Well, first of all, there is no such thing.

However, even if such a thing did exist, you still have to contend with traffic and total road capacity. So, for instance, taking local systems (details will vary by city, but not much), a double-decker bus is ten meters long and takes ~100 people. A tram is 55m long and takes about 550 people. A commuter train is 160m long and takes a thousand people (and doesn't share roads, of course). A car is 4-5m long and typically carries one or two people. Take any large city, attempt to replace the public transport with private cars, and there simply _will not be space_.

Fully-segregated metros in particular can also be much faster than cars in urban areas; they don't have to contend with traffic or intersections at all.