At the moment they work out fares for planes, trains, buses and taxis roughly on the basis of distance travelled, ie
Fare = z + Fd
Where d is 'distance travelled', z is 'base cost' (things like 'flag fall' in a taxi) and 'F' is fudge factor.
But if they worked it out in a multifactorial way, on the basis of weight of passenger and distance travelled, such that lighter passengers travelling the same distance as heavier passengers paid less, this would be more accurately reflective of actual costs. The formula would be
Fare = z + Fdw
where w is 'weight'.
Not at all complicated; you would be weighed when you stepped onto the bus, or whatever, and the ticket seller would automatically offer the relevant fare scale for your weight. Except in the case of anorexics, there would be clear health benefits to the society that would no doubt offset the initial cost of weighing equipment over time.
Think about it folks, this idea has legs!
PS And that's this blog's 1000th post.