Real Driving Emissions

Real Driving Emissions, abbreviated RDE, is a European Union test that measures the pollutants a car emits while it is actually being driven on public roads, rather than relying solely on figures obtained in a laboratory. Its purpose is to close the long-standing and damaging gap between the clean emissions cars displayed during official lab tests and the much higher levels many produced in everyday use. By taking the measurement out onto the road, RDE aims to ensure that a car is genuinely clean where people breathe the air.

The defining feature of RDE is the use of a Portable Emissions Measurement System, or PEMS. This is a compact array of analysers and a flow meter fitted to the vehicle, usually mounted at the rear, which samples the exhaust continuously and records the mass of pollutants emitted while the car is driven over a defined route. That route must include a representative mix of urban, rural and motorway driving, covering a range of speeds, gradients, ambient temperatures and altitudes, so that the test reflects realistic conditions rather than a gentle, optimised cycle.

The focus of RDE is on the pollutants that most affect local air quality, chiefly oxides of nitrogen and particle number, the very emissions that proved most prone to manipulation. The test was introduced directly in response to the dieselgate scandal, in which it became clear that some vehicles recognised the fixed laboratory procedure and curbed their emissions only under those conditions. Because an on-road route is far harder to anticipate and game, RDE makes such defeat strategies much more difficult to deploy.

Rather than imposing a completely separate limit, RDE works by requiring that real-world emissions stay within a defined margin of the laboratory limit. This margin, known as the conformity factor, was set above one initially to allow for the greater variability and measurement uncertainty of on-road testing, and was intended to be tightened progressively towards parity. In effect, a car must now perform acceptably both in the lab and out on the road, not merely on the test bench.

RDE does not stand alone but complements the laboratory-based Worldwide Harmonised Light Vehicles Test Procedure, or WLTP, which handles fuel economy and carbon-dioxide measurement under controlled conditions. Together they replaced the discredited older approach built around the New European Driving Cycle, pairing a more realistic lab test with genuine on-road verification.

The practical significance for owners and the public is considerable. RDE has pushed manufacturers to ensure that exhaust after-treatment systems remain effective across the full range of real-world conditions, helping to deliver cleaner air in towns and cities and restoring some confidence that the emissions performance promised on paper is reflected in everyday driving.