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Why Do Traffic Jams Occur?

At 65 mph, how close will you comfortably get to the car ahead of you?

Under 3 car lengths separation is not a leisurely drive for me, and I'm on edge at less than two!

A modern highway can move a lot of cars if everyone is driving at 65 mph with six car lengths of separation. With three or two car lengths separation, a highway can handle even more cars per hour! However, capacity is limited since drivers have speed limits and minimum comfortable separations. At rush hour, more and more cars on the road eventually leads to small separations between cars, drivers put on the brakes, and at lower speeds the highway's capacity declines dramatically. The only solution is for traffic to stop. This is a traffic jam!

The key idea is: "When traffic is heavy, once you hit the brakes, speeds drop, the highway's capacity drops, and the only solution is for cars to stop."

The spreadsheet below generated the graph above. The graph and the spreadsheet show how smaller separation distance and higher speed will increase the flow of cars--but there is a limit where we won't drive faster or closer, and we put on our brakes.

Construction, rubber-necking, debris on the road, sun glare, lane closures, and rush hour traffic volume all lead to increased traffic flow and smaller separations that can create a traffic jam.

Traffic jams have been compared with fluid flow when shocks occur. I'm not convinced the physics are the same. Instead, my favorite analogy is stock market panics. As stock prices drop, investors sell in a panic, further driving down prices. Yet stock prices often rise well past prudent valuations. It is risky to own overpriced stocks since the downside is nasty, brutish, and short. But please don't be contrarian in a traffic jam.

The spreadsheet below is a self-validating spreadsheet that tries to check and explain its entries and formulae.

Two Columns

Quick Start for Validating Spreadsheets
Why Do Traffic Jams Occur?
What do Rocket Scientists Do? The Calibration Mystery
What do Rocket Scientists Do? Resistance, Resistivity, Sheet Resistivity

Every effort is made to avoid errors--please tell us if you find a mistake! Any opinions expressed are not NASA's.