Assessing the potential for carbon emissions savings from replacing short car trips with walking and cycling using a mixed GPS-travel diary approach

There is a growing recognition of the role that walking and cycling can make in reducing greenhouse gas (GHG) emissions by substituting motorized travel, particularly on short trips. However, there is a lack of evidence at the micro level on the realistic, empirically derived potential of walking and cycling to displace motorized travel and thus reduce GHG emissions. The aim of this study was to investigate the potential for GHG emissions savings from replacing short car trips with walking and cycling and the extent to which high quality infrastructure for walking and cycling may influence day-to-day travel decisions, change the spatial and temporal nature of local journeys and impact on overall GHG emissions from motorised travel. To achieve this aim this study conducted an in-depth observational study of a purposively selected cohort of 50 residents in Cardiff, Wales. Using a mixed-method approach detailed quantitative and qualitative data were collected for each participant using personal Global Position System (GPS) devices, 7-day travel diaries and contextual interviews over two seasonally matching 7-day time periods in 2011 and 2012. GHG emissions for motorized travel modes were derived using journey distance, vehicle technology details and average speeds obtained from the GPS data. The spatial and contextual data provided new insights into the complexities of walking behaviour and factors influencing cycling for everyday travel or recreation, including route choice decisions, activity destinations and the role of the new infrastructure to enable change. We found significant potential of active travel to substitute short car trips, with sizeable impacts on carbon emissions from personal travel. Half of all car trips were less than 3 miles long. Taking into account individual travel patterns and constraints, walking or cycling could realistically substitute for 41% of short car trips, saving nearly 5% of CO2e emissions from car travel. This was on top of 5% of ‘avoided’ emissions from cars due to existing walking and cycling. The evolving high quality walking and cycling infrastructure in the case study area was unlikely to promote a significant reduction in carbon emissions from (displaced) car journeys on its own. The study contributes to the debate on how to achieve stringent low carbon targets in urban transport. The combination of methods for data collection developed and employed in this study also helps to inform future research on the wider environmental impacts of active travel, including ‘co-benefits’ of improved air quality, reduced noise and reduced fossil fuel use.