Road speed, free-flow speed and the jam factor, explained
What the current speed, the free-flow speed, congestion and the jam factor each mean, how they relate, how to read them for a site, and why none of them counts cars.
Three numbers describe how a road is moving. The current speed is how fast traffic is going now. The free-flow speed is how fast it goes on the same road when it is clear. The jam factor is a score from 0 (free) to 10 (standstill or closed) for how jammed it is. Traffic Flow gives all three for the roads around each place, measured at the moment the flow runs, plus congestion as a percentage and the level in words.
This article is for anyone reading those fields for the first time: site analysts, dispatchers, event and venue teams. It explains each one, works through the arithmetic, shows how the levels are named, and is clear about what live speed data does not tell you.
Current speed and free-flow speed
The current speed is an average over the roads around the place, weighted by the length of each road stretch, so a long main road counts more than a short side street. The free-flow speed is the same average for those roads when traffic is light. It is not the speed limit: a city street with a limit of 50 km/h may flow freely at 38 km/h because of crossings and bends.
Comparing the two is what makes the speed meaningful. A road moving at 30 km/h is crawling if it normally flows at 80, and perfectly normal if it normally flows at 32.
Congestion as a percentage
Congestion is how much slower than free flow the traffic is: one minus the current speed divided by the free-flow speed, as a percentage, and never below zero. A made-up example: a road that flows freely at 50 km/h and is moving at 18 km/h has a congestion of 1 minus 18/50, which is 64%. The same road at 45 km/h has 10%.
The jam factor and the five levels
The jam factor puts the state of the roads on one scale from 0 to 10. A closed road counts as 10, so a place where one of its roads is shut scores higher even if the others are moving. The block also writes the score in words:
| Jam factor | Level | What it usually looks like |
|---|---|---|
| under 2 | Free flowing | Traffic at or near its usual free speed |
| 2 to under 4 | Light | Some slowing, no queues to speak of |
| 4 to under 7 | Moderate | Clear slowing, queues at junctions |
| 7 to under 9.5 | Heavy | Stop and go, long queues |
| 9.5 and over | Standstill | Barely moving, or closed |
How Traffic Flow turns the jam factor into its traffic_level field.
Congestion and the jam factor usually move together but are not the same. Congestion only compares two speeds. The jam factor also reflects closures and how stop-and-go the flow is, which is why a road can show a moderate congestion and a heavy jam factor when one stretch is shut.
The other fields that help you trust a reading
- roads_measured: how many road stretches the average is built on. One stretch is a thin basis; six is a fair picture of the area.
- roads_closed: how many of them are closed now.
- busiest_road: the road with the highest jam factor, so you know where the trouble is.
- traffic_confidence: from 0 to 1, how sure the measurement is. Readings near 1 come from many vehicles right now; lower ones rest on fewer measurements, so treat them with more care.
- traffic_updated: when it was measured, in UTC.
The block’s single setting, Roads, chooses the circle: the roads next to the place (about 150 metres), or every road within 500 metres or 1 km. A wider circle smooths out one bad junction; a narrow one tells you about the road at your door.
A worked reading: two sites at 08:00
Here is a made-up reading for two warehouses, one in Singapore and one in Los Angeles, both checked at 08:00 local time with the roads within 500 metres.
| Singapore | Los Angeles | |
|---|---|---|
| speed_kmh | 34.2 | 21.5 |
| free_flow_kmh | 41.0 | 72.4 |
| congestion_pct | 16.6 | 70.3 |
| jam_factor | 2.1 | 7.8 |
| traffic_level | Light | Heavy |
| roads_measured | 5 | 3 |
| roads_closed | 0 | 1 |
| busiest_road | a local link road | the motorway on-ramp |
Made-up readings, to show how the fields fit together.
The Los Angeles site is moving faster in absolute terms than many city streets, yet it is the one in trouble: its roads normally flow at over 70 km/h, one is closed, and the on-ramp is the bottleneck. The Singapore site is slower in km/h but close to its own normal. Reading speed without free flow would have got this the wrong way round.
What live speed data does not do
- It does not count cars. Speed and the jam factor describe how traffic moves, not how much of it there is. A quiet road and a busy road can both flow freely.
- It has no peak hours or direction split on its own. To learn when a road is worst, run the flow at the hours you care about and keep each reading, for example as a row in Google Sheets.
- It is a snapshot. A reading at 08:00 says nothing about 17:00; see using live traffic to plan deliveries and service visits.
Questions
Is free-flow speed the same as the speed limit?
No. It is how fast traffic actually moves on those roads when they are clear, which is often below the limit in a city.
What is a good jam factor?
Under 2 is free flowing and under 4 is light. For a site that depends on passing drivers stopping, a moderate jam factor at the right hour can even help visibility; for deliveries, lower is better.
Can Traffic Flow tell me how many cars pass a site?
No. It measures speed and congestion, not volume. Hourly car counts are available only for roads in the United Arab Emirates, through Regional data sources.
Why does a place say No roads measured?
No measured road lies within the circle you chose. Try a wider Roads setting, or check that the place has a correct position.