Imagine two roads, parallel mostly, but then one takes a detour, like this:

         ____
    ____/ __ \____
 -> _____/  \______
    _______________
 -> _______________

Now imagine the cars move, at the same speed, on both roads. Same number of cars will come in on the left as go out on the right.

The cars will be bumper to bumper, both on top and below (there'll just be more cars on top).

Why should cars that come in at the same time on the left exit at the same time on the right?