Debatable, and largely depends on a person’s diet and some other factors like how much travel is getting done. If someone is fueling their biking (or walking) by flying in beef from the other side of the world, I think it is pretty safe to say that their carbon footprint is worse than a typical gas car, (because air travel and beef are just that bad) or if not that at least an electric car from renewables and ethically sourced materials. For everything else in between, we’d just be speculating and we’d have to factor in source and type of car fuel, and the source and type of additional food consumed by a cyclist where that “additional food” line lies exactly.

Controlling for diet, distance and purpose of travel, I think cycling virtually always wins over walking.