Improvement of transportation project cost estimating practices and the tracking of estimates as they evolve is critical to greater financial accountability and more accurate project scheduling. Cost estimating refers to the processes used to predict the cost of a project at various points in the project development process. Estimate tracking is the ability to document changes in estimated project costs, and the reason for these changes, over time. One key issue is developing an effective process for developing and refining project cost estimates given a process that involves the public and other agencies in decision making. A framework for accurate cost estimation must recognize that changes in the scope of project are a natural part of a collaborative decision making process. The objective of this study was to develop guidelines on improved environmental cost estimating methodologies for transportation projects.