Active travel (walking or cycling for transport) is considered the most sustainable form of personal transport. Yet its net effects on mobilityrelated CO 2 emissions are complex and under-researched. Here we collected travel activity data in seven European cities and derived life cycle CO 2 emissions across modes and purposes. Daily mobility-related life cycle CO 2 emissions were 3.2 kgCO 2 per person, with car travel contributing 70% and cycling 1%. Cyclists had 84% lower life cycle CO 2 emissions than non-cyclists. Life cycle CO 2 emissions decreased by-14% per additional cycling trip and decreased by-62% for each avoided car trip. An average person who 'shifted travel modes' from car to bike decreased life cycle CO 2 emissions by 3.2 kgCO 2 /day. Promoting active travel should be a cornerstone of strategies to meet net zero carbon targets, particularly in urban areas, while also improving public health and quality of urban life.

Active travel (walking or cycling for transport) is generally good for health, the environment and the economy. Yet the net effects of changes in active travel on changes in mobility-related CO 2 emissions are complex and under-researched. Here we collected longitudinal data on daily travel behavior, mode choice, as well as personal and geospatial characteristics in seven European cities and derived mobility-related lifecycle CO 2 emissions from daily travel activity over time and space. Fixed-and mixed-effects modelling of longitudinal panel data (n=1849) was performed to assess the associations between changes in lifecycle CO 2 emissions and changes in transport mode use (primary exposure), main mode of travel, and cycling frequency (secondary exposures). Daily mobility-related lifecycle CO 2 emissions were 2.8 kgCO 2 per person at baseline, with car travel contributing 69% and cycling 1%. At follow-up, mobility-related lifecycle CO 2 emissions were-0.52 (95%CI-0.82 to-0.21) kgCO 2 /day lower per additional cycling trip,-0.41 (95%CI-0.69 to-0.12) kgCO 2 /day lower per additional walking trip, and-2.11 (95%CI-1.78 to-2.43) kgCO 2 /day lower per 'avoided' car trip. An average person cycling 1 trip/day more and driving 1 trip/day less for 200 days a year would decrease mobility-related lifecycle CO 2 emissions by about 0.5 tonnes over a year. Those who changed from 'not cycling' to 'cycling' decreased daily CO 2 emissions by-2.54 (95%CI-3.90 to-1.17) kgCO 2 /day. Mobilityrelated CO 2 emissions decreased by-9.28 (95%CI-11.46 to-7.11) kg/day for those who changed their 'main mode' from car, van or motorbike to active travel. Extensive sensitivity analyses by city, journey purpose and key personal characteristics largely con rmed our results. Active travel is shown to substitute for motorized travel, with signi cant climate change mitigation effects. Even if not all car trips could be substituted by active travel the potential for decreasing emissions is considerable and signi cant. Investing in and promoting active travel should therefore be a cornerstone of strategies to meet net zero carbon targets, particularly in urban areas, while also improving public health and quality of urban life. 'objective' built environment characteristics included here were (see Gascon et al., 2019 for how these

Active travel (walking or cycling for transport) is considered the most sustainable form of personal transport. Yet its net effects on mobility-related CO2 emissions are complex and under-researched. Here we collected travel activity data in seven European cities and derived life cycle CO2 emissions across modes and purposes. Daily mobility-related life cycle CO2 emissions were 3.2 kgCO2 per person, with car travel contributing 70% and cycling 1%. Cyclists had 84% lower life cycle CO2 emissions than non-cyclists. Life cycle CO2 emissions decreased by -14% per additional cycling trip and decreased by -62% for each avoided car trip. An average person who ‘shifted travel modes’ from car to bike decreased life cycle CO2 emissions by 3.2 kgCO2/day. Promoting active travel should be a cornerstone of strategies to meet net zero carbon targets, particularly in urban areas, while also improving public health and quality of urban life.

Active travel (walking or cycling for transport) is considered the most sustainable form of getting from A to B. Yet the net effects of active travel on mobility-related CO2 emissions are complex and under-researched. Here we collected travel activity data in seven European cities and derived lifecycle CO2 emissions from daily travel activity. Daily mobility-related lifecycle CO2 emissions were 3.2 kgCO2 per person, with car travel contributing 70% and cycling 1%. Cyclists had 84% lower lifecycle CO2 emissions from all daily travel than non-cyclists. Lifecycle CO2 emissions decreased by -14% (95%CI -12% to -16%) per additional cycling trip and decreased by -62% (95%CI -61% to -63%) for each avoided car trip. An average person who ‘shifted travel modes’ from car to bike decreased lifecycle CO2 emissions by 3.2 (95%CI 2.0 to 5.2) kgCO2/day, and using a bike as the ‘main method of travel’ gave 7.1 (95%CI 4.8 to 10.4) kgCO2/day lower lifecycle CO2 emissions than mainly using a car or van. Investing in and promoting active travel should be a cornerstone of strategies to meet net zero carbon targets, particularly in urban areas, while also improving public health and quality of urban life.