Traffic lights (traffic signal systems)

In urban areas, traffic signal control (traffic light control) has a decisive influence on traffic flow throughout the entire road network. Traffic signal systems (traffic lights) are used to increase road safety for all road users and to improve the quality of motor vehicle traffic flow. However, traffic signal control is also an important instrument for speeding up public transport and for bundling and directing motor vehicle traffic.

Traffic control in the city of Aachen serves a wide variety of traffic types and user groups, the interests of which must be weighed up in each individual case. In principle, the signal coordination of successive junctions is planned, operated and continuously reviewed in the light of the latest findings. However, due to the complexity of the demands on the traffic area, frequent interventions in the functionality of the green waves are to be expected.

There are 228 traffic light systems in the city of Aachen. Each signal system has a control unit that performs most of the control functions on site. 181 systems are connected to a central traffic control computer. These systems can be operated and monitored from a central terminal. Faults are displayed and recorded in the control center.

The traffic computer coordinates the systems of a green wave. Depending on the time of day, it switches various signal plans whose green time distributions are adapted to the changing traffic loads throughout the day.

The control center enables remote access to the signal controls or signal plan selection and thus their prompt manual adjustment to changing traffic conditions.

144 systems provide online data via modern computer interfaces, which enables in-depth functional analysis on the traffic computer. In addition, there are 21 control units with radio interfaces and 16 control units with an outdated interface type, which only allow a very limited exchange of data between the control unit and the control center.

Planning principles

The distribution of green times and the allocation of lanes are based on the existing traffic volumes. The traffic volumes are regularly recorded in traffic counts as a necessary planning basis for all signalized junctions (traffic light intersections). Signal planning is based on these traffic counts. With the help of traffic detection, it is also possible to react to temporary fluctuations in traffic volumes at some junctions by changing the green times depending on the traffic. Weakly used side streets often only receive a green window on request.

The distribution of green times cannot, of course, be based solely on the volume of motor vehicle traffic. Other decisive specifications for signal planning result from the requirements of pedestrians, cyclists and local public transport.

With a few exceptions, all 228 traffic lights in the city are located on roads with a cross-section of more than 5000 vehicles per day. Signal systems are used in the following cases:

• High traffic volumes in several opposing traffic flows at an intersection
• Heavy traffic and / or high speeds on the higher-level road
• Insufficient visibility
• Accumulation of accidents between left turners and oncoming traffic
• Endangering vulnerable road users (cyclists / pedestrians)

Green waves

In principle, the aim is to achieve a journey that is as uninterrupted as possible. Heavy traffic flows should - in so-called green waves - be able to pass several consecutive traffic light systems without stopping. As a rule, no signal circuit is operated completely independently of the signalized adjacent nodes. Only in exceptional cases is a section or individual system not coordinated with the neighboring nodes.

There is no coordination on the following road sections:

• Roermonder Straße in Laurensberg and Richterich
• Hohenstaufenallee
• Freunder Landstraße
• Karl-Marx-Allee

on systems that are operated as single runners:

• Merowinger Strasse / Purweider Weg
• Lintertstraße / Schönforststraße
• Aachener Straße / Oberforstbacher Straße
• Eupener Straße / Weißhausstraße

and at all immediate green push-button systems (where pedestrians should get green as soon as possible after a manual request).

A green wave is activated on all other main roads as far as possible. The green waves of the opposing directions of traffic compete with each other. Green waves can only be implemented for both directions of travel if the intersections are evenly spaced. In practice, however, there are no evenly spaced junctions.

A decision must therefore be made as to whether one of the two directions of travel should be preferred. On the entry and exit roads in the urban area, this decision is relatively simple: in the morning, the main load of traffic moves into the city, in the afternoon out of the city. For this reason, the respective main load direction is preferred in the planning of the green wave in favor of a liquefaction of the traffic, which means that in the opposite direction, a coordination of the traffic light systems is often switched that is perceived as poor. This particularly affects motorists traveling out of the city in the morning or into the city in the afternoon, i.e. against the main direction of traffic.

On the ring roads, only some of the main load directions can be identified. The directions are therefore treated relatively equally in the planning.

Circulation times and daily schedules

As a rule, turnaround times of 90 seconds are used at times of high traffic volume. The cycle time is the time until a signal plan is repeated. During off-peak periods, the cycle time is reduced to 75 seconds to reduce waiting times, or the signal system is switched off (night shutdown). The switching times of signal systems are listed below. S1 is the morning program, S3 the afternoon program and S2 the off-peak program.

Special features and disruptive factors

Aachen's road network, which consists of intersecting radials and ring roads, poses a particular challenge for the planning of optimal green waves.

At junctions where radial roads and ring roads intersect, turning flows must also be coordinated in some cases. This is particularly difficult in areas with closely spaced junctions. Examples of this include

• Normaluhr with the intersections Römerstraße / Kurbrunnenstraße / Dunantstraße and Wilhelmstraße / Zollernstraße / Theaterstraße
• Trierer Straße in the area of the Brand junction with the junctions Trierer Straße / BAB slip road in the direction of Cologne and Trierer Straße / Debyestraße / BAB slip road in the direction of Belgium
• Outer ring road offset Lütticher Straße with the junctions Lütticher Straße / Amsterdamer Ring / Hohenstaufenallee and Lütticher Straße / Brüsseler Ring.

The traffic flow within the green waves is subject to considerable disruptive factors, which can have a major impact on the functioning of the wave when considered individually, but have an even more serious effect when taken together. Here are a few examples:

• Exceeding the capacity at peak traffic times (e.g. 7:30 to 8:30 a.m. and 4:00 to 6:30 p.m.) of particularly heavily loaded individual systems can lead to a build-up of backlog, which only dissipates again after several cycles. With high traffic loads, the green wave often comes to a complete standstill over a longer period of time.
• Stopping or parking in the second row for loading and unloading (especially by the various mail carriers) becomes a significant disruptive factor for the wave once a certain traffic load is reached (e.g. on Adalbertsteinweg or Wilhelmstraße).
• Equipment malfunctions / transmission faults can also have a lasting effect on the green wave. As with any technical device, malfunctions can also occur in the 228 signaling systems despite intensive and costly maintenance. Vandalism or accidents also contribute to faults in the green waves.
• If the distance from neighboring systems is more than about 750 m, a green wave is no longer feasible, since at greater distances the vehicle groups disperse to such an extent that coordination is no longer possible.

Pedestrian protection

The decision to shorten or postpone the green times for motor vehicles in favor of vulnerable road users has a considerable influence on the quality of the green waves. The consideration of pedestrian safety has a particularly strong influence on the circuits and thus on the green waves.

In Aachen, 56 systems are equipped with pedestrian push buttons. At 34 signal systems, there is a so-called immediate green switching for pedestrians. Here, the pedestrian pushbutton switches the vehicle traffic to red and the pedestrian crossing to green with a very short time delay. For reasons of pedestrian comfort and safety, no consideration is given to the green wave. Some of these systems are located on busy arterial roads, where many children also cross the road safely on their way to school, e.g:

• Hohenstaufenallee / Schillerstrasse
• Vaalser Straße / Westpark
• Vaalser Straße / Hammerweg
• Roermonder Straße / Rosenhügel
• Roermonder Straße / Schloss-Schönau-Straße
• Krugenofen / Sebastianstraße
• Blücherplatz / Eintrachtstraße
• Mozartstraße / Reumontstraße / Südstraße
• Halifaxstraße / Auf der Hörn
• Lütticher Staße / Sanatoriumstraße

Furthermore, particularly convenient pedestrian circuits (e.g. two green windows for pedestrians in a 90-second cycle) or particularly safe pedestrian circuits (e.g. all-round green) generally have a negative effect on the green wave:

At the Ludwigsallee / Ehrenmal pedestrian signal, a second green window is activated for pedestrians in a 90-second cycle to shorten pedestrian waiting times when a pushbutton is requested, which significantly interferes with the green wave.

On the Grabenring, there are eight facilities with all-round green for pedestrians. In favor of the safety of the high volume of pedestrians, a strong impairment of the flowing motor vehicle traffic is accepted:

• Templergraben / Wüllnerstraße
• Karlsgraben / Lochnerstraße
• Karlsgraben / Königstraße
• Alexianergraben / Annastrasse
• Seilgraben / Alexanderstraße
• Templergraben / Pontstraße
• Rope trench / new gate
• Seilgraben / Minoritenstraße

In individual cases, the all-round green system is also used outside the Grabenring to increase pedestrian safety:

• Wilhelmstrasse / Lothringer Strasse
• Stolberger Straße / Elsassstraße
• Alt-Haarener-Straße / Auf der Hüls
• Alt-Haarener-Straße / Haarener Gracht
• Kornelimünster Steinkaulplatz
• Lütticher Straße / Limburger Straße
• Limburger Straße / Hohenstaufenallee

Bus acceleration

In addition, a disruption of the green wave is accepted in favor of giving priority to local public transport at traffic lights. In order to allow the bus to exit a bus lane or bus stop into the flow of traffic, the motor vehicle traffic is held back for a few seconds at the traffic signals. The group of vehicles then reaches the next signal with a corresponding delay. This results in a reduction in the green time window that can be used without stopping. At 82 traffic signals, buses can interfere on request, which in exceptional cases can cause the green wave to deteriorate.

More safety at traffic lights

In addition to guidelines and attention fields, the pilot tone and the orientation signal are also very important for the orientation of blind people in the city. The aim of the city is to make it as safe as possible for blind people to use aids such as guidelines and traffic light tones. 

Walking safely through the city and always arriving at the right place is normal for many people. If you can see and are otherwise good on your feet, walking into or through the city is not a major obstacle. This is more difficult for blind people. They can't just go somewhere quickly or run through the traffic lights before the green phase ends. The route through the traffic lights has to be planned and orientation on the sidewalk is not easy either. Fortunately, there are a number of aids that help people to live independently and more autonomously in the city.

Goal: Safely through the city
Guidance strips made of ribbed plates and other changes to the pavement are used to improve orientation in the city. However, Ingeborg Jansen, Aachen's commissioner for the disabled, still sees some room for improvement. "We have been able to improve a lot, but unfortunately there are still places in Aachen where there are no guidelines. That's why it's particularly important to us that every new construction project pays attention to accessibility". This is an ongoing process. On the other hand, adjusting the traffic light tones is somewhat easier for blind people. Together with representatives of the associations for the blind in Aachen and the surrounding area, a good volume setting for the traffic light tones is being found during site inspections.

Release tone and pilot tone
There are two different tones that are used for traffic lights: the pilot tone and the release tone. The pilot tone is used to help blind people find the traffic light, or more precisely the traffic light pole. It is realized by a clicking sound. The enabling tone, on the other hand, is a rather high-pitched beep that is active during the green phase of the pedestrian light. "Green phases are often too short to navigate through the entire traffic light using the enabling tone, which is why the pilot tone must be heard again when the pedestrian light changes to red," says Jörg-Michael Sachse-Schüler, commissioner of the PRO RETINA e.V. self-help group in Aachen. This is the only way blind people can find their way all the way across the traffic lights.

Finding compromise solutions
It is difficult to extend the green phases for pedestrians at major intersections. "Traffic light times are of course always subject to conflicting interests. Due to the conflict of interests, a good balance has to be found for pedestrians, cyclists, public transport, i.e. buses or local public transport, and motorists," says Suhal Tosun from the city's traffic management department. "Traffic light control always means a compromise solution". Another compromise solution must also be found for the volume of the sounds. The orientation signal in particular can very quickly become annoying for residents. However, the technology helps here, as the sounds adapt to the volume of the junction. When the traffic calms down in the evening, the volume of the sounds also decreases.

Sometimes only short green phases
Horst Boltersdorf, Deputy Chairman of GIPS Spielen und Lernen, and Bernd Neuefeind, Second Chairman of the Blinden- und Sehbehindertenverein der StädteRegion Aachen e.V. 1907, agree: "The integrity of people is more important than making fast progress. No one is helped if a blind person is suddenly standing in the middle of the road at a red light because the signal is too quiet." Of course, this doesn't just apply to blind people, but also to older people or people who can't move quickly. "Often, especially at major junctions, the green phase for pedestrians is really very short," says Bernd Neuefeind. Suhal Tosun explains: "The green phase in Aachen is designed to give the signal to start walking. It doesn't mean that you should cross the road completely during the green phase. The green phase is always followed by a 'clearance time'. This is a kind of protection time for the pedestrian during which the cars still have red. It serves to ensure that pedestrians who step onto the pedestrian crossing during the last second of green can still cross at a brisk pace."

Obstacles on the guidelines
Another obstacle for blind people are blocked guidelines and chained bicycles. "Many people don't think about it directly, but for a blind person, the guideline is the way to find their way through the city. If there's a chair or a table on it, it's really very obstructive. That shouldn't happen," says Ingeborg Jansen. "Bicycles attached to traffic light poles or standing against a house wall along which a guideline runs are also a big problem," adds Horst Boltersdorf. Blind people cannot avoid such obstacles and collide with the objects. The City of Aachen is therefore once again asking all citizens to pay particular attention to the guidelines and ground markings and to park bicycles in the designated areas. In places in the city where there are no guiding strips, blind people use the walls of buildings for orientation. Here, objects such as flower pots or tables and chairs can become obstacles.

Positive mood
The volume of the traffic lights in Aachen is gradually being adjusted to a volume that can be compromised. Once this has been done, nothing should be changed. The settings are always made in compliance with the German DIN standard for acoustic signaling devices on light switching systems. "It's important to us to find a solution with the people affected ourselves," summarizes Ingeborg Jansen. "That works very well," adds Suhal Tosun about the collaboration.

Further topics