Traffic engineers describe a stream of vehicles with three linked quantities: flow (vehicles per hour), density (vehicles per kilometre or mile) and speed. Flow equals speed times density. As density rises, speed drops, and beyond a critical density flow falls too – that is when jams form.
How to use the Traffic Density Calculator
- Vehicles passing a point
- Counting time
- Vehicles on the stretch
- Length of the stretch
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the number of vehicles counted passing a point and over what time, and the number of vehicles on a measured stretch of road.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Vehicles passing a point | A number |
| Counting time | Units: min, h, s |
| Vehicles on the stretch | A number |
| Length of the stretch | Units: km, mi, m |
What each result means
- Flow = [Vehicles passing a point]÷[Counting time]
- Density = [Vehicles on the stretch]÷[Length of the stretch]
- Space-mean speed = [Flow]÷[Density]
- Average spacing = 1000÷[Density]
- Average headway = 3600÷[Flow]
Example values
How the main result changes when Vehicles passing a point changes and everything else stays at the example values above:
| Vehicles passing a point | Flow (vehicles/h) |
|---|---|
| 225 | 900 |
| 337.5 | 1,350 |
| 450 | 1,800 |
| 562.5 | 2,250 |
| 675 | 2,700 |
| 900 | 3,600 |
Formula
Flow q = vehicles passing ÷ timeDensity k = vehicles on the road ÷ lengthSpeed v = q ÷ k; spacing = 1 ÷ k; headway = 1 ÷ qWorked example
Inputs — Vehicles passing a point: 450; Counting time: 15 min; Vehicles on the stretch: 30; Length of the stretch: 1 km.
Results — Flow = 1,800 vehicles/h; Density = 30 veh/km; Space-mean speed = 60 km/h; Average spacing = 33.3 m; Average headway = 2 s.
Frequently asked questions
What is a typical lane capacity?
About 1,800–2,200 vehicles per hour per lane on a motorway.
What is jam density?
Around 115–150 vehicles per km per lane, when traffic is stopped.
What headway is safe?
At least 2 seconds between vehicles in good conditions.
What formula does the Traffic Density Calculator use?
Flow q = vehicles passing ÷ time.