Developing and validating a road travel time network for cost path analysis

Modelling access to services, using cost path analysis and distance decay models, can be improved if travel time rather than just distance is taken into account. If just straight line distance is used then traffic congestion in urban areas and rough bendy rural roads are not considered in the analysis and the results are subsequently distorted. An accurate road network layer that shows travel speed and travel time for each road segment is an important component of GIS techniques for modelling accessibility based on time. The problem is that developing such a road network can be an expensive exercise. This paper will demonstrate cost effective techniques that can be used to develop such a road network using a standard road network, a landcover data set, and a sinuosity index. Road layers often contain information that describes the road characteristics and additional information on the bendiness of the roads can be obtained from calculating the sinuosity of the road arcs in the network. A process for calculating sinuosity and estimating road travel speeds is described. The travel times of the road network are validated by calculating the travel times between the main towns in New Zealand and comparing this to the travel times published by the New Zealand Automobile Association (AA). The travel times using the road network, were overall 6.6% faster than the AA travel times.