The major flow of traffic along Drakeford Drive is coordinated. The following section will show that in order to give good coordination to one direction of traffic along Drakeford Drive the other direction has to be disadvantaged.
Below is an illustration of a time and distance diagram that displays the AM coordination of Drakeford Drive.
The x-axis represents distance along Drakeford Drive. The numbers along the bottom, 100, 107, 111, 56, etc, are the reference numbers of the different signalised intersections, with 100 being Erindale Drive, 111 being Athllon Drive, 64 Sulwood Drive, etc. They are plotted to scale so the distance between the intersections on the graph is proportional to the distance between the intersections on the road.
The y-axis represents time. Plotted vertically at the location of each signalised intersection is the sequence of greens and reds seen by drivers on Drakeford Drive over several cycles of the lights.
It is possible to draw a diagonal line starting at the beginning of a green at Erindale Drive with a slope that represents the speed (80km/hr) of traffic along the road. As you move up that diagonal from left to right, ie travel along Drakeford Drive from Erindale Drive to Sulwood Drive, you can see the colour of the lights as you reach each signalised intersection. The diagram represents the morning peak coordination and as such traffic heading north receives good coordination. As the traffic approaches each intersection the traffic signals are green allowing the vehicles to proceed through the intersection without stopping.
However, imagine you are travelling southbound during the morning peak. This time start your diagonal line at the beginning of a green at the Sulwood Drive lights and draw it with the same slope but from right to left. You can see that as you arrive at the first set of lights they are just about to turn red. If you are in the first vehicle in the platoon you might just get through but otherwise you will hit a red and have to stop. Move up the vertical red and then set off again on the diagonal at the start of the next green. Red again at the next set of lights, and so on. This is the sequence of lights if you are travelling in the opposite direction to the major traffic flow.
What the diagram illustrates is that it is relatively easy to provide coordination in one direction, but what happens in the other direction depends on the speed of traffic and the spacing of the intersections. The only way to improve the coordination for the southbound traffic is to spoil the coordination of the northbound traffic. With a ratio of 2:1 vehicles heading north verses south in the morning, the northbound traffic receives the better coordination.
In the afternoon peak the situation is reversed. Good coordination is provided for southbound traffic but similar problems occur for drivers travelling north.
As it can be seen from above it is the spacing of the signalised intersections (distance between the signalised intersections) and the speed limit of the road (slope of the line in the diagram above) that effects the coordination of the traffic signals. It is these physical constraints that don’t allow traffic to be coordinated perfectly in both directions.