Intersection Level of Service (LOS) Explained: HCM Thresholds, Delay and v/c
How level of service is defined for signalized and unsignalized intersections, the HCM delay thresholds for LOS A to F, and what engineers check beyond the letter grade.
Level of service (LOS) is the letter grade — A through F — that traffic engineers use to describe how well an intersection or road operates. It’s the headline number in almost every traffic impact study, and it’s defined in the Highway Capacity Manual (HCM).
LOS at intersections is based on delay
For intersections, LOS is determined by control delay: the average extra time, in seconds per vehicle, a driver loses because of the traffic control — slowing down, waiting, and accelerating back to speed.
Signalized intersections
| LOS | Control delay (s/veh) |
|---|---|
| A | ≤ 10 |
| B | > 10 – 20 |
| C | > 20 – 35 |
| D | > 35 – 55 |
| E | > 55 – 80 |
| F | > 80 |
Unsignalized intersections (two-way and all-way stop) and roundabouts
| LOS | Control delay (s/veh) |
|---|---|
| A | ≤ 10 |
| B | > 10 – 15 |
| C | > 15 – 25 |
| D | > 25 – 35 |
| E | > 35 – 50 |
| F | > 50 |
The thresholds are lower for unsignalized control because drivers expect less delay at a stop sign than at a signal.
In addition, the HCM assigns LOS F to any lane group or movement whose volume-to-capacity ratio (v/c) exceeds 1.0, regardless of the delay calculated.
How LOS is reported
- Signalized intersections are reported for the intersection as a whole, by approach, and by lane group.
- Two-way stop control (TWSC) is reported for the stop-controlled minor-street movements and the major-street left turns — there is no meaningful “overall” LOS, because major-street through traffic doesn’t stop.
- All-way stop control (AWSC) and roundabouts are reported by approach (or lane) and for the intersection.
Beyond the letter grade
A single letter hides a lot. Reviewers and good engineers also look at:
- v/c ratio — how close the movement is to capacity. A LOS D movement at v/c 0.98 is one bad day from failing.
- Queues — the 95th-percentile queue against available storage. A left-turn queue that spills out of its pocket blocks through traffic even if the LOS looks fine.
- The critical movement — at a TWSC intersection, one left turn from a driveway can be LOS F while everything else is LOS A.
- Change, not just the grade — many agencies define an impact as the project worsening a failing movement by more than a set amount of delay, not just causing a letter change.
Acceptable LOS
Each agency sets its own standard. LOS D is a common minimum for urban signalized intersections at peak hour, with LOS C often required in rural areas; some downtown areas accept LOS E or F where widening isn’t possible or desirable. Check the agency’s TIA guidelines.
Common sources of error
- Dividing by the peak hour factor twice (once when converting volumes to flow rates and again in the analysis)
- Lane configuration in the model that doesn’t match the field — a missing right-turn bay or a shared lane coded as exclusive
- Signal timing from the wrong plan for the peak being analysed
- Unbalanced volumes between closely spaced intersections
How LaneMarker handles LOS
LaneMarker AI Studio runs HCM signalized, two-way stop, all-way stop and roundabout methods directly on the mapped network. Lane geometry comes from AI Build, timing is read from controller printouts, and LOS tables by lane group update automatically when a lane, volume or timing plan changes. On published traffic studies used as benchmarks, signalized intersection delay matches within about 1.5 seconds and roundabouts within half a second.