This is the calculation I use and it seems to work out more often then not:
Observe the lanes, every person is assigned 1 point, elderly (60+) +1, buggies over 70% full get +1, smokers get +1, welfare recipients +2. Add up the totals and the line with the smallest sum is usually the fastest. Obviously you can't always tell who is on welfare or who the smokers are, but the obvious ones are usually the ones that take the most time anyway.
Observe the lanes, every person is assigned 1 point, elderly (60+) +1, buggies over 70% full get +1, smokers get +1, welfare recipients +2. Add up the totals and the line with the smallest sum is usually the fastest. Obviously you can't always tell who is on welfare or who the smokers are, but the obvious ones are usually the ones that take the most time anyway.