In 2022, a survey was developed to try to gain insight into why a significant increase in morning peak traffic volumes was observed on the I-75/I-575 Northwest Corridor (NWC) in the Atlanta metropolitan area after the opening of the Express Lanes. It seemed unlikely that the increase was due to induced demand (increased total vehicle miles traveled (VMT) that was suppressed due to congestion), as most morning peak trips are generally mandatory trips such as work trips, school trips, and daycare trips. The previous research team suspected that the increase may have come from a diversion of commute traffic from arterials onto the freeway corridor, or from a shift of traffic from the shoulders of the peak to the center of the peak once the Express Lanes opened and congestion declined. Where the significant increase in traffic volumes on the NWC came from is an important question, especially for transportation planners, because it helps decision-makers get a better understanding of what the effects of opening new managed lane capacity along a corridor might be on traffic patterns around that corridor.
Survey invitations were distributed through two different channels: email invitations and postcard invitations. Email invitations got a response rate of 3.26% and postcard invitations got a response rate of 2.88%. It was found that 53.4% of the observed 35% increase in vehicle throughput came from sources that would not be expected to increase total VMT (39.7% from route reassignment and 13.7% from departure time of day shift). The other 46.5% of the observed increase in vehicle throughput came from sources that could increase total VMT (1.7% from changes in mode split, 5.7% from trip redistribution, and the remaining 39.1% from trip generation). These numbers assume that the sample of respondents is representative of the NWC service area, which may not be the case. Additionally, the behavior of the relatively large number of Express Lanes users is not likely to be representative of the corridor users overall. In future work, this should be checked and adjusted for. These numbers also fail to capture non-commuters who may have used the NWC in the morning peak hours, which may be a source of error causing trip generation to be overestimated. Assuming that the induced demand numbers presented herein are correct, after the opening of the NWC Express Lanes, total VMT in the corridor’s service area may have increased by 16.3%.
One method that many agencies and governments are using to try and reduce total VMT is paying for incentives to encourage carpooling. This survey’s findings that 49.0% of adult carpools are fampools (carpools consisting of only family members, who are likely to have carpooled anyway without the incentive) confirm others’ findings that a substantial proportion of carpools are fampools on urban highways (Poole and Balaker, 2005; Li et al, 2007). Induced demand generating an increase in total VMT is of particular concern to transportation planners because it reduces the congestion reduction benefit that can result from increased road capacity and it increases external costs (Litman, 2001), such as parking demand, uncompensated crash damages, and environmental impacts (such as emissions and pollution). Induced demand is something that transportation planners should investigate when considering new transportation infrastructure or substantial expansions to existing transportation infrastructure.