[BIair]

Isn't this money squandered, if autonomous vehicles are likely by the time of its completion?

[tdicola]

Even if the cars drive themselves there's still not enough lanes to let them flow right now, much less in 20+ years. There was a study done that found Seattle's needs _22_ lanes of highway to meet the current demand. However because of how the highway system was originally built through the city we're stuck with 6-8 lanes at best. We need mass transit to get cars off the roads and reduce traffic.

[CydeWeys]

Really smart autonomous vehicles that automatically do ridesharing and that intelligently pack vehicles full of 4-6 people on their way in could do it. At that point you're dealing with an experience that's not too different from a bus though (i.e. sharing a space with strangers). There's not enough capacity for everyone to commute in on their own individual autonomous vehicle, nor is there enough parking downtown, so the vehicles would have to be commuting twice daily.

[transitthrow]

(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?

[timthelion]

Only if you don't care about the environment. Autonomous vehicles, even electric ones, are inefficient compared to mass transit.

[Tiktaalik]

Autonomous vehicles don't really solve the core congestion problem.

At the end of the day the road is only so large, cars are relatively big, and it's rush hour and everyone is travelling at the same time.

Public transit is more efficient.