The scenarios produce a wide range of forecasted VMT levels in the year 2030, as shown in Table 1. The business-as-usual scenario, based on the Trends of the 2000s land use and no changes in transportation pricing, is estimated to result in an increase of 9.5 million household VMT per day, a growth of 21% over 2010 levels (estimated at 44.5 million VMT per day). With this level of VMT growth, technological improvements to vehicle fuel economy would have to yield an 18% reduction in per-mile carbon intensity just to break even on GHG emissions. Note that the VMT estimates produced in this study refer solely to household travel, and thus will differ from the total VMT numbers reported by MassDOT or the Boston MPO.

Since the average fuel efficiency of the Massachusetts passenger vehicle fleet was about 24.5 miles per gallon in 2010, it would need to rise to an effective fleet-wide average of 29 miles per gallon in 2030 in order to achieve a break-even point with carbon emissions. Whether that level of reduction is possible depends on a variety of factors: federal fuel efficiency standards for new cars, the rate of fleet turnover, the share of electric vehicles, and the carbon intensity of the electrical grid. Even if the Obama-era Corporate Average Fuel Efficiency (CAFE) standards were reinstated, fleet turnover would also have to increase substantially in order to achieve this target in the next 10 years. The scope of the technical improvements necessary to achieve GHG reductions (or merely restrain GHG growth) in the face of rising VMT underscores the importance of land-use policies that support VMT reductions. These reductions will be essential to fully reap the benefits of vehicle efficiency improvements in the timeframe needed to achieve the Commonwealth’s ambitious carbon mitigation targets.  

The results of the other two land use scenarios also indicate the significant role land use patterns play in regionwide VMT growth. Relative to 2010, the base Smart Growth scenario (with no pricing changes) predicts an increase in VMT of 19.0%, the base Trends of the 2000s scenario predicts an increase in VMT of 21.4%, and the base Sprawl scenario predicts an increase in VMT of 24.2%. This 5.2% spread in the VMT growth rate between the Sprawl and Smart Growth scenarios is equivalent to nearly 2.3 million vehicle miles travelled per day. As noted previously, these forecasts do not account for any land use policies that would target growth to specific locations within a Community Type, so the differences among them are conservative, but the relative impact of the Smart Growth scenario on VMT is still quite substantial. 

As expected, the model also forecasts that certain transportation pricing policies would have a substantial effect on future VMT, especially when applied in combination. We first explored the isolated impacts of increases in the gas tax of 18 and 51 cents above the current level of 24 cents per gallon. The resulting changes under the 18-cent increase were surprisingly small, with less than a 0.5% reduction in 2030 VMT compared with the scenario of no gas tax hike (Table 1). When the tax was raised 51 cents to 75 cents per gallon, VMT growth was reduced by 1.5% for the Trends of the 2000s and Smart Growth scenarios and by 1.2% for the Sprawl scenario. Even these reductions are smaller than the difference between the Smart Growth and Sprawl scenarios when no increase in the gas tax occurs. This means that the land use patterns can have an even larger effect than modest pricing policies: the Smart Growth scenario with the existing gas tax is better at curbing VMT growth than is a tripling of the gas tax under a Trends of the 2000s land use scenario. This result clearly demonstrates the substantial value of focused land use policies in restraining VMT growth, regardless of additional pricing policies. 

In the next scenarios we modeled the combined effect of the 75 cent per gallon gas tax with either a $0.25 per mile VMT tax, a tiered freeway congestion pricing fee, or the combination of both. For all of the land use scenarios, the separate addition of either the freeway congestion pricing or the 25 cents per mile VMT tax produced similar reductions in VMT, with the VMT tax having a slightly stronger effect (Figure 3). While neither additional policy produced net reductions in 2030 VMT compared with 2010,  VMT is projected to grow by only 2.8% in the case of the Smart Growth scenario with a 25 cent VMT fee and a 75 cent gas tax (Figure 3). For the Sprawl scenario with those same pricing policies, VMT could grow by 7.3%.  

These results indicate that fees must be set at relatively high levels to reduce significantly the growth in VMT, let alone to achieve any net reductions in VMT. This is due to the low price-elasticity of vehicle travel demand – a known feature of travel behavior that can be attributed partially to the lack of competitive alternative modes of travel in much of the region. Our analysis indicates that even a tripling of the gas tax may have only a modest effect on VMT, whereas VMT fees are a much stronger price signal.  

Why is this? Assuming pump prices (including tax) of $3.00 per gallon, fuel costs for a passenger vehicle with an average real-world fuel economy of 30 miles per gallon are roughly 10 cents per mile. A 51-cent increase in the gas tax would raise per-mile fuel costs to 11.7 cents per mile. For someone who drives 10,000 miles per year, this change would increase their fuel costs by only $170 annually, not enough to prompt significant changes in the behavior of most drivers. Furthermore, the price signal of a per-gallon gas tax will weaken as drivers acquire more fuel-efficient vehicles. In contrast, a VMT fee of 25 cents per mile, applied to 10,000 miles of driving, would add $2,500 to annual operating costs – an amount significant enough to induce changes in where people live, how much they travel, and what travel mode they choose.