When the light turns red, every car will accelerate at the same moment, without a transmission delay along the string of cars.

That's not possible unless all the cars have stopped maintaining an equal distance between them that they would have when they are operating at full speed. It's not as if the guy driving the car 10 cars back doesn't see the light turn green at the same moment that the person driving the first car in line does. The delay happens because space needs to be maintained between cars. Self driving cars aren't going to operate two feet from one another. Even if the engineering allowed them to operate at some minimal safe working distance, say, 10 feet, the way engineering works is by building safety factors into the system, because there are always contingencies we can't foresee. Depending on the nature of the project, a safety factor can vary from 1.5 to 20 in some circumstances.

As to your other point, even if the cars are "aware" that another car is going to merge 5 minutes from now, they still need to adjust their speed to account for the space. Just because they do it gradually instead of abruptly doesn't mean that they can drive any faster than space limitations account for. You'll note that conservation of mass is a law of physics, and it can't be violated by computers, even if they're really, really, really smart.