Green Wave

sketch

In the case of a green wave , the traffic lights on a street are switched in such a way that when driving on the street at a certain speed, each traffic light is in its green phase. This speed ( progression speed ) is indicated in some cases on additional boards, for example green wave at 40 km / h . The advantage lies in a more continuous flow of traffic in the vehicles. While green waves in the case of one-way streets without time constraints - such as crossing green waves - can be planned and set up without any problems, green waves in two-way traffic only work equally for both directions of travel if the distances between the intersections allow this and the cycle times of the individual signal programs on each other are matched. In cases in which it is not possible to set up such an ideal green wave, one of the driving directions is usually preferred, e.g. in the morning the one in the city and the one out of the city in the afternoon.

planning

The control process of the green wave is explained here using the example of the guidelines for traffic light systems (RiLSA for short) that are valid in Germany . There a whole range of parameters must be taken into account when planning a green wave:

term	meaning	Note
Progression speed V P	actually traveling speed at which the LSA can be passed as part of the Green Wave without stopping	The progression speed is a planning variable. It is usually between 85% and 100% of the maximum permissible speed. In special cases, it has to be set lower, for example when driving significantly slower in practice due to a confusing route or because of tight bends.
Sub-point spacing l TP	Distance between two neighboring partial points	The intersection of the center lines of two green belts on two-way streets is called the subpoint (TP). The distance resulting from these two points of intersection is called the partial point distance.
LSA distance	Distance between the individual intersections of the street	The distance between two traffic lights and the progression speed jointly determine by how many seconds the green phase of the second traffic light must be delayed compared to the first traffic light so that all vehicles starting at the first traffic light when it is green can pass the second without stopping.
Orbital time	Time difference between the beginning of a green phase and the next beginning of the same green phase (" oscillation period ")	In the course of a green wave, the cycle times of the traffic lights involved must match. Often the common cycle time of the LSA in the green wave street cannot be freely chosen, because the effects on the rest of the traffic network (green waves of cross traffic) must also be taken into account. A variable turnaround time , as is sometimes found in traffic-dependent controls, cannot be used in green waves.
available green time	Green time that is available to an LSA for the direction of the Green Wave	The available green time is usually determined by the green time required for cross traffic. The less cross-traffic, the higher the available green time for the green wave, and the greater the leeway that remains for planning the green wave. Nodes at which there is little green time for the green wave are the compulsory points of planning.
signaled turning streams	Signal-controlled, turning traffic flows that leave the green wave street	The goal is usually to enable the green wave in one street for both directions of travel. Due to the fact that traffic light distances are not always ideal, there are often different green phases for direction and opposite direction at one and the same traffic light. If in such a case the traffic flows turning off do not receive their own signals, all traffic must always be released in both directions at the same time as part of the green wave (connection ban). Separate signals for turning traffic allow the green wave to be planned more freely, because then the green phases for direction and opposite direction are far less dependent on each other.
turning traffic	Cross traffic that turns into the Green Wave street and then continues on the Green Wave train	When planning a green wave, vehicles turning must be taken into account. After turning in, they usually reach the next traffic signal before the green phase begins. Subsequent traffic flowing in the train of the green wave cannot pass this traffic light from the beginning of the green because the vehicles that have turned must first accelerate. In such a case, the start of the green phase concerned must be brought forward by several seconds. The vehicles that have turned in can thus pass the intersection before the crowd of vehicles from the Green Wave reaches this traffic light. Turning vehicle flows of considerable traffic importance are given special consideration when planning the green phases. This means that a wave is planned in such a way that the vehicles do not have to stop after turning, but can continue on the train of the green wave without stopping ( green wave around the corner ).

In practice, the shaft is usually "hung up" at critical junctions between two main roads and developed from there in both directions. Depending on the distance between the cross streets, only direction-related coordination is often possible. The traffic in or out of the city is coordinated according to the load. The opposite direction has to stop frequently.

The formation of a cluster results in more even traffic. However, the traffic performance of the traffic signal systems involved usually falls, since green times must be switched to maintain the wave, which should actually be made available for cross traffic. From an occupancy rate of over 80–85%, the wave collapses due to vehicles that have to stop at the end of the crowd in front of red. In particularly busy streets, the green waves must therefore be abandoned during rush hour in order to maximize traffic performance.

A green wave in both directions is only possible if the distance between the traffic lights corresponds to the partial point distance or a multiple thereof. With a rotation time of 90 seconds and a maximum permissible speed of 50 km / h, this is 625 m, with 70 seconds and 50 km / h 486 m.

Coordination systems

There are two basic procedures for coordinating multiple nodes. A distinction is made between the simultaneous system and the alternative system .

The simultaneous system releases all crossings of a green belt at the same time. Thus the node distance can be calculated as follows:

${\displaystyle S=M\cdot t_{\mathrm {u} }\cdot V_{\mathrm {p} }}$

In the alternative system, the node release is shifted by half a cycle. The nodal distance calculation is then:

${\displaystyle S=(2\cdot M+1)\cdot t_{\mathrm {u} }\cdot V_{\mathrm {p} }/2=l_{\mathrm {tp} }}$

Variable assignment:

S = distance between nodes

M = constant

t _u = cycle time

V _p = rate of progression

l _tp = sub-point spacing

procedure

A green wave is not a wave in the physical sense. However, it has some properties in common with physical waves. The following illustration therefore serves as a clear model and less as an explanation of the green wave.

There are such things as oscillators (the traffic lights) that carry out something like oscillations and periodically go through the same traffic light phases (red, red-yellow, green, yellow, red ...). There is the period of oscillation T , the time between (e.g.) one red phase and the next. There is a frequency f , which is the frequency of the phase change. And the following applies:

${\displaystyle f={\frac {1}{T}}}$,

z. B. if red repeats every two minutes ( T = 2 min), then the frequency is f = 1/2 cycle per minute.

If you look along the street, you can see traffic lights in the same phase at certain intervals. This distance has the character of a wavelength λ .

The best way to follow the “propagation” of the wave is to follow the chronological sequence of encounters between a vehicle and the individual traffic lights. It “rides” on the wave by meeting every traffic light when it is green, ie moving along the road at the same speed as the wave. The following image sequence shows the state of the road at time intervals T / 4.

Between the first image (at time t = 0) and the last image (at time t = T ) the car has moved one wavelength, the phases of all traffic lights along the road are identical in both images; the wave has traveled a wavelength and each traffic light has passed through all phases once (each oscillator performs a full oscillation). The speed of the vehicle (and at the same time the propagation speed c of the wave) is the distance covered (i.e. λ ) divided by the time required for this (i.e. T ). Therefore applies

${\displaystyle c={\frac {\lambda }{T}}}$.

Since f = 1 / T (see above), it also holds

${\displaystyle c=\lambda \cdot f}$.

Is z. B. T = 2 min and λ = 1200 m (distance from one green traffic light to the next), then c = 1200 1 / (2 min) = 600 m / min (= 10 m / s = 36 km / h) : Green wave at 36 km / h.

Disturbing influences

Various factors can disrupt the homogeneous flow of traffic in a green wave:

• Traffic-dependent traffic light signal systems (LSA) due to the varying green times, in particular acceleration circuits of the local public transport can have a disruptive effect
• Level crossings
• Limitation of the green time for the wave due to the changing number of phases (e.g. due to separate signaling for those turning left)
• unpredictably alternating heavy traffic turning into the route (e.g. event traffic)
• Driving at speeds above the progression speed (often due to inattentiveness or lack of information about the progression speed)
• Dissolving the vehicle bunch at very large LSA distances (approx. 750 m or more)
• Obstructions to the flow of traffic due to parking in the second row (delivery traffic, taxis ...)
• Change in the cross-section of the lane (e.g. widening from one to two lanes in the same direction of travel and vice versa)
• Successive LSA with different phase sequences, e.g. B. separately signaled left-turn traffic

history

The additional sign 851 introduced in 1971 with the West German Road Traffic Regulations (StVO)

Additional sign 1012-34: The 1992 amendment to the StVO changed the size and number of the additional sign

The first manually coordinated switching of traffic lights at six intersections was introduced in Salt Lake City in 1917 . From 1926 there was a manually controlled green wave in Leipziger Strasse in Berlin, near the first German traffic light, the traffic tower on Potsdamer Platz . From the 1950s onwards, the green wave was introduced in other cities in Germany with an additional traffic light display that indicated which speed was necessary so that the passing car could optimally keep to the wave ( speed signal ). The display had three fields with 40 km / h, 50 km / h and 60 km / h (or different), with one of them glowing white. However, it was not successful in most cities, and such additional advertisements were removed again from the 1970s, with the exception of a few systems (e.g. in Hanover, Düsseldorf, Suhl or Wiesbaden).

In the GDR there were special number displays behind the intersections, which usually indicated the recommended speed in steps of five to reach the next green light. This also made it possible for the incoming cross-traffic to "tune into" the green wave and thus to reduce the waiting times at the intersections. Since these displays were not provided for in the German road traffic regulations at the time, they were switched off after the fall of the Wall and most of them have since been dismantled.

Current development

There can be conflicts when local public transport is to be given priority. At road crossings and the respective stops, the traffic then inevitably comes z. T. to stand. From an environmental point of view, too, the green wave is sometimes viewed critically.

In Copenhagen, traffic lights for cyclists are being coordinated in a green wave for the first time as part of cycle superhighways . In 2013, the first Green Wave for cyclists was set up on Belziger Strasse in Berlin .

To increase performance, traffic lights are increasingly being switched depending on the traffic - the resulting varying green times make it difficult to recommend the speed.

Norms and standards

Germany

• Guidelines for traffic light systems (RiLSA)

Austria

• RVS 04/05/30 ff. - Traffic light systems (VLSA)

Switzerland

• SN 640 840 - traffic lights; Coordination in streets with the method of partial point reserves

literature

• Frank Höfler: Transport Practice, Volume 2: Transport Technology . Bauwerk Verlag, Berlin 2006, ISBN 3-934369-53-7 , p. 148 ff .

Individual evidence

1. ↑ Conrad von Meding: Is there the green wave in Hanover or not? In: haz.de . January 22, 2010, accessed June 3, 2020 . 
2. ^ Andreas Schinkel: Green wave on Vahrenwalder Strasse. In: haz.de . August 17, 2012, accessed June 3, 2020 . 
3. ↑ Louisa Lange: Green wave or environmental zone? In: bio-hannover.de . Retrieved June 3, 2020 . 
4. ^ Peter Neumann: Traffic light for cyclists: First green wave for cyclists in Berlin. In: berliner-zeitung.de . April 1, 2014, accessed June 3, 2020 .