ADR is the European Agreement concerning the International Carriage of Dangerous Goods by Road. It sets the legal framework for how dangerous goods must be classified, packaged, marked, labelled, documented and carried across borders. In the UK it's implemented through the Carriage of Dangerous Goods Regulations, enforced by DVSA. ADR is updated every two years (odd years, effective 1 January), so always check you're revising the current version.
ADR splits every dangerous substance into one of 9 hazard classes:
Common mistake: candidates think classes are ranked by danger level 1-9. They're not — the number is just a hazard category, not a severity scale.
Every dangerous substance gets a 4-digit UN number (e.g. UN1203 = petrol, UN1830 = sulphuric acid) that identifies the exact substance regardless of language or country. UN numbers run from UN0004 up to around UN3550+. The UN number always sits inside an orange plate on the vehicle, and on packages it's shown alongside the proper shipping name. Don't confuse the UN number with the class number — they answer different questions: UN number = what is it, class = what kind of hazard.
Vehicles carrying dangerous goods in bulk or certain quantities display plain orange plates (no numbers) or numbered orange plates showing the Kemler code (hazard identification number) on top and UN number on the bottom. The Kemler code's first digit shows the primary hazard, a doubled digit means intensified hazard, and an X prefix means the substance reacts dangerously with water.
Small loads can qualify for the '1000 points' total quantity exemption system, and there's a separate limited quantities (LQ) and excepted quantities (EQ) regime for small packaged amounts — know that these exist and reduce (not remove) some requirements, they don't make goods 'not dangerous'.
Don't mix up class 2 gas sub-divisions (2.1/2.2/2.3) — they're tested individually, not as 'just gases'.
ADR sets strict rules for how dangerous goods are packed and how packages are marked and labelled so everyone in the chain - loaders, drivers, emergency services - can identify the hazard at a glance, even before paperwork is checked.
Dangerous goods can react with each other if mixed in a vehicle or store - fire, toxic gas, or violent decomposition can result. ADR sets out strict rules for which classes must be kept apart, and how far apart, both during carriage and in warehousing.
ADR Chapter 7.5 (carriage) and 7.5.2 in particular gives a segregation table showing which classes must not be loaded together. Key pairs to remember:
'Segregation' means physical separation - a solid partition, a minimum distance, or loading in separate compartments/vehicles - not just stacking apart by a few inches.
All packages must be secured against movement (sliding, tipping, falling) using blocking, bracing, lashing or fixed racking. An unsecured load is a common cause of package damage and leaks, and is a roadside enforcement offence in its own right - separate from any dangerous goods breach.
Some substances may only be loaded together up to a total quantity threshold (e.g. certain Class 1 explosives compatibility groups) - always check the specific compatibility group letters (A-S) before mixing explosives.
Anyone driving a vehicle carrying dangerous goods above the small-load thresholds must hold a valid ADR driver training certificate (the 'ADR licence'), covering the class of goods carried and the vehicle type (tank or non-tank). Certificates last 5 years and must be renewed by attending a refresher course and passing exams before expiry - if you let it lapse you must resit the full initial course, not just the refresher.
Mixing up LQ with 1.1.3.6 (they use different limits and rules); assuming a valid driving licence is enough (ADR certificate is separate); forgetting the certificate must match BOTH class and vehicle type; and leaving written instructions inaccessible.
If a dangerous goods incident happens - spillage, leak, fire, or an accident involving your load - your first duty is to stop the vehicle safely, secure the area and protect yourself and others. Never guess what to do: the written instructions (the ADR Transport Document plus the Instructions in Writing, sometimes called Tremcards) carried in the cab tell you the hazards, the protective equipment needed and the emergency actions for that specific substance. Read them before you act.
Every ADR vehicle must carry Instructions in Writing in a language the driver understands, plus usually the languages of the countries of origin, transit and destination. They cover: nature of the danger, action to take in an incident, and personal protective equipment. A common mistake is assuming the transport document alone is enough - it isn't; the Instructions in Writing are a separate, mandatory document.
ADR requires each vehicle crew member to carry: a warning vest (hi-vis), a suitable warning device (two self-standing warning triangles or equivalent), eye protection (goggles/glasses), and where the load requires it - gloves, a hand torch, an emergency escape mask (for certain toxic gas classes) and a shovel and drain seal for spill containment. Equipment needed depends on the hazard class carried - check the Instructions in Writing.
A 2 kg dry powder fire extinguisher must always be carried for small fires (e.g. tyres, brakes, engine). Vehicles over 7.5 tonnes need extra extinguisher capacity (typically a further 6 kg minimum), and combination units carrying trailers need extinguishers on both units. Never use water on a fire involving Class 3 flammable liquids or reactive substances - check the placard/label for the correct extinguishing medium; the wrong agent can make things worse.
Incidents involving loss of dangerous goods, injury, or significant damage must be reported to the emergency services immediately and, in the UK, may require a written report under ADR reporting requirements if certain thresholds are exceeded (e.g. death, injury, loss of product, or environmental damage). Always call the emergency number shown on the transport document/Instructions in Writing.