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I'm a fluid dynamicist and throughout your text you've been missing one key point about continuum mechanics. Unlike an incompressible fluid, traffic density can change, and it increases under pressure. Bottlenecks cause pressure, and where you have high pressure you have low speed (similar to the Bernoulli Equation) by law of conservation. Physically what is happening is that as cars go faster, drivers keep larger following distances. Thus it would be impossible for all cars to merge into a single bottlenecked lane without slowing down, unless they were to pack together extremely/dangerously tight. This is a simple conservation of mass flow rate. It can only happen by increasing the traffic density if you want to maintain speed while reducing the number of lanes. However, there is one more option. Vehicles could speed up before/inside the bottleneck and possibly merge without causing waves behind them (this is why holding your thumb over a garden hose causes the water to shoot out faster, because the upstream pressure at the bottleneck CAN force the fluid downstream, whereas cars in traffic cannot.

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