7. Speed-Density and Speed-Flow Curves

5. Analysis of Speed, Flow, and Density Relationship

The hypothetical diagrams connecting mean speed, density, and flow are shown in figure:

At point A density is close to zero, and there are only a very few vehicles on the road; the volume is also close to zero, and these few vehicles on the road can choose their own individual speeds.

At point B, the number of vehicles has increased but the conditions are of free flow and there are hardly any restrictions.

Around point C, traffic conditions begin to show sign of instability, and speeds and densities fluctuate with small change in volume. Point C is the point of maximum volume, and further increases in density reduce speeds considerably.

Flow near point D is reduced almost to zero, with cars stacking up almost bumper to bumper. Point D is known as jam density.

A driver would perceive excellent driving conditions from A to B, moderately good conditions from B to C but increasingly deteriorating conditions from C to D.

6. Time-Distance Diagram of Flow

A

A

B

BTime, t

Dis

tan

ce, x

Spacing

Headway

Slope = dx/dt =

speed

T

L

A

vx

vf

0