Using a natural experiment to assess the effect of spatial barriers on health service utilization.

Abstract

The closure of the Manawatu Gorge in August 2011 caused a change in the travel time for patients living in the eastern area of the MidCentral Health District to their main hospital and health services located in Palmerston North. This presented an opportunity to study the effect a change in travel time and spatial access had on a population before and after such an event. This study used a retrospective cohort design, using routinely collected data from general practice, emergency department, hospital admissions and outpatient services. The investigation was completed using novel geospatial information systems methods to produce high fidelity data for analysis with free and open source software by developing and validating two new methods of improving geocoding data quality and a new travel time prediction model. Potential and realised spatial accessibility measures were calculated for 101,456 patients over 3.5 years while the gorge was both open and closed. Catchment sensitivity analysis and two-step floating catchment area using distance decays presented complimentary evidence of accessibility change during the Manawatu Gorge closure. Analysis of utilisation measures in both primary and secondary care were correlated with travel time. Utilisation of general practice services appeared to be negatively impacted by increased travel time when comparing realised accessibility in a control and intervention group during the gorge closure. It appeared as though other factors affected access to health services to a greater degree than an increase of up to fifteen minutes travel time.