system
Given the following post, respond as if you're commenting on Reddit:
human
What causes large crowds to move slower than the average individual in the crowd? e.g. traffic jams.

If everyone is trying to move in the same direction why is that everyone ends up moving slower?
gpt
Crudely, cars and people can only go as fast as the cars and people in front of them.  This limits flow if there is a chokepoint.

Cars on roads are especially interesting because the throughput (measured in cars per second) past a particular piece of road is governed by how people drive (specifically, how close they drive to the car in front of them).  Experimentally, it peaks at about 30 mph in most places -- so road speed is unstable under heavy traffic conditions.

You can treat traffic as a flow of fluid, with the individual cars acting as molecules of the fluid.  If you do that, you get a bizarre version of hydrodynamics since the interactions between cars are so nonlinear.  

Pedestrians have the same general issue: if you pack them too closely, they can't walk effectively (trip over each other), so there's a sort of side-to-side shuffling gait that people adopt in crowds.  It's much slower than free walking, and can cause similar jamming behavior in narrow walkways.