Use of Percentages in Assessing Induced VMT
Projects that add roadway capacity will frequently induce additional motor-vehicle travel, a concern under CEQA. This is why SB 743 resulted in the evaluation of induced travel in terms of vehicle miles traveled (VMT) as the most appropriate metric for determining transportation impacts. Methods for calculating the amount of induced VMT can be found in the September 2020 guidance documents Transportation Analysis Framework and Transportation Analysis Under CEQA. Where significant induced travel is predicted, that prediction sets the goal for mitigation. The induced travel analysis should address both direct impacts as well as the potential for transportation impacts which are cumulatively considerable when viewed in connection with past projects, the effects of other current projects, and the effects of probable future projects.
How induced VMT compares to existing, total VMT in a project area is not relevant to the discussion of direct impacts. Most induced VMT from individual projects will be below 1 percent of existing VMT in a county or region. This is to be expected, as induced VMT is a function of new capacity. If a project adds less than 1 percent to the total of lane-miles in a region, it will induce less than 1 percent of new VMT.
The effect is similar for almost any direct environmental impact. Any individual transportation project will likely contribute a small amount of criteria pollutants, storm water runoff, noise, land consumption, or other impacts, compared to those impacts over a county or region.
Another analytical flaw when using percentages as opposed to absolute VMT is that they depend on the denominator chosen. A 1 percent change in a place with a lot of existing VMT implies much more absolute VMT - and hence emissions and other adverse environmental outcomes - as compared to the same percentage change in a place with low existing VMT.
In summary, the important number in analyzing the direct induced VMT from a project is the absolute value of the increase, e.g. 1 million additional annual VMT.