What Real Driving Emissions Testing Measures

Real driving emissions (RDE) testing measures how much pollution a car actually produces while driving on public roads, rather than in a laboratory under controlled conditions. Instead of the older lab-based test, a vehicle is fitted with portable equipment that records exhaust gases during normal driving — on highways, city streets, and in different weather and traffic conditions.

The equipment, called a portable emissions measurement system (PEMS), sits in the vehicle and continuously samples what comes out of the tailpipe. A technician drives the car over a route that includes acceleration, braking, idling, and steady cruising. The data collected shows real nitrogen oxide (NOx) and particulate matter (PM) levels — the pollutants that cause smog and respiratory harm.

This matters because laboratory tests, which still exist and are still used, can miss how a car behaves in actual traffic. A vehicle might pass the lab test but emit far more pollution when a driver accelerates hard, sits in congestion, or drives in cold weather. RDE testing catches that gap.

Key Takeaways

  • Real driving emissions testing measures actual tailpipe pollution on public roads using portable equipment, not in a controlled laboratory setting.
  • The test includes varied driving conditions — city streets, highways, acceleration, and idling — to reflect how people actually drive.
  • RDE results are compared to legal limits, and vehicles that exceed those limits during real driving may fail even if they passed the laboratory test.
  • Different countries have different RDE standards and timelines; the European Union began requiring RDE testing in 2017, while the United States has different rules.
  • RDE testing is one reason some vehicles have been recalled or had their emissions ratings downgraded after real-world data showed higher pollution than lab tests predicted.

How the Test Route and Conditions Are Chosen

The route for an RDE test is not random. It must include a mix of driving types: low-speed urban driving (typically 10 to 60 kilometers per hour), rural driving at moderate speeds, and highway driving at higher speeds. The test also includes a range of acceleration rates, from gentle to moderate, and must happen under normal weather conditions — not extreme heat or cold that would skew results.

The vehicle is driven by a trained technician, not the owner. The PEMS equipment records data continuously, and the route usually takes 90 minutes to two hours. The goal is to create a realistic picture of how the car performs across the kinds of driving most people actually do.

Weather, traffic, and road conditions all affect emissions. A car idling in traffic produces different pollution than one cruising at steady speed. Cold starts produce more emissions than a warm engine. RDE testing captures these variations, which is why the results often differ from laboratory numbers.

How RDE Results Compare to Laboratory Test Standards

In the European Union, where RDE testing has been mandatory since 2017, vehicles must meet emissions limits during real driving. The legal limit for NOx is set at a multiple of the laboratory limit — currently 1.5 times the lab standard for new vehicle types, and 1.43 times for individual vehicles. This margin exists because real-world driving is messier than lab conditions, but the vehicle still cannot emit unlimited pollution.

If a vehicle fails the RDE test — meaning it exceeds the legal limit during real driving — it may be recalled, have its emissions rating downgraded, or be prohibited from sale in that market. This is what happened to several diesel vehicles after RDE testing revealed they emitted far more NOx on the road than in the laboratory.

The United States has not yet adopted RDE testing as a mandatory requirement in the same way. American emissions standards still rely primarily on laboratory tests, though the EPA has proposed RDE testing for future model years. This means a vehicle sold in the U.S. may not have undergone real driving emissions testing, even if it was tested that way in Europe.

Why RDE Testing Revealed Emissions Cheating and Mismatches

RDE testing became prominent after the 2015 Volkswagen emissions scandal, when it was discovered that some diesel vehicles had software that detected when they were being tested in a laboratory and reduced emissions during that time — but not during normal driving. Real driving emissions testing exposed this because the vehicles could not cheat: they were being measured on actual roads with real traffic.

Even without intentional cheating, RDE testing has shown that many vehicles emit significantly more pollution during real driving than their laboratory test results suggested. This happens because laboratory conditions are highly controlled: steady speeds, no traffic, no cold starts, no aggressive acceleration. Real driving includes all of those things.

The gap between lab and real-world emissions has led to recalls, software updates, and in some cases, buyback programs where manufacturers repurchase vehicles that cannot meet RDE standards. It has also pushed manufacturers to redesign engines and emissions control systems to perform better under real-world conditions, not just in the test chamber.

The Equipment and Process During an RDE Test

The portable emissions measurement system (PEMS) is a box of equipment, usually placed in the vehicle's trunk or back seat, with a probe inserted into the tailpipe. The probe samples exhaust gases continuously and sends data to a computer that records NOx, particulate matter, carbon dioxide, and other pollutants in real time.

The vehicle is also fitted with a GPS unit to track location and speed, and sensors that record engine load, fuel consumption, and other operating parameters. All of this data is time-stamped and later analyzed to determine whether the vehicle met the legal emissions limits during each phase of the drive — urban, rural, and highway.

The test is repeated multiple times, often on different days or in different weather, to may support the results are consistent. A single test that happens to occur in ideal conditions might not reflect how the vehicle performs on a hot day or in heavy traffic. Multiple tests reduce the chance that one unusual condition skews the result.

Current RDE Standards by Region and Future Changes

The European Union has the most established RDE framework. Since 2017, all new vehicle types must pass RDE testing before they can be sold. The standard has tightened over time: the allowable margin between lab and real-world emissions has shrunk, and more vehicle categories are now covered.

The United Kingdom, which left the EU, has adopted similar RDE requirements. Japan and South Korea have also introduced or are developing RDE testing standards. China has proposed RDE testing but has not yet made it mandatory across all vehicle types.

In the United States, the EPA has proposed RDE testing requirements for light-duty vehicles starting with model year 2027, though the timeline and specific standards have not been finalized. Until then, American emissions standards rely on laboratory tests, which means vehicles sold in the U.S. may not have been tested under real driving conditions.

What RDE Testing Means for Vehicle Buyers and Owners

If you are buying a vehicle in a market where RDE testing is required — such as the European Union — you can be more confident that the vehicle's emissions performance in real driving is closer to its advertised rating. The vehicle has been tested on actual roads, not just in a laboratory.

If you are buying in the United States, the vehicle may not have undergone RDE testing. This does not mean it is unsafe or illegal, but it does mean the real-world emissions could differ from the laboratory test results. Diesel vehicles in particular have shown larger gaps between lab and real-world pollution in the U.S. market.

For owners of vehicles that have been recalled or downgraded due to RDE testing failures, the impact varies. Some manufacturers have issued software updates that improve emissions control during real driving. Others have offered buyback programs. The specific remedy depends on the vehicle, the manufacturer, and local regulations.

Frequently Asked Questions

Can I request that my car be tested with RDE equipment?

RDE testing is performed by manufacturers and regulatory agencies, not by individual car owners. If you suspect your vehicle is emitting more pollution than its rating suggests, you can report it to your local environmental agency or emissions authority. They may investigate or require the manufacturer to conduct testing.

Does a vehicle that passes RDE testing produce zero emissions?

No. RDE testing measures whether a vehicle stays within legal limits for pollution during real driving. Those limits allow some emissions — the goal is to reduce pollution to acceptable levels, not to eliminate it entirely. Only zero-emission vehicles like battery electric cars produce no tailpipe emissions.

Why do some cars still fail RDE testing if they passed the laboratory test?

Laboratory tests use a fixed, controlled driving pattern that does not reflect real traffic. Real driving includes acceleration, braking, idling, cold starts, and varying speeds that can cause emissions control systems to work differently than they do in the lab. RDE testing captures this variation.

If my vehicle was tested in Europe with RDE, does that explore to the U.S. market?

RDE testing standards differ by region. A vehicle that passes RDE testing in Europe meets European standards, but the United States has not yet adopted the same RDE requirements. The vehicle may still be legal to sell in the U.S., but it was not necessarily tested under U.S. RDE standards.

What happens if a manufacturer's vehicle fails RDE testing?

The manufacturer must take corrective action, which may include a software update to improve emissions control, a hardware redesign, a recall, or in some cases a buyback program. The specific remedy is determined by the regulatory agency and depends on the severity of the failure and the number of vehicles affected.