Transportation Engineering: Why New Lanes Fill Up Fast
A tour of how traffic flows, jams, curves, pavements and safety really work, from phantom jams to the fourth power law.
Transportation Engineering · 50 questions
Try 3 questions
1. On a highway, what happens to traffic flow (vehicles per hour) once density rises past the critical density?
- Flow drops even as more cars enter
- Flow holds steady at its maximum value
- Flow keeps rising until the road jams
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Answer: Flow drops even as more cars enter
Beyond the critical density, vehicles slow down faster than their numbers grow, so fewer cars pass a point each hour.
2. A lane carries 1,800 vehicles per hour at an average 60 km/h. Using flow = speed × density, what is the density?
- 3 vehicles per kilometer
- 108 vehicles per kilometer
- 30 vehicles per kilometer
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Answer: 30 vehicles per kilometer
Density equals flow divided by speed: 1,800 / 60 = 30 vehicles per kilometer of lane.
3. Why can a stop-and-go jam appear on a busy highway with no crash, merge or lane closure?
- Speed limits force cars to bunch together
- Small braking grows as it passes backward
- Drivers copy each other's lane changes
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Answer: Small braking grows as it passes backward
In dense traffic each driver brakes a bit harder than the one ahead, so a tiny slowdown amplifies into a 'phantom' jam.
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