There is significant traffic congestion on freeways during peak hours in metropolitan areas across the country that lead to increased emissions and other impacts. Both recurring bottlenecks and nonrecurring events like crashes and severe weather events, result in breakdowns on freeways conditions reducing the mobility and reliability of the freeway system. These conditions push vehicles in to non-optimal speeds, shift traffic to local streets which increases conflicts with vulnerable users, while slower speeds and stop-and-go conditions increase pollutants and GHG emissions. While the road building era of the 1950s freeway networks is essentially complete, even minor strategies and investment intended to optimize existing roadway system assets are increasingly facing opposition in the name of “induced demand”. The public indicate concerns about increased greenhouse gases (GHG) and air quality impacts of such lane projects and lack of trust that the traffic analysis is being addressed sufficiently, and proper mitigation is being applied.

While recognizing road improvements have some level of “induced” and “latent” demand, and it’s not possible to build our way out of congestion, DOTs need assistance in understanding the best investments to balance competing goals. For instance, auxiliary lanes and intelligent transportation system treatments are identified by AASHTO as operational improvement designed to reduce congestion at localized recurring bottleneck locations. Some data have shown substantial reductions in congestion with auxiliary lanes by reducing conflicts in complex-weaving areas bringing these segments back up to their theoretical capacity. However, results are complicated by changes in travel patterns (re-routing, time of day), measuring reductions in incidents, and capturing safety impacts on parallel local roads and alternative modes. Additionally, while “induced” demand has been typically applied to large roadway projects, there are new concerns that auxiliary lanes (typically less than 1 mile of “new capacity”) may also induce traffic demand, limiting their efficiency in solving congestion bottlenecks.

Most induced demand research is tied to significant new lane-mile capacity changes with limited real world case studies and evaluation of conditions, especially for smaller roadway treatments. Under what conditions do major and minor induced demand issues occur, and how can these effects be mitigated through tolling or land use controls. How can roadways be a responsible component of a transportation system intent on tackling GHG emissions that encourage spreading demand across multi-modal options, where available. A definitive study of these and other minor “capacity” enhancements on both freeways and arterials is needed to improve DOT investment decisions for the long term.