Thierry Schwartz, engineer and division head at the Department of Bridges and Roads, explains how “traffic lights” work.
Thierry Schwartz
Before answering, we need to agree on the term “traffic lights”. Why don’t we say “green lights” or “three-colour lights”? More seriously, we are talking about traffic light signalling (SLT). Luxembourg has more than 500 TSS installations. There are just over 300 on the national road network. It is also interesting to note that the three main TSS operators are the City of Luxembourg, the City of Esch and the Public Works Administration, each with its own teams dedicated to planning, implementation, operation and maintenance. The Administration des ponts et chaussées took over this task in 2015, following a political decision to relieve the burden on small municipalities and bring about a certain degree of harmonisation. To return to the question, the Administration des ponts et chaussées currently has 103 installations connected to 90 traffic light controllers. Of these, 12 are located at motorway interchanges, 68 at crossroads or roundabouts and 23 at pedestrian and cycle crossings. It should be noted that 70 new installations are being planned in the short to medium term.
Thierry Schwartz
Traffic lights follow programmes adapted to different traffic situations: rush hour, off-peak hours or weekends. The basis is a fixed-time mode, where the green/red phase durations (i.e. the state of the traffic lights for a traffic flow) remain constant. However, most installations use ‘traffic-variable’ control: sensors adjust the duration of green phases in real time or trigger a green phase on demand, for example for pedestrians or buses. These systems are controlled locally or remotely via a control centre, which allows for a rapid response to changes in traffic. To take a concrete example, when there is an event at the Luxembourg Stadium, it is possible to plan a specific programme at the SLT level to ease traffic flow at the end of the event. Similarly, we implement programmes adapted to morning rush hours for access to the capital and vice versa for leaving it. It should be noted that the three main operators use this “traffic-variable” mode, which is controlled centrally. Programmes also exist to ease traffic flow on important sections, e.g. at motorway exits, for safety reasons. However, if the network is completely saturated, even the best programming will not be able to change the situation.
Thierry Schwartz
The amber phase, which warns that the green light is about to end, generally lasts 3 to 5 seconds, depending on the speed limit and the type of road. Then, a safety period is allowed before the opposite flow turns green, in order to avoid any collisions, in other words, to clear the intersection. In Luxembourg, the systems are programmed so that no direction is green at the same time. To give an idea of the scale, for a typical cycle, i.e. the complete sequence of all the phases necessary to manage an intersection, we aim for an average of 90 seconds for all users, with a maximum average waiting time of 70 seconds.
Thierry Schwartz
An SLT installation has three objectives, in order of importance: to guarantee safety (avoid conflicts), to ensure the smooth flow of traffic for all users and to regulate/prioritise certain flows (buses, trams). Roundabouts have safety advantages but do not allow for the regulation of capacity or the effective prioritisation of public transport; on major roads, traffic lights are therefore preferred. Other factors also come into play, such as the surface area that a roundabout can occupy. Finally, there are also roundabouts with traffic lights. This is more complex, but it works quite well, as we have seen with the Robert Schaffner roundabout, known as “Iergäertchen”.
Thierry Schwartz
Indeed, some countries have an amber phase after the red phase to warn drivers to get ready to start moving. This principle is not currently provided for in national legislation. Historically, Luxembourg has looked to what was done in Germany and has gradually adopted German practices. Other countries, particularly Anglo-Saxon countries, have some differences compared to our traffic lights, but again, this is more a matter of history.
Thierry Schwartz
In Luxembourg, the green light lasts long enough for pedestrians to reach at least the halfway point, with adjustments for certain users such as the visually impaired. If a pedestrian starts crossing at the last second, they can finish crossing before the cars turn green. Countdown timers, used elsewhere, improve comfort but their impact on safety has not been proven. They are expensive to install, and adapted phases often offer a better solution. In Luxembourg, they therefore remain optional.
Thierry Schwartz
It will always depend on several factors such as the geographical location, the site and the complexity of the installation. As a guide, we can estimate between €20,000 and €30,000 including VAT for a simple pedestrian/cycle crossing and between €100,000 and €150,000 including VAT for a crossroads. Complex sites can exceed €200,000. The cost of planning, programming and civil engineering must also be added. In terms of energy, annual consumption varies between 800 and 2,300 kWh, depending on the size of the installation— , this is equivalent to a two-person household for the largest sites. The switch to LEDs has obviously reduced the consumption of the installations. It should also be noted that the Luxembourg installations operate 24 hours a day, as switching them off at night increases the risk of accidents.
Thierry Schwartz
“At APC, six people are responsible for planning, implementation and maintenance; two managers are dedicated to operations and quality. Regular campaigns recheck and update the programmes (approximately every 2.5 years for analysis and minor interventions, every 5 to 7 years for more extensive overhauls). Average lifespan: 15 years for electronics, 5 to 7 years for sensors, approximately 30 years for masts and cables.”
Thierry Schwartz
“SLT management is evolving towards smarter, more connected systems. Control centres will use predictive models and real-time data to anticipate traffic flows and even dynamically adapt phases using AI. Ultimately, installations will communicate with vehicles and smart city platforms. Please note: safety parameters (intermediate times, non-conflicting phases) remain fixed and cannot be modified by AI.”
Thierry Schwartz
As far as the control of SLT installations by emergency services is concerned, this does indeed exist. It is in operation in several countries. In Luxembourg, we do not have this type of installation, as the emergency services have never requested it. In addition, they have flashing lights and sirens that give them priority. As for cyberattacks on SLTs, what we see in films is not realistic. We have systems where it is simply impossible, even with computers, to turn all four lights at a crossroads green at the same time. It is technically impossible.
"Running a red light is extremely dangerous"
Roland Fox, Director of the Highways Agency, wishes to warn about the increase in red light violations. “We are seeing an increase in this behaviour, which endangers road users, as the consequences can be very serious. ” To combat this dangerous and illegal behaviour, installing red light cameras at critical points may be a solution. This should not prevent us from repeating a simple awareness message: “Running a red light is extremely dangerous,” emphasises Roland Fox. He also points out that, in order to better accept waiting times at red lights, it may be necessary to understand how they work. “The primary purpose of traffic lights at an intersection is to ensure the safety of road users. But if a road user is slow to react when the light turns green, it is important to understand that they are penalising all other road users at the intersection. It’s a vicious circle, as the programmes are designed to allow a certain number of cars through within a set time. But if road users are not paying attention or stop too far from the stop line, they cause other motorists to lose precious seconds and may cause frustration,” concludes Roland Fox.
Avoiding the chain reaction
In 2019, a French study by Cerema (the public authority responsible for infrastructure development in France) on the impact of the cycle on capacity, acceptability and safety showed that the acceptable waiting time at a red light is between 60 and 90 seconds. Beyond that, tolerance drops sharply: at 120 seconds, more than 50% of users consider the wait excessive, which increases the risk of dangerous behaviour such as jumping the lights.
The researchers also observed that perception varies depending on the context: in dense urban areas, drivers are more tolerant of long cycles than in the countryside. However, according to the study, the optimal cycle is between 80 and 100 seconds, as this maximises the capacity of the intersection without significantly reducing user satisfaction.
Another important finding is that every second lost at the start (e.g. a driver who is slow to react to a green light) reduces the capacity of the intersection by 2 to 3%. This is the “domino effect”: an individual delay affects the entire queue, then the next cycle, creating an overall loss of fluidity.
Finally, the study highlights that cycles that are too short (<60 seconds) can also compromise safety, as they encourage drivers to accelerate in order to “beat the red light”. It is therefore crucial to strike the right balance: neither too long to avoid frustration, nor too short to maintain safety.
