When a new car is certified for sale in the UK and Europe, it must undergo strict emissions testing. Current standards state that, under standard laboratory testing, a new diesel or petrol car must emit no more than 0.080g/km of nitrogen oxide (NOx).

RDE stands for Real Driving Emissions. This is the test that can be used by vehicle manufacturers to test the emissions a car produces under real driving conditions (e.g. not in a laboratory).

as there is an expected difference between emissions from a car in the laboratory and under real world testing, there are different stages of RDE.

The first stage allows a car to emit 2.1 times the amount of NOx during RDE testing.

Under RDE, a car is driven on public roads and over a specified range of different conditions. Specific emissions measuring equipment attached to the vehicle collects data to verify that legislative caps for pollutants such as NOx are not exceeded. RDE will not replace the new WLTP laboratory test, but complement it. RDE will ensure that cars deliver low emissions on-the-road.

Conditions include:

• Low and high altitudes
• Year-round temperatures
• Additional vehicle payload
• Up- and down-hill driving
• Urban roads (low speed)
• Rural roads (medium speed)
• Motorways (high speed)

The second stage, RDE2, allows cars to emit 1.5 times the amount of NOx during RDE testing. This becomes mandatory for all cars on sale by January 2021.

• RDE1 standards: 168 mg/km NOx emission limit (80mg/km x NOx conformity factor of 2.1)
• RDE2 standards: 120 mg/km NOx emission limit (80mg/km x NOx conformity factor of 1.5)

On paper, RDE2 diesels will be the lowest-emitting diesel cars ever on sale.

However, in reality, the issue is more complex. An RDE2 diesel will be able to emit up to 0.120g/km NOx under the RDE test. Yet, data from Emissions Analytics shows that there are already non-RDE diesels on sale that emit less than this.

The new Budget policy may have some impact on air quality. However, with RDE2 not due to come into full force for more than four years (2021), the results won’t be felt for some time.

There is also some concern within the automotive industry about the negative impact this measure will have on new diesel sales.