Promoting healthy drink choices at school by means of assortment changes and traffic light coding: A field study

•We increased the assortment of healthy drinks sold at Belgian schools.•This intervention was complemented by a traffic light system signaling drink healthiness.•Compared to a control school, unhealthy drink choices decreased in intervention schools.•The market share of red beverages dropped by more than 30 percentage points.•The intervention effect was not sustained after the invention had been discontinued.