How this calculator works
Traffic engineering links three quantities: flow, density and speed. Flow is the number of vehicles passing a point per hour. Density is the number of vehicles occupying a length of road, usually per kilometre. Their product relationship is flow = density x speed when speed is in kilometres per hour and density is in vehicles per kilometre. Average spacing is the reciprocal of density once the units match, and the time headway is the interval between successive vehicles passing a point. The model assumes a uniform stream with one representative speed; real traffic has platoons, lane changes and a mix of vehicle lengths.
Formula
Flow (veh/h) = vehicles / time (h). Density (veh/km) = vehicles / length (km) = flow / speed (km/h). Spacing (m) = 1,000 / density (veh/km). Time headway (s) = 3,600 / flow (veh/h).
Worked example
If 120 vehicles pass a point in 5 minutes, the flow is 120 / (5/60) = 1,440 vehicles per hour. At 60 km/h the density is 24 vehicles per kilometre, the average spacing is 41.67 m and the time headway is 2.5 s.
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Frequently asked questions
What is the difference between flow and density?
Flow counts vehicles crossing a point per unit time. Density counts vehicles present on a stretch of road per unit length. A congested road can have low flow and high density because the vehicles are close together but moving slowly.
Which speed should I use?
Use the space mean speed of the stream, which is the average speed weighted by the time each vehicle spends in the section. A simple average of spot speeds can overstate the speed that matters for the flow-density relationship.
Does lane count matter?
This page reports the total for the road or for the lane you counted. If you count one lane of a multi-lane road, the result is per lane. For a whole carriageway, add the counts across lanes and use the total road length.
What is headway used for?
Headway is used in capacity analysis, traffic signal timing and safety studies. The spacing shown here is the distance between vehicle fronts, not the clear gap between bumpers, and it assumes a uniform stream.