- Aug 8, 2009
Because the traffic on I-5 is compressible. I am quite sure someone will post an interesting picture of that but it is not my intent here.I know it works.
But the question is why is air meeting a bottleneck in a tube so different than cars on Interstate 5 when reduced to one lane?
Recall that the requirement of Bernoulli when applied to a gas or liquid was incompressible flow. The molecules vibrate in their own little space and bunch together to fill the total space available. For a gas this requires relatively low speeds and low mass for quick and easy accelerations.
If the speed limit on I-5 was 5 mph and the following distance was strictly enforced to 500 feet between cars there would be no compression when the pavement changed from 4 lanes to 2. Assume the average speed in the four lanes of two mph. The cars would speed up and travel at 4 mph in the two lane section in order to handle the desired (or required) flow rate and separation distance in the two lane section.
If the 4 lane speed increased to 5 mph, then you would get a bottleneck at the two lane section because the two lanes could not handle the flow with the speed and spacing restrictions imposed. This is what happens in a supersonic wind tunnel.