Typically the best return on investment is to build proper separated active travel infrastructure (e.g. cycle lanes).

From https://www.cyclinguk.org/briefing/case-cycling-economy:

> For every £1 invested, walking and cycling return an average of around £5-6. A ‘benefit to cost ratio’ (BCR) greater than or equal to 4 is considered to be ‘very high’ value for money. Putting this in some context, the predicted BCR for the HS2 (high speed railway) wasn’t nearly so impressive at about £2.30 for every £1 invested.