Multi-Regime Traffic Flow Modelling on Specific Urban Highway Infrastructures

Vehicle arrival headways are among the fundamental microscopic traffic-flow characteristics and provide important information about vehicle interactions within traffic streams. This study investigates the statistical properties of vehicle arrival headways observed under different traffic operating conditions on urban motorway facilities. Traffic data were collected at six locations in the City of Zagreb, including open motorway sections, a tunnel, a bridge, and an underpass. Vehicle speeds and arrival headways were determined from digital video recordings using a frame-by-frame traffic observation methodology. The collected data were analysed using lognormal and shifted-lognormal distributions. The results showed that the observed headway distributions can be adequately represented by shifted-lognormal models. Relationships between the distribution parameters and selected traffic-flow characteristics were then investigated. Particular attention was devoted to average vehicle speed and the share of vehicles travelling in platoons. The analysis revealed the existence of two distinct traffic regimes. The first regime was characterised by higher average vehicle speeds and lower platoon shares, whereas the second regime was characterised by lower average speeds and higher platoon shares. Based on these observations, both a single-regime model and regime-specific models were developed for estimating the parameters of shifted-lognormal headway distributions. An internal assessment procedure demonstrated that the regime-specific models provide better agreement with observed headway distributions within their respective traffic conditions than a single model estimated from the complete dataset. The results indicate that accounting for differences in traffic operating conditions can improve the representation of vehicle arrival headways on urban motorway facilities.