Fixing transport infrastructure would have the most impact. Narrower roads with fewer lanes and more complexity, 20mph/30kmph speed limits, better designed pedestrian crossings, and separated bike and pedestrian infrastructure. And requiring the vehicles themselves to be designed such that they are not just safe for the occupants, but safe for other vehicles and people too (which means lower hood heights and lower weight).
And in general, providing viable alternatives to driving so there are less vehicles on the road, making it safer to walk and bike.