Appraising an incentive only approach to encourage a sustainable reduction in private car trips in Dublin, Ireland

The research presented in this paper examines the potential for a reduction in car use in the Greater Dublin Area (GDA) as a result of introducing a range of sustainable policy incentives. A package of mode-specific incentives devised to encourage a mode shift away from single occupancy vehicle (SOV) use to walking, cycling, public transport, carpooling and car-sharing were examined for their potential to reduce car mode share. The policies tested were represented in a four-stage transportation model to generate new mode choices and subsequently new mode shares. Changes in mode share were similarly analysed in terms of tailpipe emissions savings from changes in SOV kilometres travelled and estimations of the potential monetary savings accrued from a reduction in emissions. Ultimately, an indication of possible reductions in private car use and the potential for a rise in sustainable mode choice and a reduction in car trips was ascertained in the GDA. Modelling was conducted in collaboration with the National Transport Authority (NTA) of Ireland to represent the policy changes by means of parameter modifications in the Mode Choice and Trip Assignment stages of the Eastern Regional Model. These changes were made to account for modifications made to infrastructure, frequency, time and cost parameters. A 2012 Base Year and forecasted 2035 scenario were considered in this study. Outputs from these modelled scenarios produced mode share estimates derived from new origin-destination trip demand and mode choice predictions. The results showed that the policy interventions tested incentivised more mode shifting behaviour among sustainable modes rather than from private car trips to public transport and active mode trips. Moreover, it was estimated that active mode users were more elastic or sensitive to changes made in the pedestrian network than the cycling network, which is reflected in mode shifting from cycling to walking. A reduction in the mode share of private cars of up to 1.6% and an increase in walking of up to 5% were estimated across the modelled scenarios in the GDA, and a daily monetary saving of up to €14,125 could be achieved as a result of reductions in CO2, NOx, and PM2.5 private car emissions from the introduction of the policy incentives tested. Overall, this study assesses the behavioural impacts of introducing a range of policy incentives alone on private car use and produces evidence to support the effectiveness of a carrot and sticks approach versus an incentive only method.