That doesn't matter though, as if they're going faster they're more spaced out. Only 2000 cars per hour can pass the merge point
I have witnessed eight lanes of traffic slow from 70MPH to 10MPH over a single poorly designed merge when there was more than enough aggregate free space for the merge. That happened because drivers don't accelerate hard enough on onramps and people don't redistribute themselves prior to shitty merges.
Show me a society that has no traffic jams and I'll show you one that's ready for socialism. Or vice versa.
If you close one lane, reducing the capacity of the mergepoint to 180 cars per minute while 200 cars are approaching a queue will build. The speed with which the cars mass the mergepoint is not relevant because the rate of cars per lane per minute will stay basically at 20 cars/lane/min.
As to your point about all lanes slowing down - cars will always redistribute as you can imagine. People tend to merge left as there's an additional traffic stream merging on their right. The writer made points about the capacity of the mergepoint vs. the cars approaching- not individual lanes and speeds.
1. If the flow rate changes with speed, thus merging at 5 mph instead of 50 is bad.
2. What will prevent accidents. I suggest start-stop traffic causes more accidents than free flow.
3. Fuel efficiency, which would depend on many things, eg the percentage of cars in traffic that stop their engines when the car is stopped.
So if the car is travelling at .5 carlengths per second then the duration per car should be 4 seconds whereas if the car was travelling at 10 carlengths per second the duration per car should be only 2.1 seconds.