Last reviewed 21 July 2023

In light of the VW emissions scandal in 2015, the UK’s Department for Transport carried out a substantive vehicle emissions testing programme and found large discrepancies between real-world and in-lab testing across Euro 5 and Euro 6 diesel passenger vehicles. This article by Lisa Bushby discusses how industry can move beyond these and bring about improvements in local air quality.

Achieving national and local air quality objectives requires a continuing effort to reduce vehicle emissions, with this being particularly important within urban, densely populated areas. Regulation and local restrictions have largely driven this to date. New vehicles must be designed to comply with a comprehensive set of legislative requirements. These are manufactured to ensure vehicles are safe and their environmental impacts are controlled. Prior to placing a new vehicle design on the market manufacturers must secure type approval to demonstrate it conforms to the relevant safety and environmental standards.

Euro 6 to Euro 7

The Euro 6 standards are one of the key measures designed to reduce in-use emissions of air pollutants such as particulate pollutants (PM) as well as ozone precursors such as nitrogen oxides (NOx) and hydrocarbons in new vehicles. Euro 7 standards are now proposed. If approved, these targets will apply from 1 July 2025 for cars and vans and from 1 July 2027 for buses, coaches and heavy goods vehicles.

One of the biggest differences with Euro 6 buses compared to any previous legislation, is that engines are required to comply during real world usage, including at different temperatures and under various load conditions.

To ensure their compliance, Euro 6 bus and coach operators must adhere to the correct maintenance schedules, such as oil changes, cleaning of the diesel particulate filter and AdBlue quality.

Low emission zones

At a local level, city administrators see considerable benefits in establishing low emission zones (LEZs). The number of these has increased dramatically in recent years, and there are now hundreds of LEZs across Europe and the UK encouraging heavy diesel vehicles, including passenger vehicles such as buses and coaches to become cleaner by imposing local zoned bans on those vehicles that do not meet specified emissions targets.

For buses and coaches, many city centre low emission zones require vehicles to meet Euro 6 levels, for example in London, those over 5t gross vehicle weight must meet Euro 6 for nitrogen oxides and particulate matter and Glasgow has imposed a similar zone and limit from 1 June 2023. Other city centre restrictions apply in other UK cities.

Older coaches can be tested for a low emission certificate for Euro 4, 5 or 6 engine limits. Operators should contact an authorised testing facility or Driver and Vehicle Standards Agency test station to book the test. In many cases, engines can be retrofitted with conversion kits to allow them to become, say Euro 6 compliant.

Alternative considerations for reducing real-world emissions

A substantive vehicle emissions testing programme was conducted by the UK Department of Transport in 2016 in response to emissions tampering exposed in the Volkswagen emissions scandal. The programme identified large emissions discrepancies between real-world and in-lab testing across a range of Euro 5 and Euro 6 diesel passenger vehicles. Policy makers and industry see this as a catalyst to improve vehicle emissions testing beyond standards and to aid in the development of better climate change mitigation strategies and to bring tangible air quality improvements to the environment.

In the specific case of bus and coach operators, lower fuel consumption means lower emissions and can also represent lower costs. Optimised use of the companies’ available resources, ranging from vehicles to drivers, represents one of the main concerns when it comes to reducing fuel consumption. Improving the driving style of bus drivers and implementing rigorous maintenance practices are the main ways to achieve not only fuel cost reductions, but also to mitigate environmental impacts and improve road safety.

This is backed up by a large number of studies that have confirmed the fuel consumption of buses depends on many factors, such as road type, road slope, air conditioning, speed, acceleration, load mass or driving style. Therefore, identifying the factors that affect significantly fuel consumption in bus operation is a meaningful management tool.

Vehicle type

Vehicle type is considered to be the most influential variable, although the use of smaller and lighter vehicles during periods whenever passenger load is reduced has inherent limitations for operators, who will be reliant on a fixed fleet. Though appealing, having available a range of sized vehicles significantly increases operators’ fixed and investment costs and may not be an option for some operators.

In terms of reducing emissions through vehicle maintenance measures, operators should:

  • maintain engine oil levels and clean air filters to keep vehicles running efficiently (the on-board diagnostics unit required by the Euro 6 regulations alerts operators to a potential engine fault. Acting on a fault alert to have the engine checked will likely result in improvements to fuel economy and emissions levels, while replacing a faulty sensor could result in a fuel economy improvement of as much as 40%)

  • use the fuel type recommended by the manufacturer

  • consult the vehicle owner’s manual for maintenance guidance

  • keep tyres inflated to at least the pressure recommended by the manufacturer as this can reduce fuel consumption by up to 4%. Over-inflation of 10‒15% may save fuel, but it will increase braking distances, while under-inflated tyres wear more rapidly and increase fuel consumption owing to greater rolling resistance.

Driving style

During operation, steady, higher speeds typically mean lower fuel consumption and the distance between stops strongly influences commercial speed.

Studies have shown that the number of training sessions per driver is highly correlated with fuel consumption and confirms the success and impact of training practices on the energy efficiency of bus and coach operators.

Drivers should be trained to:

  • accelerate smoothly from a standstill and brake softly to save fuel

  • aim for a constant speed

  • maintain a steady speed by using the highest gear possible

  • avoid unnecessary acceleration and braking

  • anticipate traffic flow by looking ahead as far as possible

  • maintain a safe distance between vehicles and anticipate traffic conditions to allow for more time to brake and accelerate gradually

  • use the retarder and engine braking to reduce speed when approaching traffic lights

  • apply the brakes to bring the vehicle to a standstill when close to the traffic lights

  • drive at the maximum legal speed to save fuel and improve the flow of traffic

  • not to overtake other vehicles at unnecessarily high speeds

  • use cruise control where appropriate on motorways to assist with smooth driving

  • reduce speed in strong headwinds or heavy rain

  • avoid driving with the windows open

  • use heating and air conditioning selectively to reduce the engine load

  • avoid idling the engine.

Route planning

Planning the route a driver takes is another critical consideration when aiming to reduce emissions and lower operating costs. Factors of fuel-efficient routing include total drive time, road speeds, traffic flow, road roughness, weather and short stops.

There are a variety of route management software tools available, eg elogii or Trapeze Group.

Other general tips include:

  • minimising overall drive time to destinations

  • avoiding congested areas

  • finding an alternative motorway solution rather than going through city centres

  • avoiding narrow lanes, sharp turns or small roundabouts

  • pre-planning pick up and drop off points

  • parking the coach in a shaded area wherever possible.


To truly ensure greener, cleaner cities, going forward, policy makers should be sceptical of enforcing any strategy of regulating lower levels of NOx and PM from vehicles and relying on in-lab testing measures alone to prove this. Bus and coach operators can play a crucial role in helping city administrators improve local air quality without relying on local emissions zones or Euro standards by changing the behaviour of drivers, carefully planning trip routes and ensuring their vehicle fleet is subject to effective maintenance.