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6.2 Preliminary Engineering The selected alternatives (Alternatives 2 and 3) discussed in Chapter 4 of this document will need to be further developed at a preliminary design level to more accurately understand the project footprint and potential environmental impacts. As part of the preliminary design phase, a best -fit alignment and method of widening the corridor will be further evaluated. There are multiple areas along the 1-90 corridor that will require interim alternative assessments to determine the best methods for reducing environmental impacts and minimizing interruptions to the traveling public. The overall goal is to identify an initial design footprint for the project to better analyze environmental and traffic outcomes. The design footprint will show the locations of improvements in the corridor, including stormwater facilities, earthwork, and CHAPTER 6: NEXT STEPS connection methods to the interstate. While no new access points to 1-90 have been proposed during the feasibility study, modifying existing access to accommodate additional lanes will be required. Future design work will include plan and cross-section views of the improvements and construction staging schemes to identify staging areas within or adjacent to the WSDOT right-of-way. Preliminary engineering, typically at the 5% to 10% level will be developed to determine potential environmental impacts, which is necessary in determining the level of environmental analysis and documentation that will be required for the project. Once the level of environmental analysis and documentation is identified, engineering design (typically developed to the 15% to 30% level) will move forward concurrently with the environmental analysis. VOLUME I: FEASIBILITY REPORT 1-90 Corridor - Easton to Cle Elum Feasibility Study 1 83