Study finds improving bike, pedestrian infrastructure cuts driving, CO2 emissions

By Chet Edelman
In an attempt to meet CO2 reduction targets, both mandatory and self-administered, cities worldwide are attempting to overhaul their transport infrastructure to limit private vehicle use and encourage more active forms of travel (i.e., walking and biking). While the common assumption among planners is that greater rates walking and biking will lead to subsequent decreases in driving, there is in fact very limited evidence to suggest that this is the case. A new study from New Zealand, however, may shed light on the matter. According to the authors’ findings, investing in bike and pedestrian infrastructure does in fact reduce driving-related CO2 emissions. The authors found that systematic improvements to walking and biking networks in two New Zealand cities led to a 30 percent increase in active travel, which in turn reduced distance traveled per vehicle by 1 percent and CO2 emissions by 1.6 percent.
To evaluate the impacts of infrastructure improvements on driving behavior, researchers identified two similarly sized cities in New Zealand in the process of updating and improving their biking and pedestrian network in addition to two control cities where no intervention was in place. Using odometer readings collected through annual New Zealand Transport Agency vehicle inspections, the authors compared vehicle distance traveled in both the intervention and control cities to identify whether travel did in fact drop due to infrastructure improvements. Additionally, the researchers conducted in-person household interviews to determine the socioeconomic characteristics and travel behaviors of people within the study area. Through survey data, the authors could detect changes in travel behavior that the odometer readings would be unable to measure, such as walking and biking.
After comparing the odometer and survey data over the course of three years, the authors found that active travel increased by almost 30 percent in the cities where the intervention took place. The increase in active travel, however, only led to a 1 percent decrease in distance traveled per vehicle and a 1.6 percent reduction in CO2 emissions. While this number may seem low given the large increase in walking and biking, a relatively small reduction is not surprising given that walking and biking trips typically replace shorter car trips. More specifically, the survey data found that while driving trips less than five kilometers made up about 57 percent of total vehicle trips, they accounted for less than 13 percent of total vehicle distance traveled. Therefore, a 30 percent increase in active travel only lowering vehicle travel by about 1 percent is to be expected.
Considering this is one of the only studies to date demonstrating reductions in CO2 emissions are achievable through improvements to biking and walking networks, it is difficult to say whether the results are case specific or can be generalized on a larger scale. Regardless, the findings suggest that while efforts to reduce CO2 emissions through active travel infrastructure are encouraging, a host of other interventions must also take place simultaneously in order for cities to seriously address their respective carbon footprints. Greater pedestrian and bicycle mobility is just one piece of the puzzle.
Chet Edelman is a Project Assistant at SSTI.