Real-world driving emissions tests – what are they? And why is it important?

Real-world driving emissions tests have been given widespread publicity ever since the diesel emissions scandal of the 2010s. But how exactly do they differ from the old lab-based tests?

Here we offer a simple explanation of the term 'real-world driving emissions' and carefully look at how the implementation of these tests has affected the average road user.

What are real-world driving emissions tests?

Real-world driving emissions tests are tests that measure the pollutants a car produces while being driven on real roads, as opposed to being tested in a lab that simulates real-life driving conditions.

When a car manufacturer makes a car it has to meet a Euro emissions standard, set by the EU. The UK continue to mirrors these standards despite leaving the European Union. 

These standards are in place to ensure the cars they are producing aren’t polluting too many harmful substances into the environment.

To guarantee the cars meet the standard, each engine and variant has to be tested thoroughly. Real-world driving emissions tests are the latest, and in theory most reliable, of these tests.

Why do we have Real-world driving emissions tests?

Previously, for a car to meet its Euro standard and be made available for mass consumption, it had to pass a test known as the New European Driving Cycle (NEDC) type-approval test, which dates back to the 1980s and was last updated in 1997.

However, motorists were generally not able to achieve the fuel economy figures claimed by manufacturers in their advertising and literature because the tests were carried out in laboratory settings, not on real roads.

Furthermore, consumer confidence in fuel economy and efficiency data hit an all-time low following the ‘dieselgate’ diesel emissions scandal, when ‘defeat device’ software was used to detect when a vehicle was running on a rolling-road within a laboratory.

This led to the emergence of tough new tests which were implemented in September 2017.

From this point the NEDC regime was replaced with the new Worldwide Harmonised Light Vehicle Test Procedure (WLTP) and the Real Driving Emissions test (RDE1) (more on that in the next section).

The WLTP is still conducted under laboratory conditions. However, the new test is faster, longer and more dynamic, with the introduction of new variables to deliver results that are more reflective of real-world conditions.

The Real Driving Emissions (RDE) test complements the WLTP, because it is conducted on the road, using a Portable Emissions Measurement System (PEMS). This is an acknowledgment that no laboratory test can ever replicate what happens on the road.

Europe was the first region in the world to introduce on-road testing, which features a car being driven on public roads for between 90 minutes and two hours over a route of at least 29.92 miles.

Combined, the WLTP and RDE tests should narrow or eliminate the gap between theory and reality.

The new tests measure everything from fuel consumption and carbon dioxide (CO2), to nitrogen oxides (NOx), particulates and carbon monoxide (CO). The ultimate aim is to improve air quality and tackle climate change.

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How do real-world emissions tests work?

Crucially, the RDE test includes urban and rural roads, along with motorway driving, of which five minutes must be at speeds of 62.13mph. Throw into the mix a number of different conditions – such as uphill and downhill driving, and additional vehicle payload – and you have the makings of a far more realistic set of results.

The PEMS analyses tailpipe emissions of NOx and particulates to ensure all cars meet strict Euro 6 emissions standards on the road as well as in the lab. Better for the environment and better for the motorist.

The PEMS used for testing are sophisticated pieces of equipment, but there is no 'standard' equipment, which means different suppliers might deliver different RDE test results.

Different temperatures and altitude also have an impact on the results.

With this in mind, the tests include a 'conformity factor' to allow for a margin of error. This allowed emissions levels to be exceeded by 2.1 times for new-to-market cars from 1 September 2017, with this figure reduced to 1.5 times from September 2019.

The 'conformity factor' is essentially a 'not-to-exceed' limit, but in reality, the RDE results were expected to fall below the current NEDC levels.