Last reviewed 25 September 2018

In this feature, Richard Smith summarises Government initiatives to reduce roadside nitrogen dioxide (NO2) levels and outlines what choices operators have when operating in low emission and clean air zones (CAZs).


In July 2017, the Government published its plan to reduce roadside NO2 levels as part of which the authorities in Birmingham, Leeds, Nottingham, Derby and Southampton were required to introduce CAZs by 2020. A large number of other urban local authorities had to produce local action plans, most of them on the basis of a single stretch of road that exceeds the mandated limits.

Following a case brought by an environmental pressure group later in 2017, the Supreme Court ordered that 33 of the 45 authorities that the Government had excused from conducting feasibility studies should also be included. In the remaining 12 authorities, compliance is expected by the end of 2018 as a result of current actions and in any event, even the other 33 are expected to be compliant by 2012.

What’s the problem?

It is well known that some products of combustion are harmful and can cause breathing difficulties, heart problems, cancers and death. Not all of these harmful products come from road vehicles (shipping and aviation can be major sources near such terminals and even wood-burning stoves are now under attack) but in the great majority of cases, road vehicles are the cause and the reduction of such emissions is good sense anyway.

The problem is predominantly an urban one since there are large concentrations of vehicles in congested conditions and the presence of tall buildings lining the roads contributes a “canyon effect” preventing the pollution from dispersing more widely. By contrast, even heavily used stretches of open road do not suffer the same concentrations of pollutants because they can disperse readily.

Speed also plays a role and slow moving and stationary vehicles are likely to produce greater amounts of pollutants because the engine is not operating in its optimum state, which will usually be at a steady speed near the middle of its revolutions per minute range.

In this case, it is NO2 that is the main focus, although particulate matter (PM) is also a problem. Both of these are particularly associated with combustion in the compression-ignition engine (diesel engine) by virtue of the nature of the combustion process.

For more details on exhaust emissions see the topic on Control of Emissions from Vehicles.

The solution

The Government’s plan adopts a multi-faceted approach and is not confined to road transport but also includes reduction of emissions from other forms of transport, industry and non-mobile road machinery, and commercial and domestic buildings. Within the road transport sector, much of the plan focuses on achieving cleaner buses and other passenger-carrying vehicles, although incentives for the adoption of alternative fuels will benefit all types of vehicle. Nevertheless, the adoption of such fuels has great implications for the refuelling infrastructure that are more readily solved for passenger vehicle operators with a restricted radius of operation around a central depot where refuelling can be concentrated, though at considerable cost.

A further strand involves investment in the national and local road network to tackle congestion and reduce stop-start traffic, something that will be welcomed by all road users.

Low emission zones and CAZs

Low emission zones (LEZs) and CAZs appear to amount to much the same thing with both attempting to improve air quality through a range of measures. The London LEZ has been in effect since 2008 and other cities in the UK and elsewhere also have LEZs. One difference is that while the current LEZs have all been introduced by individual local authorities on their own account and to their own specifications, CAZs are Government mandated and to a specified framework.

This framework provides for four different classes of CAZ according to the type of vehicles that are covered. A class A zone will control only buses, taxis and private hire vehicles while a class D zone will involve all vehicles including private cars. In a CAZ, the emission standard is also laid down: Euro VI/6 for diesel engines and Euro 4 for petrol ones, while LEZs operate to varying standards. The full details of the different classes of CAZ are in the Factsheet: UK Clean Air Zones.

CAZs, like LEZs, may be charging or non-charging. In a charging zone, compliant vehicles may circulate freely but non-compliant vehicles may only do so on payment of a fee, with fixed-penalty charges being levied if the fee is not paid. In a non-charging zone, there is no charge even for non-compliant vehicles, but a range of other incentives are used to encourage the use of lower emission alternatives.

While LEZs may have restricted times of operation, CAZs are intended to be 24/7. When the ultra-low emission zone (ULEZ) comes into effect in central London next April, it will effectively be a class D charging CAZ. From October 2020, when the same standards are extended to the whole of the current LEZ for buses and lorries, it will to all intents and purposes be a class B charging CAZ and from October 2021, the area within the North and South Circular Roads will upgrade to a class D zone with the remaining “doughnut” of the LEZ remaining a class B zone.

Full details of the London LEZ, ULEZ and the proposed changes are in the topic Environmental Restrictions on Vehicle Operation.

CAZs: the state of play

Of the five authorities directed to introduce CAZs, only three have so far announced clear plans. Birmingham proposes a class D charging zone within the Middle Ring Road and including the A38 Queensway, while Leeds and Southampton are both intending to introduce class B charging zones: city-wide in Southampton’s case, in the northern part of the city in Leeds. Derby and Nottingham are both proposing to use other means of achieving the necessary reductions.

This is a fluid situation and the Factsheet: UK Clean Air Zones will be updated from time to time with the latest developments.

Other measures

The London Borough of Hackney has introduced (since 3 September 2018) two Ultra-Low Emission Vehicle (ULEV) zones where access between the hours of 0700 to 1000 and 1600 to 1900 on weekdays will be restricted to vehicles emitting less than 75gm/km CO2 — effectively a ban on heavy vehicles as the FTA has noted, although the A1202 that separates the two zones is not included.

Oxford is planning to go even further in 2022 with a zero emissions zone and a complete ban on all diesel and petrol vehicles. It will at first affect only some (unspecified) vehicle types but will extend to all vehicles in 2035.

What can operators do?

Trunking operations are relatively unaffected at the time this feature is published. However, those operators within urban areas have a choice. They can either:

  • continue to operate older vehicles up to their planned replacement date and accept that they may have to pay a fee to use them in certain areas


  • accelerate their replacement with latest Euro VI standard vehicles, perhaps with hybrid power units and/or using alternative fuels. Considerations of corporate social responsibility may play a part in this decision but at its heart will be a straightforward economic decision on the relative costs of the options. Unfortunately, haulage operators do not seem to have the same range of incentives to do this that bus operators do.

A more drastic solution in some cases might involve an effective rearrangement of the whole operating methodology along lines that have been suggested for a long time. This involves breaking bulk at the edge of an urban area and using smaller vehicles for the final stage of the journey. This will enable the use of ULEVs to universal benefit.