A section of an arterial road has been found to have a speed-density relationship that was modelled using the following form:

where v is the space-mean speed in kilometres per hour (km/h), and k is the density in vehicles per kilometres (veh/km).

The relationship was found to have the best fit to the data by using the following coefficients:

a = 3.5

k_m = 126 veh/km

Which is the critical density (ie the density at which the capacity is obtained) of the above arterial road?