(I pulled these numbers off Wikipedia, but they seem about right.)
Light rail can carry up to 13,000 passengers per direction per hour.
A highway lane carries up to 1,900 cars per lane per hour.
It takes about 6 lanes of highways to match 1 lane of light rail given current car usage (1 passenger per car).
If usage patterns change (say, an automated carpool that maximizes the capacity of the vehicle -- say 5 people per car), you still have 1900 * 5 = 9,500 passengers per lane per hour, which is good, but still worse than a single lane of light rail. Hey, worse is better right?
However, the bottleneck in many cities isn't the highway, but rather the smaller city streets the highway feeds into. Transit can sidestep this bottleneck altogether with subway tunnels, which automated vehicles can't. Even after everything else, light rail is still more economical and more environmentally friendly.* Finally, light rail converts to automated train operation quite handily (further increasing capacity), especially if all the other cars around it are automated too.
* I was about to say more comfortable, but is being crammed into a small car more or less comfortable than standing on a cramped train?
I felt compelled to create an account because of this misconception (not really your comment, just within the whole thread) that mass transit isn't already automated.
Metros in most major cities already have trains that drive themselves. There may be a driver, but all they do is open and close the doors. Some systems (like Vancouver) don't have drivers at all.
Light rail can carry up to 13,000 passengers per direction per hour. A highway lane carries up to 1,900 cars per lane per hour.
It takes about 6 lanes of highways to match 1 lane of light rail given current car usage (1 passenger per car).
If usage patterns change (say, an automated carpool that maximizes the capacity of the vehicle -- say 5 people per car), you still have 1900 * 5 = 9,500 passengers per lane per hour, which is good, but still worse than a single lane of light rail. Hey, worse is better right?
However, the bottleneck in many cities isn't the highway, but rather the smaller city streets the highway feeds into. Transit can sidestep this bottleneck altogether with subway tunnels, which automated vehicles can't. Even after everything else, light rail is still more economical and more environmentally friendly.* Finally, light rail converts to automated train operation quite handily (further increasing capacity), especially if all the other cars around it are automated too.
* I was about to say more comfortable, but is being crammed into a small car more or less comfortable than standing on a cramped train?