How can we prevent a traffic jam?

A transport engineer reveals the efforts going in to making the commute more bearable...
28 May 2019

Interview with 

Richard Llewellyn, Edinburgh Napier University


Busy traffic-congested streets


How much time do you spend stuck in traffic - late for a meeting, or trying to get your kids home from school? You could be forgiven for thinking not much thought goes into controlling traffic in urban areas. But behind the scenes, highly complex systems are at work. Katie Haylor heard more from Edinburgh Napier University's transport engineering lecturer Richard Llewellyn...

Richard - If you take any big city within the UK, pretty much all of them these days will have something called an urban traffic control system, controlling all of the traffic signals within that city. Now traffic signal timings are set based on traffic flow. Historically we did that many years ago just manually and taking an average on a particular day, but these days it's much, much more high tech. So we've got detectors in the road and above the traffic signals they detect the flow at any one time and they adjust those green times that you see on a cycle by cycle basis. A traffic signal cycle is where we go from from red to amber to green and back again. And when you're driving in a city, if you're driving on on the main road into a city centre, what should be happening is you should be seeing some sort of linkage between those signals. So if you hit a green signal then as you get further down the street the timing should be such that your progression along that road is unimpeded.

Now unfortunately, it doesn't always work that way and it doesn’t always seem that way because lots of different things are happening on our streets. Buses are stopping, people at delivering goods, the flow doesn't quite work as well as it should do. But there is a lot of intelligence within those systems that make those changes to try to get the maximum amount of traffic through, and they can also do clever things as well like prioritize emergency vehicles, public transport, even respond to pollution. We have systems that can measure pollution and start to adjust the traffic patterns, adjust which streets get priority to try and maintain or reduce those pollution levels.

Katie - So I guess if you've got something like an organized parade and you've got days of notice, that's one thing. But if you've got an emergency services vehicle coming in you might only have a few minutes of notice so how quick are they at reacting, I guess, to the changing traffic conditions?

Richard - Well they can actually make changes to the signal timings on a cycle by cycle basis, so every time those lights change one amount of green time might be different from the next amount of green time. They tend to be more responsive in the off peak periods when things aren't as busy. The problem with the peak period is basically our networks are so congested there isn't a lot of slack, and the systems are very limited in terms of what they can do.

Katie - Considering that challenge, what is going on in the area of traffic control research to try and improve congestion, bottlenecks, or make journeys more reliable?

Richard - Yeah well I mean reliability that's the key word. People will tend to accept that congestion is a fact of life these days in cities. But what people find difficult to accept is that the journey time from day to day will change and they might allow 45 minutes to get to a meeting one day, and the next week it'll take them 90 minutes and this is a real problem to people.

One of the issues we've had historically is the ability to collect traffic data. We've been always reliant on roadside detectors or just traffic counts, but these days the number of people that are carrying around smartphones that are monitoring where they're travelling to how long it's taking is a great source of data. And companies like Google for example now are providing journey time information which allows traffic engineers to start to build a picture of journey time information and start to try to react to that and at least disseminate it to those that need to know. Ultimately though the problem still remains that that network is a finite resource and we really need to question who should be using it and when, that's really the big challenge.

Katie - Considering that vehicles are becoming increasingly more autonomous, is this set to impact upon how we control traffic on the roads?

Richard - Yeah, well, I mean autonomy brings with it some potential advantages in terms of driving style. It removes some of the unpredictability from the system. So if you imagine people driving along a motorway, one driver to the next, everyone leaves a little bit of a different gap between the vehicle and front. But if you have a machine in control of the distances between vehicles on, for example, a motorway, you've potentially got a much more stable flow there. There has actually been some suggestion that if we had fully autonomous vehicles the need for things like traffic signals could be completely removed from our cities, in that the vehicles themselves would communicate with each other and that we wouldn't need this technology by the side of the street. But that one's quite a long way off.

Katie - As you're a traffic expert I've got to ask, what is the best etiquette in terms of managing traffic flow if you come to a bit of a standstill on a motorway? You've got someone who's edging really close to you, someone who's trying to leave a bit of space and there's always that person who will go in the gap! Actually for efficient movement, what is the best way to behave in that situation, assuming you're not in an autonomous car?

Richard - A motorway works at its best when you've got a steady state flow. Now these shock waves and issues that occur on a motorway occur when there's been some event that's happened on the motorway. Now that can sometimes be something like an accident, but it can also be things like merging vehicles or someone changing lane or someone driving a little bit too close and braking. So if you want to adopt a good driving style and try to get the maximum benefit from motorways, the best way of driving is try to allow as much distance as possible from the vehicle in front, to brake very gently, within reason, and to accelerate away very smoothly. And by maintaining that smooth traffic flow, it might not completely prevent that type of congestion, but certainly it would go some way to help.

Katie - How do traffic control systems make sure that if there's a diversion, traffic is being sensibly and evenly distributed? Because I guess what you don't want is to think ‘OK, that route is cut off, let's send all of the cars in the city onto a different route and just cause a massive jam.’ How does that work?

Richard - Within traffic models, what the model will try to do is it uses something called the generalised cost of travel in terms of the vehicles on the network so tries to minimise the amount of delay within that network. So it will try many iterations to try to distribute that traffic as best as it can minimising that cost. And once it has found that solution that is the form of diversionary route that will be signed. Sometimes if we know of an event that's happening we might manually put something in and come up with a plan in advance but if it's in real time that's the way it would work.


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