlikely to be touched with two hands (if a car, why not a caravan or a bus-stop) and it is arguable the great (wet) British Outdoors would require a lower touch voltage than indoors, rather than a higher one. I suspect however  there is a desire to co-ordinate with other standards related to substations, and also to set the limit as high as reasonably possible without appearing too reckless. 

In practice factors we do not usually like to rely on come into play, like the ground surface (gravel is safer than a lawn, as it drains better, and is sharp, so bare feet are less likely paved surfaces are not such a good connection to terra firma as a bare earth surface), the fact that cars are painted, and door handles are probably not shiny bare metal, though they may be - all these unreliable factors do help to reduce the risk, but not always and are not guaranteed.

Note also that the effective body resistance is a voltage dependent thing - or at least the breakdown of the surface skin is - once under the skin we are all pretty wet and salty, and  the current that flows through a person in a particular configuration at 110V is not half the current in the same victim at 220V, but in fact rather less. Also voltages that force enough current  and are present for a long enough to heat the entry and exit areas are more dangerous, as that heating lowers the resistance, leading to a positive feedback, and in extremis creating a burnt area.

M.

Great, thanks for all your help, it`s starting to make a lot more sense now, I agree, the science behind this can be unreliable in reality faced with all the many varied situations.

Great, thanks for all your help, it`s starting to make a lot more sense now, I agree, the science behind this can be unreliable in reality faced with all the many varied situations.