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.
But buses actually don’t. you just pay for it later. Roads are one of the fastest deteriorating forms of infrastructure. the hole “ but rail dosnt make money” argument ignores the fact that… roads don’t ether
The time frame before a rail line will save money over buses is measured in decades.
With buses, you don't include the cost of road maintenance because buses share the roads with millions of other vehicles so the allocable cost is neglible.
Most European cities are transitioning towards dedicated bus lanes. In that case, in principle, that can be light rail or tram too. The biggest problem is crossing with normal roads. Trams are more tricky to implement than busses.
Roads for buses and trucks are far more expensive than for cars. Car drivers are absolutely subsidizing bus passengers by order of magnitude. Per passenger mile even Uber is cheaper.
A one lane road with minimal parking can handle a crazy number of deliveries and busses. Allow cars on that road and suddenly it’s almost useless as a transportation option.
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.