Alleviating the negative side effects of ramp metering: development of a supplementary ramp metering algorithm

Ramp meters reduce the total travel-time when aggregated across all freeway users. However, the distribution of this benefit is highly skewed, with some freeway users experiencing an increase in travel time. The bias in benefit distribution can have negative side-effects such as user dissatisfaction, encouraging trips from outer-rim suburbs, and exacerbating urban sprawl. These side-effects can restrict road authorities in realising the full potential of ramp meters and are in conflict with demand management strategies of expanding cities. The root cause of this issue is the variation in ramp metering delay across the on-ramps, which real-world ramp metering algorithms do not consider for the primary control decisions. In this research, ramp metering delay is considered as a form of congestion-toll that is paid at the on-ramps. A model based algorithm is developed that dynamically controls the ramp metering delay at each on-ramp according to the level of congestion it is predicted to cause. Predictive ramp metering can be inefficient given the stochastic nature of bottleneck capacities; therefore, the proposed algorithm is deployed as a supplementary algorithm that extends the functionality of existing reactive control methods. As a by-product a framework for evaluation and development of coordinated ramp metering algorithms is introduced, in addition to a practical method for measuring and controlling ramp metering delays. Examination of the proposed supplementary algorithm in combination with the well-known HERO algorithm indicates that the benefit distribution bias is significantly reduced, whilst the total travel time can be reduced because the controllability of the system is increased. This method aligns the short-term control problem with long-term planning concepts such as demand management and equity, which is expected to provide road authorities with greater leverage to realise the full potential of the ramp metering system