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| Site | Road | City | Setting | JTI | Congested | Risk | Collisions | £m / year |
|---|
Congestion against journey time
Bubble size is traffic volume, colour is corridor. Each bubble is one site.
Journey times rise steadily with congested share, but cost rises far
faster. The 14 sites above 15% congested hours average £36.3m a
year each, against £10.1m for the 42 sites below 5% — roughly 3.6× the
cost from the same class of asset. The driver is volume multiplied by delay, not a
collapse in throughput: no site here reaches the hypercongestion threshold where
flow itself starts falling. That matters for intervention design — these are
demand-management sites, not capacity-failure sites.