18.–21.05.2022 #polismobility

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About the discourse on nationwide 30 km/h speed limits in city centers


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Around 100 cities and municipalities (as of March 7, 2022) in the city initiative "Liveable cities through appropriate speeds - a new municipal initiative for more urban-friendly traffic" are calling for more freedom when it comes to ordering inner-city speed reductions.

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Text: Prof. Johannes Schlaich, Professor of Mobility and Traffic at the Berlin University of Applied Sciences.

Discussions about speed limits have polarized our society for many years: On the perennial topic of 100/130 km/h speed limits on highways, some see their freedom and the German auto industry threatened, while others counter with arguments such as CO2 reductions and greater road safety. With the conclusion of the coalition agreement, it is clear: For the time being, there will be no speed limit on highways.

For advocates of comprehensive speed reductions in inner cities, however, there is hope in the coalition agreement. Adjustments to the road traffic regulations are not only intended to make traffic smoother and safer: By giving states and municipalities more decision-making leeway, they should also be able to better promote climate and environmental protection, health aspects and urban development in the future. Until now, their options have been severely limited. This is because Section 3 of the German Road Traffic Act stipulates that the maximum permissible speed "within built-up areas for all motor vehicles is 50 km/h". Deviations from this are only possible in justified exceptions, for example in front of schools.

The 100 or so cities and municipalities (as of March 7, 2022) in the city initiative "Liveable cities through reasonable speeds - a new municipal initiative for more urban-friendly traffic" are also calling for more freedom. They would like to be able to impose a 30 km/h speed limit wherever they deem it appropriate, if necessary also citywide as the new maximum speed limit.

Traffic models can determine effects

Many effects of 30 km/h can be estimated with traffic models, as is common in traffic planning (Schlaich & Koesling 2014).

Here, traffic modelers can store changed travel times for motorized private transport (MIV). Among other things, this allows them to check how large the shifts in mode choice are. Model results often come up with about a two to three percent reduction in MIV trips and a slightly higher decrease in miles traveled (e.g., Ritz 2019; Krajzewicz et al. 2016). It is important to be realistic about the background speed: recent measurements of major roads in Berlin show that travel speeds on five major roads in Berlin are reduced by only one to six kilometers per hour (SenUVK 2021). Ultimately, however, such deteriorations for MIV should not be viewed only negatively, as they make cycling, walking and public transport relatively more attractive. This strengthens the environmental alliance.

Speed limit 30 also has a negative effect on the speeds of buses and road-bound streetcars. Travel time losses can be derived from existing measurements (approx. 30s per 1 km, SenUVK 2021) or microscopic traffic flow simulations and then appropriately considered in the traffic model (cf. Thomsen & Schlaich 2021). At the latest in a practical implementation of 30 km/h, possible delays, connection losses, and inefficient personnel and vehicle deployment must be analyzed in detail. The argument that such time losses can be compensated for by suitable acceleration measures (e.g., priority at traffic signals, bus lanes) must be examined on a case-by-case basis. In many cases, the potential of bus acceleration has already been exhausted.

Based on the changes in mobility behavior identified in the traffic model, traffic modelers can then determine changes in noise (according to RLS-19; FGSV 2020) and CO2 emissions (according to HB EFA, INFRAS 2022). While Tempo 30 can reduce noise by about 3 dB (UBA 2016), no clear statement can be made for CO2 emissions: Here, much depends on whether a good traffic flow is achieved at speed 30 - and there is certainly still a need for research on CO2 emissions at lower speeds (Deutscher Bundestag 2019).

Do not get lost in the details

Such a need for research should not, however, prevent cities, municipalities and communities from pursuing, adopting and implementing 30 km/h speed limits if democratically legitimized representatives consider this to be sensible. In addition to the effects on mobility behavior, noise and CO2 emissions that are often taken into account in traffic planning, two other effects must in fact be particularly emphasized:

1. the increase in road safety, especially for vulnerable road users (pedestrians, cyclists), has been proven in many studies from Europe and Germany (McKibbin 2014, UBA 2016). In addition, reducing speed differences of bicycles and MIV can increase subjective road safety and thus motivate increased use of bicycles.

2. cities can become more livable through slower, quieter and reduced MIV: Street spaces gain in quality of stay and local mobility is strengthened by better crossing possibilities for pedestrians.

To realize these significant benefits, cities and municipalities need more freedom: By adapting the road traffic regulations, 30 km/h could not remain an elaborately justified individual decision on short stretches of road, if decision-makers give sufficient weight to the clear advantages over the existing disadvantages.

Traffic models can and should support the political decision-making process, not stop it. It is important that traffic modeling does not become an end in itself, but answers decision-relevant questions, e.g., whether and, if so, which roads should continue to have a speed limit of 50. Traffic modeling can also provide answers to questions during actual implementation: for example, whether through-road closures in residential areas make sense to prevent through traffic.

In this process, decision-makers must have the courage not to want to clarify all questions in every detail before the first decision is made. Sometimes, results from other cities or from simple, low-cost implementations are enough to move on to the next implementation phase with new insights.

Prof. Dr. Johannes Schlaich

has been Professor of Mobility and Transport at the Berlin University of Applied Sciences since 2018. In teaching and in projects, he deals with the optimization of multimodal and sustainable mobility. His focus is on traffic modeling to determine in advance the effect of traffic measures on mobility and the environment.

environment. He is also an independent consultant for traffic modeling and digitalization in transportation. Previously, the graduate civil engineer was Product Manager for the digitalization of timetabling at Deutsche Bahn (DB Netz AG) and at PTV Group as Director Product Management & Services for traffic planning and simulation software as well as Senior Consultant Traffic Planning and Traffic Engineering.


Deutscher Bundestag: Wissenschaftlicher Dienst (2019), Fahrzeug-Emissionen bei 30 km/h und 50 km/h, Berlin.

FGSV (2020), RLS-19: Richtlinien für den Lärmschutz an Straßen, Köln.

INFRAS (2022), Handbuch für Emissionsfaktoren des Straßenverkehrs (HBEFA), Bern, Schweiz.

Krajzewicz, D., Cyganski, R., Heinrichs, M., Erdmann, J. (2016), Benefits of using microscopic models for simulating air quality management measures, Washington, D.C., USA.

McKibbin, D. (2014). Impact of 20mph speed limits. Northern Ireland Assembly

Research and Information Service Research Paper 17/15, Dublin, Irland. Ritz, C. (2019). Modellierung und Wirkung von Maßnahmen der städtischen Verkehrsplanung, Dissertation, Universität Stuttgart.

Thomsen, N., Schlaich, J. (2021), Modellierung von Maßnahmen zur deutlichen Reduktion des Pkw-Verkehrs in Städten, Tagungsbericht Heureka 2021, FGSV Verlag, Köln.

Schlaich, J., Koesling, S. (2014), Staugefahr in der City, der gemeinderat, Ausgabe 12, S. 68 - 69, pro Verlag und Service GmbH & Co. KG, Schwäbisch Hall.

Senatsverwaltung Umwelt, Verkehr und Klimaschutz (SenUVK 2019), Untersuchungskonzept zur lufthygienischen und verkehrlichen Wirkung von Tempo 30 mit Verkehrsverstetigung als Maßnahmen des Luftreinhalteplans zur Reduzierung von NO2, Berlin.

Umweltbundesamt (UBA 2016), Wirkungen von Tempo 30 an Hauptverkehrsstraßen, Dessau-Roßlau.


Prof. Johannes Schlaich, Professor für Mobilität und Verkehr an der Berliner Hochschule für Technik