Managed motorways - science for optimisation (paper ID 525)

Metropolitan motorways must facilitate 'Good quality journeys to many people', e.g. in the order of 1 million trips in case of Melbourne's M1 corridor. The relationships between the classical traffic flow theory parameters speed, flow rate (including Capacity as the 'maximum sustainable flow rate') and density need to be well understood so that adverse phenomena such as flow breakdown and congestion can be avoided. The concept of Capacity needs to be understood for geometric and generalised operational design of motorways. A refined methodology to determine urban motorway capacity was utilised and critical findings have emerged to inform decisions at various stages of a motorway asset's life cycle. Real-time control requires an understanding of the dynamic nature of traffic at small time intervals. Detailed data demonstrates the dynamic nature of motorways, which should be expected given the uninterrupted flow environment with many degrees of freedom for individual drivers. VicRoads findings indicate that traffic flows around capacity values as classically understood (i.e. the flow rate at the apex of the speed-flow relationship) are inherently unstable - relatively high probability of flow breakdown over extended peak periods. Controlling a motorway to achieve maximum throughput can produce sub-optimal reliability, increased travel times, potentially unsafe operating conditions and may yield sub-optimal vehicle kilometres travelled. Capacity values determined from extended data sets represent historical and statistical outcomes. Therefore the capacity determination process masks the fine grain variations that are inherent in motorway operations that are representative of the true dynamics of traffic flow. A revised approach to optimising urban motorway operations is emerging based on optimisation of productivity. A deep understanding of the fundamental traffic relationships is required to determine the unique tuning points for every site in a control led motorway network. The presentation will include latest updates compared to previous VicRoads publications.