Look at Pittsburgh if you want to see so-called bus rapid transit
Starting in the 1963, Pittsburgh (Allegheny County to be exact) created a public transit agency which took Pittsburgh Railways, the region's streetcar company, by eminent domain and combined it with 33 local bus companies. Pittsburgh Railways fought it because they were still making a profit running the largest streetcar system existing in the US at the time. The bus companies were either bankrupt or struggling.
The new agency began ripping out the rails and eliminating street car service. The intention was to build a new overhead transit system which was bogus and eventually PA Dep. Sec. of Transportation Ed Tennyson killed the project, insisting the money be used to build a light rail line which has been treated more or less as an orphan system. Around the same time a new bogus idea called "busways" was created here, where they take existing railroad rights of way and convert them into exclusive bus highways at a cost of 5 to 10 times what it would cost to reuse them for rail transit.
Even with extravagant mismanagement, the rail operations have been more efficient than the bus operations. The rail serves the better off communities to the south of the city while the poorer, urban communities to the east are stuck with buses, even though the busway ridership numbers are double the amount where the FTA says rail would be more efficient.
The many disadvantages of buses over rail (e.g., noiser, air polluting, oil dripping runoff, poorer ride, to name a few). One thing that is seldom mentioned is that a fully loaded bus is way over the highway load limits -- if it were a truck with comparable weight on each axle, it would be heavily fined. The result is that buses destroy road surfaces faster than 18 wheelers, adding a hidden cost that is not included in their cost accounting. Add that to the accounting which already shows the various forms of rail to be more efficient than buses, and hands down the notion of bus efficiency is a deceptive illusion that keeps getting projected by those who subscribe to the bus only ideology. (The solution for idiocy is ideology; it doesn't cure it, it just gives them something to do.)
There can be a place for buses in a transit mix, such as feeding rail trunk lines. However, when relied upon as the sole component, it's chief claim to superiority, its flexibility and ability to service lower density settlement patterns, actually undermines its own viability by facilitating more and more dispersal to the point that high density, viably serviceable areas have their profitability eroded away and the whole system becomes ineffective. Even though there is a large percentage of the population in the Pittsburgh area who are transit dependent, the result is that their transit service is currently being decimated because it has destroyed its efficiencies.