NCHRP report shows high variation, but general decline, in VMT forecasts

By Chris McCahill

A new tool, called Impacts 2050, provides important insight into the uncertainty associated with conventional travel demand forecasts by allowing users to model different future scenarios while taking socio-demographic trends into account. In a report for the National Cooperative Highway Research Program, the tool’s developers describe the outcomes for a wide range of possible scenarios and suggest that demand for automobile travel will likely stay at its current level or drop markedly in the coming decades. VMT per capita decreased in 13 of the 20 scenarios tested, often by more than 50 percent. This represents a major shift from past trends, as documented by SSTI, and has proven difficult to predict by conventional means.

As the NCHRP report points out, complex travel demand models are typically poor at predicting future passenger travel or adapting to different scenarios. Official VMT forecasts from USDOT seem to suffer from this shortcoming, as previously noted by SSTI. The Impacts 2050 model is simpler—an important strength, according to high-stakes forecasters and related literature—and uses a system dynamics approach, which can handle time series data and account for feedback loops reasonably well.

Figure 1. Historical VMT per capita and average model output for each test scenario. Source: NCHRP 750 Vol. 6

The researchers modeled four scenarios in five major metropolitan areas through the year 2050. VMT per capita increases in only one of the scenarios, called Tech Triumphs, by as much as 6 percent. In that scenario, technological innovations and private sector transportation investments lead to economic growth, declining energy costs, and further decentralization of metropolitan areas. In the business-as-usual model, called Momentum, VMT per capita decreases by 5 to 12 percent. In two of the scenarios, called Global Chaos and Gentle Footprint, VMT per capita decreases by more than 50 percent. The authors explain:

The decrease in auto VMT per capita is strong in the Global Chaos and Gentle Footprint scenarios due to distinct reasons. In the Global Chaos scenario, fewer trips are made due to poor economic conditions. In Gentle Footprint, we see an increase in environmental consciousness that is associated with both a decrease in reliance on the auto and an increase in the use of alternative modes of travel.

As with any long-term forecast, the actual outcomes under any scenario are highly uncertain; but taken together, the scenarios give a reasonable representation of likely outcomes. Many of the key factors driving these trends—economic growth, job creation, age of the population, migration patterns, and environmental attitudes—are largely outside the control of DOTs and MPOs. However, the scenario planning approach allows stakeholders to work toward a preferred scenario (through long-range plans, policies, and infrastructure investments) and to understand the tradeoffs that come with each.