Size matters! Using conjoint analysis to uncover public preferences for design optimisation in road infrastructure EIAs

The planning and realisation of road infrastructure projects is often societally controversial because of the impacts and the difficult trade-offs between alternative project designs. Neither in current literature nor in Environmental Impact Assessment (EIA) practice do we find any clear method to engage the public in integral trade-off choices. This paper aims to uncover public preferences about integrated road infrastructure projects, not by focusing directly on the - often contested - alternative project designs but by focusing on the size of the impacts. This paper examines the preferences for combinations of impacts and the corresponding orders of magnitude. We do so by employing Conjoint Analysis (CA), a well-established method in consumer studies which, to date, has rarely been used in infrastructure planning. CA systematically estimates public preferences for (1) the relative importance of selected impacts and (2) the corresponding Willingness-to-Pay and Willingness-to-Accept. We used the total range in sizes of impacts of three existing EIAs for infrastructure projects and asked 600 citizens to trade off the impacts involved. First, we find that public and policy preferences do not necessarily align. The CA results show that loss of 'forest and green structures' and 'agricultural land' are considered to be the most important impacts. Conversely, in each of the projects, travel time gain is regarded as the least important impact. Second, the order of magnitude of the impacts seems crucially important: the greater the magnitude of the impact, the higher its relative importance. Third, when comparing our results with the alternatives in the original EIAs, it appears that the project alternative with the highest weighted public preference required an optimised project design. We find that our systematic approach to uncover public preferences at this stage in the EIA-process can contribute to further design optimisation of the project. This stage is often overlooked in the process while it is the last EIA stage before the final approval decision is made, but the first time that the forecasted size of the impacts of the different alternative designs of a project are available and presented. Such an approach may enhance the development of alternatives that are more widely accepted by the communities and that thus potentially alleviate or even prevent social contestation.