For BRT, it doesn't make any sense to take the vehicle type that is the most expensive to operate and has pretty much the lowest quality rider experience, and build a really big expensive system around it.

Streetcars are more comfortable to ride than buses, but their service is slower and less reliable than buses. Except under certain conditions where there are a lot of passengers, they cost more to operate, and they have much much higher capital costs. They make sense when the city already has a legacy streetcar system, since you don't have to pay to build what is already built.

Light rail on roads, even in the center with its own lanes, doesn't really make much sense, because being in the road its still constrained by being in a mixed environment. It's not safe for light rail to go much faster than cars even if they're not sharing lanes. And the train headways have to correspond to sensible traffic light timings. It just costs way too much for the quality of service being provided.

Light rail that is built somewhere with existing grade separation, like an existing rail right of way, or freeway median or undeveloped land, can be cheap to build and can have good service quality. A lot of money can be saved by going onto a street sometimes instead of going over or under it. There's two issues though. First is that most of the time these cheap ROWs aren't located where people want to go, and there's not the land use planning and development activity to change that. Second, the better it is for light rail [[the more grade separated it already is), the easier it is just to go all the way and make it completely grade separated, and do it as a metro. Metros in general have better service quality and are cheaper to operate. And if it's 100% grade separated you can automate it, which dramatically lowers operating costs, and even reduces capital costs.

So for most situations BRT and light rail end up being in this awkward middle where they often get the worst of both worlds.


Regardless, any city that plans on having transit in the long term should not have all of their routes being buses. It's like buying cheap used clothes, wearing them once, and throwing them away, because it's too expensive to install a clothes washer and dryer. Or eating fast food for every meal because it's too expensive to buy a stove. Investing in the appropriate modes saves money in the long run.