Traffic Flow Calculator Formula

Understand the math behind the traffic flow calculator. Each variable explained with a worked example.

Use the Traffic Flow Calculator

Formulas Used

Traffic Flow Rate (q)

flow = density * speed

Average Time Headway

headway = 3600 / (density * speed)

Average Spacing

spacing = 1000 / density

Variables

VariableDescriptionDefault
densityVehicle Density (k)(veh/km)30
speedSpace Mean Speed (u)(km/h)60

How It Works

Fundamental Traffic Flow Equation

Traffic flow, density, and speed are linked by a fundamental relationship.

Formula

q = k u

where q is flow rate (vehicles/hour), k is density (vehicles/km), and u is space mean speed (km/h).

Time headway = 3600/q seconds. Spacing = 1000/k metres. At capacity, flow is maximised with moderate speed and density.

Worked Example

30 vehicles per km travelling at 60 km/h.

density = 30speed = 60
  1. 01q = 30 x 60 = 1800 vehicles/hour
  2. 02Headway = 3600 / 1800 = 2.0 seconds
  3. 03Spacing = 1000 / 30 = 33.3 m between vehicles

Ready to run the numbers?

Open Traffic Flow Calculator