Then why underground? Just put car on electric platform and ferry across existing highways.
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How would that add capacity? Plus there’s a speed limit and traffic jams on existing highways.
In theory anyway the future of self-driving cars means all cars communicate with each other. Their intentions are known. This means they can drive in much greater/tighter alignment, and maneuver and change lanes without causing a chain of break lights behind it. All cars can start to roll when a light changes green, go up to speed together, and arrange themselves by order of priority (e.g. cars exiting soon move to the right, cars continuing on move to the left all automatically). If a hazard is detected, all the cars are made aware and course correct automatically. No need for platforms.
But this is probably a decade away from implementation.