In Part 1 of this blog series, we explored how Activity-Based Models (ABM) and trip-based models could be used to assess the impact of new toll lanes on SR167 in Seattle, Washington, and reinforced that an understanding of economics – specifically the value of time and money – is important to using them to analyze, justify, and communicate project benefits.

This post expands on those concepts by integrating elasticity data to gauge how price changes may affect traveler behavior on SR167 under the previously examined road pricing scenario. Price elasticity measures how changes in price, such as a new toll, influences travel demand for a thing, in this case a travel corridor. In congestion pricing, both the monetary and travel time costs change, as pricing aims to reduce travel time and improve reliability. The price elasticity of the SR167 corridor users can offer additional insights into modeled results, revealing the cost increase that may be required to achieve the observed travel reduction in each model.

For further context, we looked at recent research summarizing roadway pricing experiences and a local example, the tolling of the new SR99 tunnel through downtown Seattle in 2019. Once completed, the SR99 tunnel operated toll-free from February until November 2019, providing an excellent case study for understanding the price sensitivity of local drivers. The cost-recovery pricing (tolls that repay the costs of building and maintaining the tunnel) through the SR99 tunnel informed the initial toll rates of $1 during off-peak hours and $2.25 during peak hours. While the Washington State Department of Transportation (WSDOT) anticipated a 50% diversion of the 75,000 daily vehicle trips through the tunnel, only about 30% of pre-toll daily traffic were found to divert following introduction of the toll.[1] According to WSDOT, “While daily volumes indicated that drivers experimented with alternative routes and modes, most changes were temporary and remained close to baseline averages.”[2] The 30% diversion rate aligns well with draft research from worldwide toll road sector expert, Robert Bain, who found that about 28% to 40% of baseline traffic diverts after a roadway is tolled, based on over 20 case studies.

The north portal of the SR 99 tunnel with a toll rate display. (Photo by Doug Trumm)

Research by Robert Bain

With a collective understanding of transportation economics, we can drive towards a more equitable, efficient, and informed future. Join the conversation today!