A town engineer writes: 'A footbridge over the level crossing would take away every risk of a pedestrian being struck there. The town should therefore build one, whatever it may cost.' Which of the following is the chief weakness of the engineer's reasoning?

A town has one purse and many dangerous places, so a safety work is worth building not merely because it is safe but because nothing else would buy more safety for the money. The words 'whatever it may cost' shut that question out. Which department executes the work and whether an engineer should decide are matters of procedure, and the conclusion is about this one crossing only, so nothing is generalised from it.