Jamie Volker, researcher with the National Center for Sustainable Transportation at UC Davis, told the Transportation Commission webinar that empirical studies consistently show substantial long‑run induced travel after roadway capacity expansions and explained how those findings underlie the NCST calculator.

Volker described induced travel with simple economics: when roadway supply increases and perceived cost of driving falls, driving increases. “If a lane magically appeared, you would be moving faster. But what happens in the long run is that you often end up back where you started,” he said, summarizing empirical patterns seen in U.S. and international studies.

He summarized core empirical results: short‑run elasticities (generally measured over months to a few years) are typically in the 0.3–0.8 range; long‑run elasticities (three years plus) commonly approach or exceed 1.0 for interstates, freeways and arterials (FHWA facility classes 1–4). Volker also said the evidence shows induced travel generally adds VMT to the system rather than merely shifting traffic from one road to another.

Volker explained two main project‑level estimation approaches. Travel‑demand models can capture local context and short‑run feedbacks if they are activity‑based and include many feedback loops, but they do not always capture longer‑term land‑use responses. Elasticity‑based approaches use peer‑reviewed elasticity estimates to produce an order‑of‑magnitude estimate of induced VMT; NCST’s calculator is one such tool and was designed as a repeatable ‘‘back of the envelope’’ method for projects in urbanized counties.

“Elasticity based estimates are grounded in empirical evidence, and they do fully capture the sources of induced VMT, which travel demand models do not do a good job of,” Volker said. He also acknowledged limitations: the NCST tool is designed for California’s 37 urbanized counties and is not recommended for non‑MSA rural counties because less empirical evidence exists for those areas.

Volker reviewed remaining research needs: more facility‑level, context‑sensitive empirical studies to identify how factors such as baseline congestion, lane type (for example, managed lanes), facility design and regional land‑use pressures alter induced demand; and updated datasets that reflect post‑COVID travel patterns. He said preliminary evidence suggests managed lanes and tolled/managed lanes can produce induced VMT similar to general‑purpose lanes, but more project‑level study is needed.

Volker listed existing resources and studies and said researchers are continuing empirical work and collaborations with state agencies to refine tools and methods for project‑level analysis.