Getting Started

Scope
To begin the case study, as with the traffic impact assessment, decisions have to be made about a variety of study issues. One is the scope of the analysis: that is, the intersections to include, the components of the freeway interchange that might be necessary to consider, and the time periods for which the analysis is to be performed. Additionally, identification of the stakeholders, their issues, and how their issues translate into goals, objectives, and performance measures for the study will need to be considered, as will who the audience is. Each of these will affect other issues, like the analyses to perform, sequencing, tools to use, and data to collect.

Let’s think about the scope first. How should the system be defined? What facilities should be involved beyond the main site-access at intersection C? What about intersections further away? Often, you look at the amount of traffic contributed to the total intersecting traffic by the site-related traffic, and include the intersection if the resulting percentage increase is above a given threshold. You also might decide on which facilities to include based on whether a LOS change is likely. In the latter case, you might decide a performance assessment is necessary for a decision. For the purposes of this case study, we’ve decided that intersections A through F should be included, as shown in Exhibit 2-1. When we get to the end of the analysis, we’ll reflect on whether that decision was correct.

The time periods to consider are a second important aspect of the scope. We’ve indicated that the AM peak, PM peak, and peak hour of the generator should all be considered. Sometimes it is important to look at the Saturday midday and the Friday peak hours as well (e.g. for a shopping center) or a collection of heavy use days during the year (e.g., for a sports facility). In the case of this particular system, it’s also important to study peak shopping days in November and December because the traffic volumes are significantly above average then.