You need roads anyway! Most of the businesses need to get trucks in to deliver the things they need. Thus you can reuse those roads for buses for places that are less busy and this is cheaper than a train. The real trick is how to get private one passenger automobiles off the roads.

In very dense areas you should have both roads and trains. However only the most dense cities in the world can support that, and then only in their most dense which is often small. For everyone else buses save money over trains.