Office of Planning, Environment, & Realty (HEP)
One of the challenges is explaining the results of models. Policy officials may not have confidence in the results, and may not know how to interpret them. Many of the stakeholders, such as university faculty, are extremely knowledgeable and will challenge modeling development and applications. Advocacy groups may try to use the models and their results for their own purposes, particularly when a controversial project is being planned. Ron Chicka echoed a point made by other participants regarding the importance of high-quality analysis, stating that "dealing with the public will be messy...make sure your work is not the weakest link in the chain."
Simple tabulations such as district-to-district flow tables, frequency distributions of travel time savings due to a new project or operational improvement, or GIS-based thematic maps can provide useful visualization of model results. Developing such summaries also provides an opportunity to verify model and analysis results, and to ensure that the results can be explained and defended, and that they support planning and decisionmaking needs.
Many small MPOs are finding that the traditional problem of meeting future travel demand is only one of several major challenges they face. There is great interest in using models to help analyze economic impact, operations, land use, air quality, safety, and other issues. "You need a whole range of measures to evaluate projects." (Bart Benthul, Bryan, TX)
Many participants expressed an interest in using models to improve operations on existing facilities. Often the MPO is in a low growth area, where system preservation and corridor management are economically feasible options, and where limited resources for highway infrastructure expansion are available. Such considerations are especially important in areas where limited economic and population growth prevent substantial investment in new infrastructure. One participant remarked that in a small area, the questions that matter to a policy board are very specific, e.g., "How to deal with the traffic on Main Street?" Therefore, models that deal with intersections and other small areas are often used.
Traditional 4-step models in use at small MPOs may not be detailed or flexible enough for complete analysis of operations. For operational analysis, a greater level of temporal resolution is often needed. Four-step models at small MPOs have sometimes been extended with advanced approaches such as Dynamic Traffic Assignment (DTA) and microsimulation, enabling modelers to investigate solutions such as ramp metering, signal synchronization, and other methods for increasing road capacity, such as geometric improvements.
DTA techniques can be used to obtain better information about traffic patterns across peak hours, and applied to support Planning for Operations or demand management strategies such as Commuter Trip Reduction. The four-step model can support such analyses by providing regional travel patterns and total traffic during a peak hour or peak period, but such models have generally been extended or supplemented with additional calculations or standalone tools to perform LOS analysis or analysis of queuing and bottlenecks. DTA is most suitable when large portions of the roadway system are operating near capacity. For a single facility where traffic diversion is not a significant concern, highway capacity analysis may be a simpler alternative. Concerns about traffic diversion to alternate routes typically will require analysis with a travel demand model.
In areas with extremely low transit mode shares, modeling has traditionally been focused on the automobile mode. However, there is increasing interest in addressing transit, bicycle and pedestrian modes. Some small MPOs have already done planning studies in these areas, but others face challenges in procuring the resources, data, and technical expertise required to expand into additional modes.
For many small MPOs, transit ridership is too low to have any noticeable impact on traffic volumes, and demand may not be growing. Even so, transit models have sometimes proved beneficial in other efforts such as cost/benefit analysis, transit service planning, or air quality planning. Similarly, there is an interest in the non-motorized modes: bicycling and walking. In some of the small MPO areas, particularly those in college towns, non-motorized mode shares have proved significant. Interest in non-motorized modes may also be motivated by decisions made outside the MPO planning process. For example, if the major university in a college town has chosen to pursue a policy of intense parking management, there may be reason to examine the impact of these policies on multimodal travel in the MPO area.
Full-scale regional modeling efforts are certainly not the only available approach to these challenges. Given the specialized nature of these challenges (for example, modeling a shuttle used by college students), "out of the box" modeling approaches may not produce useful results. Simpler analyses based on comparisons with similar situations, consultation with transit employees, or other 'back of the envelope" techniques, may produce equally good estimates of transit demand. As with other modeling and analysis challenges, effective solutions have started with the question, "What do we want to evaluate?", and only when that question is answered to do they move on to, "What are the appropriate tools?"
Another aspect of being in a low growth area is that projects may have objectives other than increased capacity. One important objective is improved safety, something that is not covered in traditional demand models (even the typical traffic simulation will assume that drivers behave safely). Safety analysis techniques are discussed in the Highway Safety Manual1 and implemented in various supporting tools. Safety analysis techniques often make use of traffic estimates such as average daily traffic and thus may require information from other models or traffic estimation strategies.
Participants in the meeting expressed an interest in performance-based planning.2 A number of the participating MPOs are using, or are considering using, elements of a performance-based planning approach. Participants noted the desirability of defining performance measures that are relevant and measurable, within the constraints of limited resources for data collection and analysis. They also felt that the selection of planning models should be made not simply because certain tools such as a regional demand model are available or are traditionally considered necessary. Rather, tools were selected based on their ability to identify, evaluate and prioritize projects with respect to estimated impacts on the chosen performance measures.
1 AASHTO, 2010: http://www.highwaysafetymanual.org/Pages/default.aspx (Accessed 7/5/2012) and the FHWA Highway Safety Manual site: http://safety.fhwa.dot.gov/hsm/. (Accessed 7/5/2012)
2+ For one definition of performance measurement in transportation, see http://www.ops.fhwa.dot.gov/perf_measurement/fundamentals/index.htm. (Accessed 6/22/2012)