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HCM LOS criteria for signalized intersections.
| LOS | Average Control Delay (s/veh) | Description |
|---|---|---|
| A | ≤10 | Free flow |
| B | >10–20 | Stable flow (slight delay) |
| C | >20–35 | Stable flow (acceptable delays) |
| D | >35–55 | Approaching unstable flow (tolerable delay) |
| E | >55–80 | Unstable flow (intolerable delay) |
| F | >80 | Forced flow (congested and queues fail to clear) |
