The claimant, an environmental association, requests the updating of the Clean Air Plan (Luftreinhalteplan) for the city of Stuttgart, as a third party summoned to attend the proceedings as a party whose rights may be affected (hereinafter summoned third party), in order to include measures to comply with immission limit values for nitrogen dioxide.

A Clean Air Plan has been in place for Stuttgart since 2006 which provides for numerous measures to reduce levels of particulate matter and nitrogen dioxide pollution. Due to the change in the immission limit values for nitrogen dioxide as of 1 January 2010, the defendant updated the Clean Air Plan in February 2010. Further measures included are, inter alia, a transit ban on lorries, year-round gradual driving bans for motor vehicles according to groups of pollutants being introduced gradually over time, and speed limits on certain major roads. A second update of the Clean Air Plan in October 2014 expanded the catalogue of measures again.

On 18 November 2015, the claimant brought an action and in support of it, he claimed that the annual average limit value for nitrogen dioxide (NO₂) was in some cases exceeded by more than 200 percent in 2013 at all measuring stations close to traffic, and that in 2014, too, it was significantly above the limit values at certain measuring stations. Clean air planning failed to make full use of the existing instruments and to specify the potential effect of most of the measures.

At the beginning of May 2017, the defendant submitted a draft "3rd update of the Clean Air Plan for the administrative region of Stuttgart/subplan for the federal state capital of Stuttgart for the reduction of PM10 and NO₂ pollution". In this updated clean air planning that was based on an expert overall impact analysis, a total of 20 new measures were described with which the air pollutant limit values are to be met by 2020 - or 2021 at the latest. Should a so-called "blue sticker" (blaue Plakette) be introduced from 1 January 2020, these measures would include, for instance, the imposition of all-year-round traffic bans on all vehicles that do not meet the requirements for such a sticker. Another precondition for this measure is, according to the planning authority's intentions, that only 20% of the fleet of motor vehicles registered in Stuttgart will be affected.

In its judgment of 26 July 2017, the Administrative Court (Verwaltungsgericht) upheld the action and ordered the defendant to update and/or amend the subplan for the federal state capital of Stuttgart of the Clean Air Plan for the administrative region of Stuttgart, which entered into force on 1 January 2006 in the current version of the first and second updates of February 2010 and October 2014 respectively, so that it will contain the necessary measures to comply - as quickly as possible - with the immission limit value for NO₂ of 40µg/m³, averaged over one calendar year, and the hourly limit value for NO₂ of 200µg/m³ with a maximum of 18 permitted exceedances in the Stuttgart Low Emission Zone (Umweltzone) per calendar year. The defendant's leapfrog appeal met only with partial success.

13 The admissible appeal on points of law is mostly unfounded. In line with federal law, the Administrative Court decided that the defendant is obliged to update and/or amend the subplan for the federal state capital of Stuttgart of the Clean Air Plan for the administrative region of Stuttgart, which entered into force on 1 January 2006 in its current version of the first and second updates of February 2010 and October 2014, respectively, so that it will contain the necessary measures to comply - as quickly as possible - with the immission limit value for nitrogen oxide (NO₂) of 40µg/m³, averaged over one calendar year, and the hourly limit value for NO₂ of 200µg/m³ with a maximum of 18 permitted exceedances in the Stuttgart Low Emission Zone per calendar year. In the context of this update, the defendant must consider a year-round traffic ban in the Stuttgart Low Emission Zone on all motor vehicles with petrol or gas-fuelled engines (including hybrid vehicles) below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard. However, the appeal on points of law is well-founded inasmuch as the Administrative Court did not take full account of the principle of proportionality when examining the permissibility and design of a traffic ban.

14 A.1. If immission limit values established by an ordinance issued pursuant to section 48a (1) of the Federal Immission Control Act (BImSchG, Bundes-Immissionsschutzgesetz) are exceeded, the competent authority must draw up a clean air plan pursuant to section 47 (1) first sentence BImSchG which specifies the measures necessary for the permanent reduction of air pollutants. Pursuant to section 47 (1) third sentence BImSchG, the measures of a clean air plan must be suitable to ensure that the exceedance period of immission limit values that must already be complied with is kept as short as possible.

15 Pursuant to section 3 (1) of the Thirty-Ninth Ordinance on the Implementation of the Federal Immission Control Act - Ordinance on Ambient Air Quality Standards and Emission Ceilings (39. BImSchV, Neununddreißigste Verordnung zur Durchführung des Bundes-Immissionsschutzgesetzes - Verordnung über Luftqualitätsstandards und Emissionshöchstmengen) of 2 August 2010 (Federal Law Gazette (BGBl., Bundesgesetzblatt) p. 1065), as last amended by article 1 of the Act of 10 October 2016 (BGBl. I p. 2244), the immission limit value for nitrogen dioxide (NO₂), averaged over a full hour, totals 200µg/m³ for 18 permissible exceedances in one calendar year in order to protect human health. The immission limit value for nitrogen dioxide (NO₂) for the protection of human health, averaged over one calendar year, totals 40µg/m³ (section 3 (2) 39. BImSchV).

16 The 39. BImSchV serves, among other things, the transposition of Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe (OJ L 152 p. 1), whose Annex XI, section B sets forth the limit values which must be complied with from 1 January 2010 and which have been adopted by the issuer of the ordinance. Article 13 (1) of Directive 2008/50/EC requires Member States to ensure that, throughout their zones and agglomerations, levels of sulphur dioxide, PM10, lead and carbon monoxide in ambient air do not exceed the limit values laid down in Annex XI and that the limit values for NO₂ and benzene specified in Annex XI may not be exceeded from the date specified therein (1 January 2010). Article 23 (1) first subparagraph of Directive 2008/50/EC requires Member States to ensure that, for zones or agglomerations where levels of pollutants in ambient air exceed any limit value, air quality plans be established in order to achieve the related limit values. In the event of exceedances of those limit values, the air quality plans shall set out suitable measures, so that the exceedance period can be kept as short as possible (article 23 (1) second subparagraph of Directive 2008/50/EC).

17 According to the (...) factual findings of the Administrative Court, which are binding for the Senate (section 137 (2) of the Code of Administrative Court Procedure (VwGO, Verwaltungsgerichtsordnung), both the annual average for NO₂ of 40µg/m³ and the hourly limit value for NO₂ of 200µg/m³ are not adhered to at a maximum of 18 permitted exceedances per calendar year at several measuring stations within the Stuttgart Low Emission Zone.

18 Furthermore, the Administrative Court - based on the overall impact expert report submitted by the defendant - determined that a year-round traffic ban in the Stuttgart Low Emission Zone on all vehicles with petrol or gas-fuelled engines (including hybrid vehicles) below the Euro 3/III exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6/VI exhaust emission standard - assuming that these motor vehicle groups account for 20% of the vehicle fleet registered with the summoned third party and further assuming 20% exceptions to the traffic ban - would mean that the nitrogen dioxide immission limit values will be met in 2020 at all measuring stations in the entire Stuttgart Low Emission Zone. The only exception is the "Am Neckartor" measuring station where the values will still be slightly exceeded (42µg/m³). The Court also held that such a traffic ban is the most effective and most suitable measure and that no other equivalent measure is available to achieve the objective (...). The Senate is also bound by these factual findings.

19 2. On this basis, the Administrative Court in any event rightly concluded that a traffic ban on the aforementioned motor vehicles can be lawfully ordered and must therefore be considered by the defendant. It is true that the currently applicable provisions of federal immission control law do not in themselves permit such traffic bans. However, their permissibility results from EU law.

20 a) The measures established in a clean air plan pursuant to section 40 (1) first sentence BImSchG must be enforced pursuant to section 47 (6) first sentence BImSchG by orders or other decisions by the competent public administration bodies in accordance with the Federal Immission Control Act or other legal provisions. The competent authority is neither obliged nor authorised to implement any measures which are disproportionate or unlawful for other reasons (Federal Administrative Court (BVerwG, Bundesverwaltungsgericht), judgment of 27 September 2007 - 7 C 36.07 - Rulings of the Federal Administrative Court (BVerwGE, Entscheidungen des Bundesverwaltungsgerichts) 129, 296 para. 26). The measures must therefore be capable of being implemented; immission control or other provisions must permit their implementation (Bundestag printed paper (BT-Drs., Bundestagsdrucksache) 14/8450 p. 14). Measures that encroach on basic rights require a separate basis for authorisation under (specialist) laws (BVerwG, decisions of 29 March 2007 - 7 C 9.06 - BVerwGE 128, 278 para. 27, and of 11 July 2012 - 3 B 78.11 - (...)).

21 b) Such a basis for authorisation exists in section 40 (1) first sentence BImSchG. According to this, the competent road traffic authority restricts or prohibits motor vehicle traffic in accordance with road traffic law provisions to the extent to which a clean air plan or a plan for measures to be taken at short notice as contemplated by section 40 (1) or (2) BImSchG provide for this. In this context, the measures pursuant to section 47 (4) first sentence BImSchG must address all emitters contributing to the immission values being exceeded in accordance with the respective polluter's share and taking the principle of proportionality into account.

22 Contrary to the opinion of the defendant and the summoned third party, ordering measures, which differentiate according to the contribution of motor vehicles to pollution on the basis of section 40 (1) first sentence BImSchG, does not at the same time necessarily require the adoption of an ordinance pursuant to section 40 (3) BImSchG. With the authorisation to issue ordinances in subsection 3, the legislature merely gave the issuer of the ordinance the possibility to determine that motor vehicles that only make a low contribution to pollution are or may be excluded from traffic bans wholly or in part and can stipulate the decisive criteria for this. This does not mean, however, that if the issuer of the ordinance does not make use of this authority, traffic bans linked to the pollution levels of motor vehicles are ruled out. This finding, which can already be taken from the wording of the provision ("is authorised"), is confirmed by the legislative materials. According to these materials, the legislature expressly assumed that the competent authorities may order restrictions and bans of motor vehicle traffic even before an ordinance will have been issued pursuant to section 40 (3) BImSchG (see BT-Drs. 14/8450 p. 21 and BT-Drs. 14/8895 p. 7 et seq.).

23 c) By enacting the Thirty-Fifth Ordinance for the Implementation of the Federal Immission Control Act (35. BImSchV, Fünfunddreißigste Verordnung zur Durchführung des Bundes-Immissionsschutzgesetzes), the issuer of the ordinance conclusively made use of the legal authorisation under section 40 (3) first sentence BImSchG to exclude motor vehicles that only make a low contribution to the pollution from traffic bans wholly or in part.

24 It is the essential content of the 35. BImSchV that motor vehicles which are marked with a red, yellow or green sticker pursuant to Annex 1 of the Ordinance are exempted from a traffic ban within the meaning of section 40 (1) BImSchG if a corresponding traffic sign provides for this (section 2 (1) 35. BImSchV).

25 In detail, section 2 (2) first sentence 35. BImSchV classifies motor vehicles as belonging to groups of pollutants 1 to 4 on the basis of their pollutant emissions. The classification is laid down in Annex 2 of the Ordinance and follows the exhaust emission standard tiers Euro 1 to Euro 4 laid down in EU law. For both vehicles with positive-ignition engines (section 2 (2) second sentence in conjunction with Annex 2 (2) of the 35. BImSchV) and vehicles with compression-ignition engines (section 2 (2) second sentence in conjunction with Annex 2 (1) of the 35. BImSchV), this classification is based on the respective overall exhaust emission behaviour of the vehicle, including the - in the present case relevant - nitrogen oxide emissions according to the relevant exhaust emission directives of the European Community (see BT-Drs. 162/06 p. 25). The latter also refer to the emission of pollutants in the form of carbon monoxide, hydrocarbons and particulate matter. Contrary to the claimant's argument, the issuer of the 35. BImSchV therefore did not limit its regulation to the objective of reducing traffic-related particulate matter quantities (...).

26 Motor vehicles without internal combustion engines, such as electric engine or fuel cell vehicles, are classified as group of pollutants 4 pursuant to section 2 (2) second sentence in conjunction with Annex 2 (3) of the 35. BImSchV. On the one hand, this makes it clear that a motor vehicle's type of drive as such is not a decisive differentiation criterion of the 35. BImSchV. On the other hand, it also becomes clear that the scope of the 35. BImSchV also covers particularly low emission motor vehicles and also provides for a conclusive regulation in this respect.

27 The issuer of the ordinance's historical intention also supports the view that the 35. BImSchV represents a conclusive regulation for the classification of motor vehicles in categories M and N with regard to an exemption from traffic bans (...).

28 The issuer of the 35. BImSchV emphasises that the sticker system provides for a uniform means of motor vehicle identification, which enables differentiating intervention into the vehicle fleet, simplifies the monitoring of motor vehicle traffic and leads to easily recognisable equal treatment of all vehicles from EU Member States (Bundesrat printed paper (BR-Drs., Bundesratsdrucksache) 162/06 p. 1, 21).

29 d) That being said, it can be stated that the 35. BimSchV with its conclusive classification of motor vehicles with regard to the exemption from traffic bans pursuant to section 40 (1) BImSchG, does not differentiate solely according to a motor vehicle's engine, i.e. compression ignition (diesel) or a positive ignition (petrol). The same applies to a distinction according to whether motor vehicles with petrol or gas-fuelled engines only comply with exhaust emission standards below the Euro 3 exhaust emission standard. Instead, differentiation between certain groups of motor vehicles is based solely on criteria that follow the emission behaviour of the motor vehicles according to their assignment to groups of pollutants 1 to 4 (see section 2 (2) in conjunction with Annex 2 of the 35. BImSchV).

30 This result is equally applicable to traffic bans in the form of low emission zones and route-related traffic bans. According to their unambiguous wording, both the authorisation provision in section 40 (1) first sentence BImSchG and the authorisation to issue ordinances in section 40 (3) BImSchG refer to traffic bans in general. The 35. BimSchV also does not give any indication that the scope of the Ordinance is limited to zone-related or route-related traffic bans.

31 3. However, the conclusive character of the 35. BImSchV does not rule out traffic bans linked to the type of drive. In view of the obligation under EU law to keep the exceedance period of nitrogen dioxide limit values as short as possible, the application of federal law contrary to this obligation cannot be applied.

32 a) According to case-law of the Court of Justice of the European Union (CJEU), exceeding the limit values to be complied with is sufficient in itself to constitute an infringement of article 13 (1) of Directive 2008/50/EC in conjunction with Annex XI of this Directive (judgment of 5 April 2017 - C-488/15 [ECLI:EU:C:2017:267], Commission/Bulgaria - para. 69). According to this, even a possible partial downward trend in ambient air pollution, which does not, however, lead to compliance with the limit values, cannot invalidate the finding of failure to fulfil obligations attributable to a Member State (CJEU, judgment of 22 February 2018 - C-336/16 [ECLI:EU:C:2018:94], Commission/Poland - para. 62 and 65). According to case-law of the CJEU, it is also irrelevant whether the Member State to which the violation is attributable was committed intentionally or negligently or whether it is based on technical difficulties which the Member State may have encountered (CJEU, judgment of 5 April 2017 - C-488/15 - para. 76 with further references). The Member State may plead insuperable difficulties only in specific cases, notably in cases of force majeure (see CJEU, judgment of 19 December 2012 - C-68/11 [ECLI:EU:C:2012:815], Commission/Italy - para. 64 with further references).

33 However, infringement of article 13 (1) of Directive 2008/50/EC does not yet imply an obligation to act in relation to a specific individual measure.

34 Accordingly, the fact that a Member State exceeds limit values in ambient air is not in itself sufficient to establish that the Member State has violated its obligations under article 23 (1) second subparagraph of Directive 2008/50/EC. Instead, Member States have some scope for action in determining the measures to be adopted. Notwithstanding this, the measures laid down must in any case allow the exceedance period of limit values is kept as short as possible. In these circumstances, it is necessary to examine on a case-by-case basis whether the plans drawn up by the Member State concerned comply with article 23 (1) second subparagraph of Directive 2008/50/EC. The length of the period of time during which a limit value has already been exceeded must also be considered. In line with this, the CJEU held in its judgment of 5 April 2017 - C-488/15 (para. 115) that a systematic and continuous violation of the obligation under article 13 of Directive 2008/50/EC to keep the exceedance of limit values as short as possible was in fact committed because the limit values for particulate matter were still not met in 2014 in all six Bulgarian zones and agglomerations. In its judgment of 22 February 2018 - C-336/16 (para. 99 et seqq.), the CJEU criticised the fact that the plans adopted by the Republic of Poland made it possible for the Member State concerned to put an end to such exceedances only 10, or even 14, years after the date on which those exceedances were recorded. In this context, the CJEU refused to accept Poland's statement that the time limits which it set were fully adapted to the scale of the structural changes necessary to bring an end to exceedances of the limit values in ambient air, and to overcome difficulties in conjunction with the socio-economic and financial challenges of the major technical investments to be carried out; the CJEU argued that it was not established that the difficulties referred to would not rule out the possibility of having set shorter time limits.

35 On the basis of this case-law, clean air planning violates article 23 (1) second subparagraph of Directive 2008/50/EC at least if such planning fails to take the currently most suitable clean-air measures in order to comply as quickly as possible with the limit values exceeded, and instead excludes the taking effect of such measures before 1 January 2020 and additionally makes them contingent upon conditions the occurrence of which is uncertain and which cannot be caused by the planner himself.

36 (b) If clean air planning fails to comply with the obligations under Directive 2008/50/EC, the national courts having jurisdiction, should a case be brought before them, are obliged to take any necessary measure with regard to the national authorities to ensure that the authority draws up the plan required by Directive 2008/50/EC in accordance with the conditions laid down therein (CJEU, judgment of 19 November 2014 - C-404/13 [ECLI:EU:C:2014:2382], Client Earth - para. 58). The national court which is called upon is required, within the exercise of its jurisdiction, to ensure that the provisions of EU law are given full effect by not applying, where necessary, any conflicting provision of national legislation on its own motion, without having to request or await the prior setting aside of such provision by legislative or other constitutional means (CJEU, judgment of 20 December 2017 - C-664/15 [ECLI:EU:C:2017:987], Protect Umweltorganisation - para. 55 et seq. with further references).

37 Hence, it does not need to be decided whether the obligation to help enforce EU law can be fulfilled by an interpretation of the 35. BImSchV in conformity with EU law (see on its limits BVerwG, judgment of 5 September 2013 - 7 C 21.12 - BVerwGE 147, 312 para. 36 with further references) or by the fact that the provisions are not applied to the extent to which they conflict with a traffic ban on all motor vehicles with petrol or gas-fuelled engines below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard in the Stuttgart Low Emission Zone. Contrary to the view taken by the defendant and the summoned third party, the latter does not mean that ordering traffic bans completely lacks a basis for authorising traffic bans. As explained earlier (para. 21 et seq. above), section 40 (1) first sentence BImSchG constitutes a basis for authorising traffic restrictions provided for in a clean air plan, which applies irrespective of whether the Federal Government has issued exceptions for motor vehicles with a low contribution to pollution by means of an ordinance. In view of this, the present case does not constitute a methodologically inadmissible further development of the law contra legem (see on this BVerwG, judgment of 5 September 2013 - 7 C 21.12 - BVerwGE 147, 312 para. 36 with further references).

38 B. Ordering a year-round traffic ban in the Stuttgart Low Emission Zone on all motor vehicles with petrol or gas-fuelled engines (including hybrid vehicles) below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard must hence be subject to the principle of proportionality. The Administrative Court did not fail to recognise this. However, its statements on this do not stand up to federal law standards in every respect. This applies, in particular, in as far as the Administrative Court assumes that there are no legal objections against an immediate traffic ban on motor vehicles with diesel engines below the Euro 6 exhaust emission standard (...).

39 1. The principle of proportionality dominates all state action and has constitutional rank. It results from the principle of the rule of law and from the nature of the basic rights themselves, which, as an expression of the general right to freedom of the citizen vis-à-vis the state, may be restricted by public authority only to the extent that is indispensable for the protection of public interests (Federal Constitutional Court (BVerfG, Bundesverfassungsgericht), decisions of 15 December 1965 - 1 BvR 513/65 - Rulings of the Federal Constitutional Court (BVerfGE, Entscheidungen des Bundesverfassungsgerichts) 19, 342 <348 f.>, of 12 May 1987 - 2 BvR 1226/83 et al. - BVerfGE 76, 1 <50> and of 17 June 2004 - 2 BvR 383/03 - BVerfGE 111, 54 <82>). With regard to clean air planning measures, the non-constitutional law in section 47 (4) first sentence BImSchG also expressly provides that measures are to be determined according to the polluter's share and in line with the principle of proportionality. According to the principle of proportionality, even a state measure that is suitable and necessary to achieve a legitimate purpose may not be disproportionate in relation to the purpose and/or objective of the measure. The principle of proportionality requires a weighing of the benefits of the measure and the burdens it creates, thereby setting a limit to the burdens (...).

40 The general principles of law, including the principle of proportionality, form also part of the European Union legal order (see, e.g., CJEU, judgment of 9 October 2014 - C-492/13 [ECLI:EU:C:2014:2267], Traum - para. 27 with further references). At the same time, Directive 2008/50/EC itself expressly refers to the principle of proportionality in recital 25 second sentence of the Directive. Even according to the case-law of the CJEU, clean air plans may be adopted only on the basis of the balance between the aim of minimising the risk of pollution and the various opposing public and private interests (CJEU, judgment of 22 February 2018 - C-336/16 - para. 93 et seq.)

41 a) The details of the traffic ban to be considered must therefore be appropriate and reasonable for those affected by the ban. This requires the defendant to weigh the risks to human health associated with exceeding the applicable NO₂ limit values against the burdens and restrictions associated with a traffic ban, in particular, for the vehicle owners, vehicle keepers and vehicle users concerned - and also for the supply of the population and the economy. A distinction must be made between traffic bans that only affect individual roads or road sections (route-related bans) and traffic bans that are intended to apply to an expansive, coherent transport network consisting of a large number of main and secondary roads (zone-related bans). The former merely mean that the motorists concerned cannot reach individual destinations or only by taking more or less long detours and cannot park their vehicles on the roads (and/or road sections) covered by the ban. The intensity of such restrictions does not go beyond the other prohibitions established by road traffic law on driving and stopping on roads which motorists always expect and which they must generally accept. This also applies to authorised users visiting a property or a frontage resident (Anlieger, hereinafter authorised user) and residents affected by a route-related traffic ban. Unrestricted access to a property "right up to the front door" is not part of the core area of use by authorised users in urban agglomerations, even for owners of a residential property. Authorised users and residents have no claim to a specific design and extent of the property's connection to the road as long as the road is maintained as a means of transport. In this respect, special situations can be adequately addressed by granting exceptional approvals (see BVerwG, judgment of 8 September 1993 - 11 C 38.92 - BVerwGE 94, 136 <139, 142> with further references). The situation for the drivers, vehicle keepers and residents concerned differs from this if a traffic ban covers large parts of an urban area. Such a ban not only means that the inhabitants of this zone are no longer allowed to drive their vehicles affected by the ban into an expansive area , but also that they cannot park their vehicles in the public traffic space there. This will often induce residents of such a zone to sell the vehicle concerned. However, a zone-related ban constitutes a significant encroachment upon the basic right of general freedom of action (article 2 (1) of the Basic Law (GG, Grundgesetz)) of drivers that do not live in the respective zone. It is true that the legal system does not provide for a claim to the unlimited use in terms of time and space of a motor vehicle on public roads once the motor vehicle has met the registration criteria. However, the principle of proportionality must always be observed and prohibits the imposition of such far-reaching traffic bans without considering the economic consequences for the persons concerned.

42 In order to do justice to the necessary balance of interests, a phased introduction of zone-related traffic bans must hence be examined in such a way that, in a first phase, only older vehicles (for instance, up to the Euro 4 exhaust emission standard) are covered by traffic bans (...). Zone-related bans can be considered, however, not before 1 September 2019, for the still newer Euro 5 vehicles (the Euro 5 exhaust emission standard has applied to all vehicles since 1 January 2011). This date is four years after the Euro 6 exhaust emission standard entered into force on 1 September 2015 for all vehicles. This ensures that owners of a Euro 5 vehicle will have an unrestricted minimum useful life that extends beyond the first three years during which, as experience has shown, the loss of value is particularly high. In setting the time limit, the Senate took into account the fact that buyers who purchased a new diesel vehicle that only met the Euro 5 exhaust emission standard immediately before the Euro 6 exhaust emission standard entered into force could have readily understood that this vehicle would soon no longer comply with the latest emissions regulations. Therefore, more far-reaching protection of legitimate expectations cannot be granted to these buyers. This ultimately applies to all buyers who purchased a new diesel vehicle that met the Euro 5 exhaust emission standard after 1 September 2014. This is because from that date and in accordance with article 10 (4) of Regulation (EC) No 715/2007 of the European Parliament and of the Council of 20 June 2007 on type approval of motor vehicles with respect to emissions from light passenger and commercial vehicles (Euro 5 and Euro 6) and on access to vehicle repair and maintenance information (OJ L 171 p. 1), national authorities were no longer able to grant type approval for new types of vehicles which did not comply with the Euro 6 limit values set out in Table 2 of Annex I of that Regulation. Owners of diesel vehicles who purchased diesel vehicles with the Euro 5 exhaust emission standard between 1 January 2009 and 31 August 2014, can, however, be generally expected to accept their limited usability due to traffic bans in view of the higher age and mileage and the resulting lower residual value of such vehicles.

43 There is no need for transitional periods for diesel vehicles that only meet the requirements of the Euro 4 exhaust emission standard and for petrol or gas-fuelled engines below the Euro 3 exhaust emission standard. Type approval for these vehicles could only be granted until 31 December 2010 and 31 December 2000, respectively (see Directive 70/220/EEC as amended by Directive 98/69/EC). According to the factual findings established by the competent court, there are also no indications that a significant reduction in pollution levels would not be achievable if possible immediate traffic bans were limited to diesel vehicles meeting the Euro 4 or a lower exhaust emission standard.

44 When determining the date of entering into force of any traffic bans on diesel vehicles, in particular, the Euro 5 exhaust emission standard, the defendant must also consider the development of limit value exceedances by that date according to current surveys. If the exceedance of limit values decreases much more than previously forecast, the response could be to waive or defer the introduction of a traffic ban, at least for diesel vehicles that meet the Euro 5 exhaust emission standard.

45 b) It must also be considered which groups, such as craftspeople or certain groups of residents, and which individuals could be granted exceptions from a traffic ban in order to respect the principle of proportionality. Exceptions can be granted within the scope of section 40 (1) second sentence BImSchG and section 1 (2) 35. BImSchV. Section 1 (2) 35. BImSchV, in particular, also permits, in accordance with the expressly declared intention of the issuer of the ordinance, exceptions to the benefit of individuals or specific groups, such as frontagers or craftspeople (see BT-Drs. 819/07 p. 9 et seq.). Exceptions in the form of granting transitional periods for the retrofitting of diesel vehicles, in particular the Euro 5 exhaust emission standard with suitable exhaust emission control technology, can also be a component for establishing the proportionality of the traffic ban to be considered.

47 2. The introduction of a traffic ban on all motor vehicles with petrol or gas-fulled engines below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard in the Stuttgart Low Emission Zone in phases in conjunction with the granting of exceptions is consistent with the obligation of the competent authorities under article 23 (1) second subparagraph of Directive 2008/50/EC and section 47 (1) third sentence BImSchG to keep the exceedance period of limit values as short as possible. As the Senate has already decided, neither national nor EU law requires that the measures to be taken lead to the immediate achievement of the objective. Instead, in accordance with the principle of proportionality, it is possible to provide for a procedure in several phases (see BVerwG, judgment of 5 September 2013 - 7 C 21.12 - BVerwGE 147, 312 para. 59 and CJEU, judgment of 22 February 2018 - C-336/16 - para. 93 with further references).

48 3. A traffic ban on all motor vehicles with petrol or gas-fuelled engines below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard in the Stuttgart Low Emission Zone, which is designed in accordance with the principle of proportionality, does not require a compensation scheme for the motor vehicle owners concerned. A proportionate restriction of the use of a motor vehicle by means of a local traffic ban and the potentially associated loss of market value of the motor vehicle falls within the definition of the content of property within the meaning of article 14 (1) second sentence GG which the respective owner must accept without compensation.

49 The protection of property anchored in article 14 GG primarily requires, as far as possible, that provisions that interfere with property should be designed proportionately, without recourse to compensatory settlement payments (see BVerfG, judgment of 6 December 2016 - 1 BvR 2821/11 et al. - BVerfGE 143, 246 para. 260 with further references). This must be achieved, in particular, by creating transitional and exceptional provisions. In contrast, the legislature does not have to provide for compensation for any reduction in value caused by the state. Article 14 (1) GG does not generally provide protection against a reduction in economic efficiency nor does it guarantee that every opportunity for a favourable exploitation of property can be taken (see, e.g., BVerwG, judgment of 24 May 1996 - 4 A 39.95 - (...)).

50 Notwithstanding this, it cannot in any case be assumed under the given circumstances that a locally introduced traffic ban - also in conjunction with other local traffic bans - especially for (certain) diesel vehicles will lead to a collapse of the second hand car market for the motor vehicles concerned or to disproportionate burdens due to particularly high market value losses. After all, traffic bans will only apply to a fractional part of the road network in Germany and a few agglomerations.

51 C. If a traffic ban on all motor vehicles with petrol or gas-fuelled engines below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard proves to be necessary under EU law, its implementation will not fail because of road traffic law provisions.

52 1. The Administrative Court rightly assumed that for the traffic bans to be considered the traffic signs used to designate low emission zones in the Road Traffic Ordinance should be used in the first instance in order to comply with the exceeded nitrogen dioxide immission limit values as quickly as possible. The Administrative Court also correctly found that the traffic ban to be considered could not be based on section 45 (1f) StVO and the combination of traffic signs provided for therein. Pursuant to section 45 (1f) StVO, the competent road traffic authority orders the necessary traffic bans by installation of the signs 270.1 ("Start of a zone in which traffic is banned in order to reduce adverse effects of air pollutants in a zone") and 270.2 ("End of a zone in which traffic is banned in order to reduce adverse effects of air pollutants in a zone") in conjunction with the appropriate supplementary sign in order to mark the low emission zone stipulated in a clean air plan. This supplementary sign "Exemption from the traffic ban pursuant to section 40 (1) BImSchG" in no. 46 of Annex 2 (to section 41 (1) StVO) allows exemptions only for motor vehicles that have a red, yellow or green sticker according to section 3 (1) third sentence 35. BImSchV. The issuer of the ordinance does not provide for the use of other supplementary signs in the context of establishing low emission zones. This results not only from the systematic connection of the provision with the 35. BImSchV, but also from the use of the singular in the wording of section 45 (1f) StVO ("in conjunction with the appropriate supplementary sign") and applies irrespective of whether the reference in section 40 (1) first sentence BImSchG ("in accordance with the provisions of road traffic law") constitutes a reference to constituent elements or, as is predominantly assumed, a reference to legal consequences (...). This is because section 45 (1f) StVO does not lay down any (additional) requirements under road traffic law for ordering traffic bans pursuant to section 40 (1) BImSchG, but, with the way in which they are marked, only concerns the legal consequences and in this respect is covered by the reference of section 40 (1) BImSchG to the provisions of road traffic law.

53 However, section 45 (1f) StVO is not a final regulation for the notification of traffic bans in low emission zones, simply because this would violate the obligation to comply with the immission limit values as quickly as possible as stipulated by EU law. In this respect, it can also be left open whether section 45 (1f) StVO - as the Administrative Court assumes - can be interpreted in conformity with EU law (see above, para. 37) or whether also in this respect - as in the case of the 35. BImSchV - the provision must remain partially inapplicable because it restricts the traffic signs in question in contrast to the obligations under EU law. In both cases, the "blocking" of a combination of traffic signs 270.1 and 270.2 with supplementary signs other than the one provided for in no. 46 of Annex 2 of the Road Traffic Ordinance becomes obsolete.

54 2. On this basis, the Administrative Court also assumed that the necessary restrictions on traffic bans could be implemented by the competent road traffic authority by creating supplementary signs that were not previously regulated in the Road Traffic Ordinance. This is also compatible with federal law.

55 The Road Traffic Ordinance does not provide a suitable supplementary sign to specify the motor vehicles to which the traffic ban under consideration is limited. The same applies to the official catalogue of traffic signs to which section 39 (9) StVO refers. However, the catalogue of supplementary signs according to the Road Traffic Ordinance is not exhaustive (see section 41 (2) third sentence StVO) and can be complemented by suitable supplementary signs.

56 With regard to the requirements for additional signs, the General Administrative Regulation regarding the Road Traffic Ordinance (VwV-StVO, Allgemeine Verwaltungsvorschrift zur Straßenverkehrs-Ordnung) of 26 January 2001 in the version of 22 May 2017 (Official section of the Federal Gazette (BAnz AT, Amtlicher Teil des Bundesanzeigers) of 29 May 2017 B8) determines, under the heading "Regarding sections 39 to 43 - General Information regarding Traffic Signs and Traffic Installations", no. III.16 (a), that additional signs, in as far as possible, should only show symbols rather than text or letters. According to this Administrative Regulation, additional signs which are not included in the official catalogue of traffic signs also require approval - to be obtained as required - by the competent supreme federal state authority or the body designated by it. According to "Regarding sections 39 to 43 (...)", no. III.16 (b) VwV-StVO, no more than two additional signs should be attached to a post in consideration of the principle of visibility (Sichtbarkeitsgrundsatz).

57 In view of the existing difficulty in developing generally understandable symbols for diesel engines as a type of drive or for the different Euro exhaust emission standard tiers, the priority of using symbols ("in as far as possible"), which is in any case applicable under the General Administrative Regulation regarding the Road Traffic Ordinance with binding effect within the administration, should not prevent the use of an additional text sign, such as "Diesel Euro 6 and other Euro 3 and higher vehicles permitted", as the Administrative Court considered. Such an additional sign would also be consistent with section 41 (2) third sentence StVO, according to which the contents of additional signs are limited to general restrictions of orders or bans or general exceptions to these.

58 Since traffic signs must be immediately complied with (see section 80 (2) first sentence no. 2 VwGO), regulations announced by the installation of traffic signs must be clear and unambiguous. Traffic signs must therefore be set up or attached in accordance with the principle of visibility in such a way that an average motorist can understand them with a quick and casual glance while exercising due care pursuant to section 1 StVO. Under this condition, they have legal effect for every road user affected by the regulation, regardless of whether or not they actually perceive the traffic sign (established jurisprudence, see, e.g., BVerwG, judgment of 13 March 2008 - 3 C 18.07 - BVerwGE 130, 383 para. 11 with further references). In any case, sufficient understandability appears to be ensured with the signage mentioned as an example.

59 3. It is not necessary to use traffic signs in order to identify the exceptions from a traffic ban on all motor vehicles with petrol or gas-fuelled engines below the Euro 3 exhaust emission standard and on all motor vehicles with diesel engines below the Euro 6 exhaust emission standard, which are necessary to comply with the principle of proportionality. Nothing else results from the provision of section 45 (4) StVO. According to this provision, road traffic authorities may only regulate and direct traffic by means of traffic signs and traffic installations, with the exception of the cases contemplated in section 45 (1) second sentence no. 5 StVO. This is generally also applicable to the permission of exceptions from traffic bans (see BVerwG, judgment of 13 March 2008 - 3 C 18.07 - BVerwGE 130, 383 para. 20 et seqq.) However, the underlying principle, according to which a road user must be able to rely on the exhaustiveness of the regulation of a traffic sign which is to apply to everyone, does not claim, as can be seen from the consideration of section 45 (4) StVO in connection with section 46 (1) first sentence no. 11 StVO, within the scope of application of the Road Traffic Ordinance, validity for exceptions which are not intended to apply to everyone, but which apply in certain individual cases or to a certain group of persons (see BVerwG, judgment of 13 March 2008 - 3 C 18.07 - BVerwGE 130, 383 para. 25 et seq.).

60 In this sense, section 1 (2) 35. BImSchV and section 40 (1) second sentence BImSchG authorise exceptions in individual cases or in relation to a specific group of persons which do not require notification in the form of a traffic sign (...). With regard to zone-related traffic bans, this result is confirmed by the official notes on traffic sign 270.1 in column 3 of Annex 2 of the Road Traffic Ordinance, according to which exceptions may be permitted in individual cases or generally by means of supplementary signs or a general administrative act. This result is also indirectly confirmed by the wording of section 40 (1) second sentence BImschG and section 1 (2) 35. BimSchV which - in contrast to section 40 (1) first sentence BImschG - do not provide for the decisiveness of the provisions of road traffic law. According to the explanatory memorandum of the First Ordinance Amending the Thirty-Fifth Ordinance for the Implementation of the Federal Immission Control Act (Erste Verordnung zur Änderung der 35. BImSchV), the issuer of the ordinance also assumes that an immission control order on the basis of section 1 (2) 35. BImSchV does not have to be implemented by a traffic sign (BR-Drs. 819/07 p. 10).

61 D. Any difficulties in the enforcement of the traffic ban under consideration do not lead to the unlawfulness of it being ordered. The introduction of a ban regulation does not fail due to a lack of controllability.

62 It is true that the enforcement of traffic bans without identifying the motor vehicles exempted from a traffic ban - in particular, by means of a suitable sticker to be introduced in conjunction with an adaption of the 35. BImSchV (for instance, a "blue sticker") - would probably be considerably more difficult. However, this does not mean that a ban regulation would be unlawful. There can be no question of a structural enforcement deficit causing the unlawfulness of a regulation. Such a structural enforcement deficit would presuppose a normative-based obstacle which, due to its structure, leads to deficient enforcement practice (see BVerwG, judgment of 16 December 2016 - 8 C 6.15 - BVerwGE 157, 127 para. 47 with further references). In contrast, mere enforcement deficits or the empirical inefficiency of legal provisions are not sufficient. What is necessary is a contradiction between the normative command of substantive law and the regulations not designed to enforce it (see BVerfG, judgment of 9 March 2004 - 2 BvL 17/02 - BVerfGE 110, 94 <113> with further references).

63 A normative deficit of this kind, which is designed to frustrate the regulatory effect, cannot be established in the present case. In stationary traffic, effective checks on motor vehicles appear to be possible by means of checks on vehicle keepers. In order to prosecute administrative offences, the police and administrative authorities are authorised to retrieve the necessary data from the Central Vehicle Register (see section 36 (2) first sentence no. 1 and 1a in conjunction with section 35 (1) no. 3 StVG). This also includes data on condition and features as well as identification and registration details, including the exhaust emission standard applicable to the vehicle (section 24 (1) first sentence StVG in conjunction with section 49 (3) no. 4 StVO, section 33 (1) no. 1 StVG in conjunction with section 30 (1) no. 1 and 9 of the Ordinance on the Registration of Vehicles to Road Traffic - Vehicle Registration Ordinance (FZV, Verordnung über die Zulassung von Fahrzeugen zum Straßenverkehr - Fahrzeug-Zulassungsverordnung) - of 3 February 2011 <BGBl. I p. 139>, last amended by article 1 of the Ordinance of 31 July 2017 <BGBl. I p. 3090>, section 39 (2) no. 1 (a), (3) no. 1 FZV). Checks can also be carried out in moving traffic, for example, by inspecting Part I of the Registration Certificate. Moreover, other traffic bans and restrictions in force, such as speed limits, are only randomly checked by the competent authorities.

64 Furthermore, the supreme federal state authorities would be responsible for enforcing traffic bans effective throughout the respective federal state, including the creation of a uniform framework for the design of exceptions from imposed traffic bans, so that any (other) enforcement deficits could be prevented.