Let me think a minute...
We have multiple limiting factors-
Number of roads / dendrites
Number of shuttles / dopamine supply

There is no limit on passengers because the other bottlenecks kick in well in advance. So we want to max out the number of shuttles at any one time.

When we come in to the office in the morning, it takes a little time to get shuttles out the door and onto the roads. During this phase, extra dopamine helps becaue the roads are clear. But as the roads fill, the shuttles have to line up at the gate and watch for a chance to merge into the traffic. In this phase additional shuttles do no good. In fact, it might make it worse. But as things fluctuate the line of shuttles drops and more gaps open up. During this phase, extra dopamine can be used and more shuttles hit the road.

So long as we keep buying shuttles our benefits are limited. If we could find a way to build more roads, however....