From the introduction:

In this paper Madhumati Dutta and Joysankar Bhattacharya explore how, in Kolkata, the modal structure can be changed, and the resultant reduction in pollution. Given this as their basic objective, they proceed as follows: The first step is to determine the total pollution caused by the existing modal structure. For this they selected a representative sample of 750 households and 280 non-resident commuters (amounting to 3000 individuals) with the help of appropriate sampling techniques. They obtain, for each mode, the distance traveled by the sample in one year. Using data on pollution (per person per unit distance) by mode, they calculate the total pollution caused by the transport used by their sample, and from here extrapolate for the city. Dutta and Bhattacharya then determine, with the help of their representative sample, several ways in which the modal structure may be changed. They determine the benefit in terms of emissions reduction for each of the alternative modal structures, and see, for each feasible modal change, what requires to be done by the state to implement it, and the cost involved. From the cost figures and the benefits they arrive at the most cost effective (i.e. least cost per unit benefit) modal structure for the city.