A London study recently investigated 6 years of travel survey data for major active travel infrastructure upgrades in Outer London. The active travel infrastructure upgrades included improvements to walking and cycling facilities and low traffic neighbourhoods. The study determined that 20 year health benefits yielded a ten-fold return on investment in relation to implementation cost.

Health benefits were primarily due to uptake of active transport modes (walking/cycling) and reduced car ownership. The research found that after one year, people living within the upgraded areas were, on average, walking and cycling for 41 minutes a week more than those living in comparable areas.

For more information, see:
https://www.theguardian.com/environment/bike-blog/2018/jun/26/mini-holland-schemes-have-proved-their-worth-in-outer-london-boroughs
and
https://www.sciencedirect.com/science/article/pii/S2214140524000173#fn7