Fundamental Diagram of Traffic Flow
Updated 2026-08-03
INTRODUCTION
English translation pending.
CORE DEFINITION
An empirical relationship in traffic engineering plotting flow against density, typically as an inverted V or a curve with a peak. Below the critical density, adding vehicles increases throughput; beyond it, speed drops sharply and flow falls even as more vehicles enter, producing a jam that propagates backward. The same shape appears in many flow systems where crowding degrades the medium itself. The practical rule is that maximizing throughput requires staying below critical density, which means tolerating visible spare capacity.
SCAFFOLDING EFFECT
Reduce cognitive load
- Load ceiling: treat full utilization as dangerous and keep density below the critical point. - Flow diagnosis: when throughput falls, check whether density crossed the critical threshold rather than blaming demand. - Ramp control: slow the inflow so the downstream section can recover.
Anchor fast decisions
Vehicles need spacing that grows with speed. As density rises, drivers reduce speed to keep a safe gap, so flow is the product of two terms moving in opposite directions. Past the critical density the speed term falls faster than the density term rises, so total flow declines. A disturbance then grows instead of dissipating, and the jam travels upstream against the direction of travel.
MINIMUM ACTION
In progress 0/1Practice this model in one real situation:
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Source support: Explicit
- en.wikipedia.orghttps://en.wikipedia.org/wiki/Fundamental_diagram_of_traffic_flowverified
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