The following is originally from the FHWA
Public participation is more than just a hearing, or one meeting near the end of the project development process. Public involvement needs to be an early and continuing part of the transportation and project development process. It is essential that the project sponsor knows the community's values in order to avoid, minimize, and mitigate impacts, as well as to narrow the field of alternatives (for planning) and alignments (for projects). The community also needs to understand the constraints and tradeoffs of the transportation planning and project development process and to "buy-in" to the transportation needs and purpose.
Public participation allows the State, Metropolitan Planning Organizations, public transportation providers and resource agencies to know the impacts to the community from the transportation project as viewed by the community. Early and continuing public involvement allows the project sponsor to be aware of the problems and impacts and to deal with these issues early. In that way, attempts can be made so that the impacts can be avoided, minimized or otherwise designed in a manner acceptable to all parties involved. If involved early, the public can provide insight (directly or indirectly) into what their community would find acceptable in the way of mitigation. Often, there are designs or enhancements that will allow the project to fit more harmoniously into the existing community.
If the demographics, values, impacts, and desires of a community are made known early and on a continuing basis through an effective public involvement process in both the transportation planning and the project development processes, the project sponsor can better incorporate them into the design of the project. Design options can often also add enhancements into the project.