So ....
There are a limited number of cars.
They only hold 4 passengers
.... and yet there are no queues.
Welcome to the world's first personal rapid transport system (PRT).
Excuse me, but the world's first personal rapid transit system (PRT) is generally accepted to be the one at West Virginia University, built in the 1970s. http://web.presby.edu/~jtbell/transit/Morgantown/ which is far more complex than the one at Heathrow. I worked on it at the time of installation. It was a right waste of money, but has lasted.