Perhaps carpooling hasn't been incentivized enough: the current in-app economics for 'pooling' appears to be an increased time to destination but decreased fare (+/- social interactions with driver and passenger(s)). If congestion is a function of cars on road, car size, number of passengers, and path to destination, then perhaps shorter rides with more passengers in smaller cars should be incentivized more. This see…
Even with a self driving carpool option the bus still takes off 9 other vehicles (assuming 5 passengers in a vehicle).