An environmental impact study is the significant part of any transportation project development. In general, environmental assessment is a process to find out the possible impact on environments due to the effects of proposed initiatives before they are carried out. In [the] transportation sector, construction of new roads or highways may minimize congestion and reduce travel path and time but may also have an effect on [the] environment. So it is necessary to develop the best alternative routes so that natural, cultural, [and] social environmental impacts are minimized. In recent years geographic information systems (GIS) have become increasing[ly] popular for environmental studies. GIS can play a vital role for analysis and in formulating the quick mitigation plans for high-risk environments. This study articulates what environmental impacts need to be assessed in transportation corridor planning, what geospatial data are needed to support these identified impact assessment activities, and how and what GIS tools are required to facilitate the corresponding assessment activities. The Mid-Peninsula Transportation Corridor (MPTC) planning project is analyzed as a case study.