Does micromobility always contribute to traffic emission reduction? Modeling vehicle interactions in heterogeneous traffic flows

Micromobility, comprising small, lightweight vehicles, has gained popularity globally as a sustainable solution for achieving climate neutrality in metropolitan areas. While micromobility offers environmental benefits such as energy savings and emission reductions, interactions between micromobility vehicles (MMVs) and motor vehicles (MVs) can disrupt regular traffic flow and undermine these efforts. Given the dominance of regular bicycles (RBs) and electric bicycles (EBs) in China, this study focuses on these two modes as representatives for MMVs and proposes an improved heterogeneous flow simulation model to analyze the impact of MMV behaviors on MV operating dynamics. Observed MMVs behaviors (e.g., invasion and spilling) are modeled and defined as distinct lateral and longitudinal movement rules for RBs and EBs due to their different movement characteristics. A vehicle-specific-power-based (VSP-based) model with localized emission parameters is applied for accurate tailpipe emissions estimation, contrasting with traditional regression model. Field data from Beijing, China were collected for model calibration and validation, achieving an average deviation in simulated vehicle speeds of 4.70%. Results indicate that MMV interactions contribute up to 6.26% and 17.16% increases in NOx and CO emissions, respectively. Additionally, the adoption of EB reduces CO2 and HC emissions by 18.22% and 8.48%, respectively, compared to the RB-only scenario. This study further discusses the applicability and effects of various micromobility policies, including travel demand management, installing dedicated lanes, increasing e-powered penetration, and traffic calming. Adapting micromobility policies according to MV and MMV densities is crucial to preventing disproportionate emissions reduction efforts. The proposed model enhances the understanding of potential negative effect of micromobility on decarbonization objectives and serves as an effective tool for policymakers to assess and promote the effectiveness of micromobility policies.