Deterrence functions

If the method of determining deterrence parameters was repeated using an even larger survey size, then statistically significant results by trip purpose may be determined. This would allow modelling of separate parameters based on each land use (journey purpose) and transport mode, leading to an improvement in the quality of the derived accessibility analysis.

An increased survey size could be achieved by either immediately expanding the sample size of the NZHTS, or waiting until the addition of successive years of data has sufficiently increased the size of the NZHTS dataset.

Therefore, the following recommendations are made:

·         The deterrence parameters that have been developed for the transportation mode types for various OD types should be revised when updated travel survey data becomes available.

·         The analysis of the deterrence parameters should be extended to include other modes of transport such as taxi and train once the sample size for these modes reaches an acceptable size.

Even so, the measurement of the deterrence parameters using the NZHTS assumes ‘measured’ behaviour reflects ‘desired’ behaviour. Strictly speaking this is incorrect because measured behaviour already includes the spatial distribution of the transport network and land use. Consequently measured behaviour is already influenced by ‘need’, ‘accessibility’ and a number of other factors specific to the individual undertaking the specific journey.

Although, in the interest of pragmatism, deterrence parameters derived without reference to the NZHTS would be more ideal, the current approach is considered appropriate. This is an area of future study although in the interim, it is hoped that further analysis and understanding of the NZHTS to account for the difference between measured behaviour and desired behaviour will assist to determine and refine the appropriate deterrence parameter values. Additionally it might be that research regarding travel adaptive capacity assessment can be used to supplement the NZHTS data (Krumdieck et al 2012).

Regardless, if the NZHTS is used in its entirety or supplemented with ‘desired’ behaviour, the data should be tested using logistic decay functions that are expected to produce better mathematical fitting models than a negative exponential function (λ).