Scenario planning

Scenario planning arises from the need to look at a complicated system with many variables and decide a course of action.

Scenario planning exists in a broader context (begin with the origins of military planning and moving mostly towards business and environmental planning)^[1]. There are many applications for scenario planning in the context of transit as well.

Transit has wide reaching effects on an urban area. Scenario planning allows for a more deliberative consideration of what those effects might be and hence steer agencies toward a most desired course of action.

Scenario Planning Process

1.Cause of new scenarios

To decide which scenarios to test and what possible outcomes they may produce, it is useful to first take account of what is driving change in the system. Some examples may include:

Opportunities for expansion
- Ex. approval of a grant or additional revenue that allows for a new bus line or subway line
A need for service cuts
- Ex. reduced budget that requires removing a line or reducing its service frequency
A desire to shift service outcomes
- Ex. a neighborhood is not accessible to transit and the agency is exploring ways to reach it.
External circumstances change the function of a transit system
Ex. projection of a population boom, or population shifts that push certain communities to different parts of town.

Often there are multiple drivers that need to be addressed at the same time. For example, when TriMet built the orange line, it was both an opportunity for expansion and a reaction to an anticipated population boom in Milwaukee over the next 20 years.^[2]

2. Outcomes to evaluate

One of the advantages of looking at scenarios, especially in the age of modeling technology, is the ability to see how a variety of variable change and interact with each other as options shift. However, agencies must choose which variable they wish to track, both in order to know to input those variables, and track how they shift. Some variables that transit agencies often care about include:

Total system ridership
Ridership on specific lines, broken up by time of day, day of week, time of year, and compared with capacity
Total regional transit shed
Travel time between certain key points that have been identified
Transit access in different communities, paying particular attention to access in communities with a concentration of protected classes, ie. communities of color and low income communities (Title VI)
Proximity of transit system to essential services (schools, grocery stores, employment centers, etc.)

3. Potential Scenarios

Transit agencies need to decide what potential options for new projects, cuts, or switches are. Knowing the limits of their funding and have some knowledge of their system. Agencies determine a handful of potential scenarios of what the altered system might look like.

4. Running the scenarios

Once various options have been decided on, the scenario is run. Some technologies today allow transit systems to run scenarios in minutes. In this context, running the scenario means inputting one of the options letting the software calculate the potential outcomes.

5. Analyzing the scenario

Results can come in the form of summary statistics, heat maps, and more. If the agency established some specific goals ahead of time, with regard to what positive changes they wish to see and what negative changes they wish to avoid, they can evaluate the extent to which a scenario meets those goals. Furthermore, they can run multiple scenarios and determine which one has the best overall effect. An agency can also look at a test scenario’s results and identify unanticipated problems.

6. Developing new scenarios

If none of the scenarios adequately meet the goals of the transit agency, or unanticipated problems arise that offer new insights on alternative plans, and agency may choose to develop a new set of scenarios and repeat the process until they can find satisfactory results.

Benefits of scenario planning

Scenario planning allows an agency to evaluate the effect of multiple changes on a system. For example, a scenario may include building a subway line, removing multiple bus lines, rerouting a number of buses, and changing the service frequency on several others. That scenario can be built into a system to evaluate its effects.

Challenges with scenario planning

In the past, drafting scenarios and calculating their potential impacts required a lot of time. Current tools help to reduce that effort. Scenarios can only be as accurate as the data that is in place and as projections it is given. For example, TriMet’s orange line project has attracted fewer riders than it anticipated because of certain inaccurate projections. Some elements, such as total transit shed, are likely to be reliable because there are few external implications. However, even projections on the communities served can fail to account for the time it takes to implement projects and the possibility of shifting populations.