**3. Materials and Methods**

Based on qualitative research, three case studies were selected. Case studies have the distinctive characteristics of dealing with many more variables of interest than simple data points (cases) and of relying on multiple sources of evidence [59]. For this study, three kinds of information sources were used: documentary information, visual inspections, and interviews. The interview questionnaire started by requesting interviewees to tell the story of how green streets emerged in their cities. Following up on responses, clarification questions were added to complete the understanding of the design process. Follow up questions covered federal and state stormwater regulations, implementation within ROW, design standards and manuals, factors affected by the process, and coordination of participating agencies. In addition, interviewees were asked to provide relevant literature and official documentation. Figures 1–3 present some of the on-site visits for visual inspection.

**Figure 1.** Green strip in Seattle (WA). The curb is lowered to allow runoff to infiltrate through a green trench.

**Figure 2.** Curb extension in Portland (OR) to catch runoff from the roadway and the sidewalk.

**Figure 3.** Rain garden in a former parking area in a residential neighborhood in Philadelphia (PA).

Case studies were selected based on the "similar to another and both least similar to a third case" approach suggested by George and Bennett [60]. This approach allows us to learn from controlled similarities and differences. The cases were selected from the same country, with similar total annual average rainfalls, comparable areas, the same types of sewer systems, and with ongoing green street programs. Seattle, Portland and Philadelphia were selected from a list of 20 potential cities. Portland and Seattle, the similar cases, are both green streets pioneers in the US, and both are located on the Pacific coast. Philadelphia, in contrast, is a late implementer and is located on the eastern coast. In the end, over 100 documents and websites were reviewed and nine phone or in-person interviews were conducted in the three cities. This research is presented as parsimoniously and succinctly as possible here. The stakeholders interviewed were officials of environmental, transportation, utility, and planning departments, a delegate of the office of the mayor in Philadelphia, advocacy group members, academics, and practitioners (consultants). Interviews were recorded to be later transcribed.
