Not only can you reach large audiences most effectively, but you can really engage with pedestrians the most in this case due to them waiting on their bus/tram/or train.

A lot of metro areas are adopting multi-use trails that contain residential, commercial, and outdoor activities all along a single corridor. These are great areas to show off your business to people, especially in a growing part of the city.

City centers are hotspots for office workers, party people, and all in between. Get the most eyes here from not only pedestrians but from passing cars as well, making sure you have the most utility from your ad space.