When estimating value of tile improver unit (Workers), consider
unit to have only half of the food upkeep it has. That makes
positive effect from planned +1 food tile improvement to
outweight -1 food upkeep unit has. Halving the negative value is
equivalent of doubling the positive value. Doubling the positive
value is like estimating that unit will improve two tiles during
its lifetime instead of just the one it's being evaluated for.

See gna patch #3693

(Browse SVN revision 22358)

As I said in patch #3692:
"As value of the improver unit is taken from single tile, and further possible actions ignored, settler consuming one food is considered zero value when it could only improve tiles to produce +1 food (no matter how many)"

Further, food is considered most valuable output, so settler producing +1 shield or +1 trade would have negative value (food upkeep bigger loss than tile improvements gain)

Attached patch just divides unit food upkeep by 2 in want calculation. That's about equivalent of expecting it to improve 2 tiles during its lifetime instead of just one.

Needless to say, this patch require rebalancing of _WEIGHTING constants in patch #3692 once again. Otherwise AI builds far too many workers.