Traffic Signal Management needs understanding and skills
By David Brown
Major social changes such as COVID-19 need new thinking.
But major changes also show up how good we have been at configuring and operating traffic management systems, in the first place, that can cope with change.
A fantastic example is the Sydney Coordinated Adaptive Traffic System (SCATS) which gives us lessons in: how to use simple things which are understood in a clever way; the need to record and keep corporate history; and the need to keep enough technical expertise to make the most of the system into the future.
We have looked at its history in the past but planners and managers will benefit from understanding what it can and cannot do.
Flowing down a road with all green lights is the ultimate goal.
How coordinated is SCATS? AITPM Fellow, Alan Finlay, spent many years as a traffic engineer working on traffic signal operations with SCATS.
“It's coordinated in the sense that there's the possibility to connect individual sets of signals together to achieve a “Greenwave” effect so that ideally traffic that's travelling along an arterial road in the heavier direction is likely to receive successive green signals and therefore minimise the number of stops.
“SCATS has the capability of being connected in fairly long strings of intersections along an arterial road or in groups of intersections in a CBD situation. But in general, the connected strings are not so long that somewhere on the other side of Sydney is going to affect something else. In fact, it should be set up so that there is flexibility and that that sense of coordination only occurs when it's absolutely necessary to occur”.
SCATS is not the big brother system where one overall computer controls the lot.
Its strength is that it has strong local controller power that can immediately respond to varying situations.
First and foremost, it looks closely at the local situation.
Ken McCallum who is one of the heroes of developing the programming for SCATS and still consults to users around the world says:
“Well, essentially, it only controls single traffic lights but the information that we gather at every junction can be projected forward to any intersection downstream”.
How adaptable is SCATS to specific situations?
“The whole idea of the “A” in SCATS means adaptive, and that means that the operator will set some outer boundaries or limitations, and within those limitations, the system is free to optimise and try and produce the best outcome in accordance with the overall control strategy.
SCATS doesn’t require constant attention in the sense that if a system has been well set up to suit the particular conditions, then there shouldn't be much need for day to day intervention in terms of its operation. But from time to time, there will be changes in the road network. For example, a new freeway might be constructed or there might be a change which converts a network of two-way streets into some one-way streets. They're the sort of things that when the network changes, then the SCATS operator will have to go back in and havea fresh look at the at the whole operating philosophy and make some adjustments so that it works optimally in the new road network arrangements.
But there is a danger if we set it in motion and just let it go. This might reduce short term budgets by not employing professional people but nothing stays the same forever.
Ken McCallum said:
“One of the things that we always realised was that if a person has local knowledge, that's better than anything that you could measure. If you know, you need a particular cycle length at a set time of day, there's nothing wrong with ensuring that that cycling is operational based on a human being’s inbuilt knowledge about how the system works. You could rely on a completely automated systems. But the only artificial intelligence we have is the real intelligence of the operators, and SCATS doesn't have anything that is good enough to do things like that yet.
SCATS relies on detecting traffic through a system that has been around for some 50 years – loop detectors.
These are wires cut into the road pavement in a rectangular pattern that can detect vehicles by the magnetic effect from the metal in the vehicle.
You can tell that a vehicle has not only arrived but how long it remains detected. It’s not perfect because it could range from a short vehicle that has stopped to a long vehicle that is travelling slowly.
This alone was not enough to make the system work. The really clever step, that Peter Lowrie identified, was to also measure the gap between cars; that is recording the time before the next vehicle was on the detector. The space between vehicles indicates how the traffic is flowing compared to the capacity of the lane.
This is one of the great examples of Australia developing cleaver technology with potential international markets because Peter developed the logic and algorithms so that the system would work anywhere in the world. It can cope with local driver idiosyncrasies.
Coordinating close-by streets
What happens when two or more intersections are close together?
The closer the intersection spacing, the more rigid the coordination has to be between the intersections.
A forced marriage but can you divorce?
In situations where the intersections are more widely spaced, typically, the marriages would occur subject to certain conditions, such as a higher volume on the through road, on the arterial road.
So it's possible to have the two groups of intersections join up together to form one very long group or to, as we say, divorce, to operate separately at different cycle times.
Does SCATS set parameters for different times of the day?
“We try not to be too didactic about the times of day that these things will occur. Rather, we try and define the sorts of traffic conditions that will make these things happen.
These are some of the parameters that SCATS can use:
The maximum cycle length (which can be set based on nearby intersections)
Where any vehicles are present on an approach
The maximum total amount of gaps between vehicles (the ‘wasted’ time)
The flows in other approaches
Essentially, SCATS works to equalise the Degree of Saturation (as measured via the stop line loop detectors) on conflicting approaches at the intersection. It does this by making small adjustments to the green times, on a cycle – by – cycle basis.
There is a much greater focus on pedestrian movements which is good. But a myopic ideology can make it worse for everyone
There have been calls recently for all pedestrian phases to be called up on every cycle so that people don’t have to touch a button and risk COVID-19.
The downside is that if every pedestrian crossing is called up every cycle, it's actually counterproductive for pedestrians. The longer the cycle time, the fewer number of cycles per hour, and therefore the longer that pedestrians, on average, will have to wait to get their turn. This is especially the case if one crossing at an intersection is busy and the others are not.
Ken McCallum has a nice metaphor
“I actually liken it to walking into a high-rise building, on the ground floor and the lift is there with a door open. So you walk in, you don't have to touch anything. But when you look at the buttons, every single floor is pressed”.
Things to remember
Here are a few things to keep in mind if we are to have a constructive input to traffic light management control.
If you coordinate traffic in one direction, what happens to the other direction? The optimum solution is to first and foremost look after the heavier direction of flow
Security is an increasing issue. How do we keep the systems safe from interference?
There are new ways of monitoring traffic that can give good information such as video and Google earth. But at this stage, none of these collects and processes information quickly enough for SCATS.
New technology can bring more accuracy but is better? Ken McCallum has another metaphor that puts that in perspective.
“I can feel it's a bit like the swimming events in the Olympics in the early days. Nowadays with timing to the nearest thousand of a second, someone can get beaten by 1000th of a second. But is that relevant to anyone?”
Traffic on the roads has often been compared to the hydrological flow of fluids through pipes. Look how autonomous systems are portrayed with vehicles flowing uninterrupted down a motorway. But coordinating traffic when there are a lot of vehicles entering and leaving the system becomes much more complicated
SCATS does keep some historic data but not for extended periods so it will take a little time (measured in days not weeks) to adapt to the fluctuations from events such as COVID-19; but it will adapt relatively quickly
SCATS is a world renowned and respected system.
SCATS is not just clever software; it reflects a human understanding of a range of complex issues.
It is very adaptable but you still need skilled input to ensure its greatest efficiency.
No system will give you utopia, and a balanced approach is critical.