the height of the demand curve for a conventional good such as trips from home to shoppig center by bus measures the willingness to pay for an additional unit of that good at the margin. therefore willingness to pay for a price reduction is correctly measured by the change in consumers"surplus,which is the area under the demand curve and above a horizontal line indicating the current price.this equivalence applies whether the demand curve results from one category gf trip-making to another.similarly,willingness to pay by suppliers is measureed by the change in producers"surpls,which is the area above the supply curve and below the price line.