A research project was undertaken to appraise international evidence on the effects of changes in urban public transport systems and services on the extent of switching to/from car travel and on total road traffic volumes, and to develop guidelines for use in the evaluation of urban transport projects in New Zealand.
The major part of the project involved collection and appraisal of international evidence, for situations where changes have been made to the urban public transport system, on the proportion of additional public transport trips that would otherwise be car driver trips, and on the effects of the mode switching on overall road traffic volumes.
Evidence was collected mainly from Europe, USA and Australia and appraised by type of public transport change, ie. major new corridor projects, service enhancements, fare changes and on-road priority projects.
It was found that the 'diversion rate' (ie the proportion of additional public transport trips that would otherwise be car driver trips) varied by country, dependent on initial mode shares, car availability, urban density, alternative modes and other factors.
With a given country, similar diversion rates applied to major new projects, service enhancements and general fare charges; but with higher rates for projects particularly oriented to motorists and with lower rates for projects with a more 'social' focus.
Recommendations were made in regard to the most appropriate 'diversion rates' for use in New Zealand's major urban centres; and as to when case-specific surveys should be undertaken instead of or to supplement such 'diversion rates'.
The international evidence found that major public transport projects can have significant effects on road traffic, with between 2% and 10% of motorists in the corridors affected switching to public transport.
However, in practice, surveys have rarely been able to detect significant changes in overall road traffic volumes: the extra road capacity made available as a result of the mode switching appears to be taken up by additional car traffic (through additional trip generation, changes in the time of travel and changes in routing).
Keywords: mode switching, urban public transport, traffic volumes, diversion rates