The Potential of social routing advice
Mariska van Essen is a PhD student at the Centre for Transport Studies. Her supervisors are prof.dr.ir. E.C. van Berkum from the Faculty Engineering Technology (ET) at the University of Twente and prof.dr.ir. C.G. Chorus from the Faculty of Technology, Policy and Management (TPM) at the Delft University of Technology.
Traffic congestion is one of the main problems of today’s society. It arises when many travellers want to use the same low-cost route at the same time. Such selfish route choice behaviour leads to a so-called user equilibrium; a network state in which all travellers use the shortest available route and therefore travel times on all used routes are equal. However, the existing road network is not used to its fullest potential. A more efficient network state, the so-called system optimum, arises when travellers take into account the additional travel time they impose on others. As such, the total travel time within the road network will be minimised. However, travel times on used routes are no longer equal. As a consequence, some travellers need to act non-selfish and take a detour to the benefit of others (i.e. the network as a whole). An important question in this regard is how to motivate those travellers to take this detour. This thesis deals with the problem of improving road network efficiency by stimulating social choice behaviour using social routing advice. Stated choice and revealed choice experiments provide insights into individual compliance behaviour, while network simulation indicates the resulting network impacts.
At the individual level, results show that travellers are sometimes willing to choose the social detour over their usual route when they are being asked or advised to do so. Nonetheless, in daily-life travellers comply less with the advice than stated. Results from the stated choice experiment indicate a significant difference in compliance behaviour across different information frames, societal goals, sizes of travel time sacrifices and personality. More specifically, travellers are most likely to choose the social route when small travel time sacrifices are involved, when information messages are aiming at alleviating congestion as opposed to safety or environmental concerns, and when these messages are framed according to certain strategies that nudge travellers towards the social route, reinforce social norms towards taking the social route or educate travellers on the importance of taking the social route. Moreover, travellers are most likely to comply with the received advice when they are cooperatively oriented and when they make their choices in a non-habitual manner. Nonetheless, above findings are less evident from the results based on revealed choices. That is, the main motivation for revealed compliance seems to be an intrinsic motivation to contribute to improved throughput. The main motivation for revealed non-compliance with social routing advice relates to perceived traffic conditions; i.e. the social route is believed to be crowded as well or the usual route is not busy at all.
At the network level, results show that if all travellers comply with the social routing advice, 3.6% of total travel time at current traffic conditions could be potentially saved in the research area during the morning peak; this corresponds to regional economic benefits of about €2,600,000,- each year. However, an information-based demand measure using social routing is expected to direct only a minority of trips towards social routes (taking into account market penetration, compliance behaviour and detour alternation among travellers). As such, potential travel time savings could only be achieved to some extent. Moreover, individual benefits will be marginal; the majority of travellers experience a travel time gain of less than two minutes. However, structural congestion and unstable flows would be reduced, especially on major city roads, while trips seem to be redirected through minor local roads, increasing cut-through traffic. Moreover, it seems that the lower the compliance among travellers, the more social travellers need to compensate for the selfish behaviour of others by taking longer detours. Finally, it is demonstrated that impacts on network performance highly depend on the spatial distribution of social travellers among origins and destinations.
