You may use hazard warning lights only in specific situations where your vehicle (or something ahead) is a genuine hazard to others, not for everyday convenience.

Core rule (UK-style Highway Code approach)

In normal driving, you should only use hazard warning lights when:

  • Your vehicle is stationary and is a potential danger or obstruction to other road users (for example, broken down, accident, or stopped in an unusual/dangerous place).
  • You are on a motorway or unrestricted dual carriageway and need to warn traffic behind of a hazard ahead , and then only for a short time while you slow and until following drivers clearly see the danger.

Outside of these kinds of situations, hazard lights are generally considered misuse and can be dangerous or even illegal.

Quick Scoop: When may you use hazard warning lights?

1. When your vehicle is a hazard

You may use hazard warning lights when your vehicle itself has become a temporary hazard. Typical examples:

  • Breakdown at the side of the road or on the hard shoulder.
  • Stopped in an unexpected or dangerous position (narrow road, blind bend, crest of a hill, just after a collision).
  • Roadside emergency stop (medical emergency, serious mechanical fault) where you must remain stopped and other drivers may not expect a stationary vehicle.

In these cases, the lights say to others: “My vehicle is a hazard, slow down and pass carefully.”

Key points:

  • Switch them on when you realize your stopped vehicle is a danger.
  • Switch them off as soon as you are no longer a hazard (moved to a safe place, breakdown recovered, or traffic conditions normalize).

2. While you’re driving (the exam-style bit)

The classic theory-test wording is:

“When may you use hazard warning lights while you’re driving?”
Correct answer: On a motorway or unrestricted dual carriageway, to warn of a hazard ahead.

This means you may:

  • Briefly turn them on to warn drivers behind of sudden danger or heavy, unexpected braking ahead , such as a queue around a bend or after a blind crest.
  • Use them only long enough for drivers behind to register the warning, then turn them off once the risk of rear‑end collision has reduced.

You should not cruise with hazards on for long distances; that confuses others about whether you’re moving or stopped.

3. When you should NOT use hazard warning lights

Even though people often do this in real life, you should not normally use hazards for:

  • Double parking or stopping in no‑parking zones “just for a minute”.
  • Saying “thank you” to other drivers.
  • Driving in normal heavy rain or darkness when your normal lights are enough.
  • Ordinary slowing in traffic where brake lights already give a clear signal.

Using hazards like this can:

  • Mask your indicators, so others can’t see which way you are turning.
  • Make drivers think you are stationary when you’re actually still moving.
  • Lead to penalties if you’re effectively advertising an illegal stop or parking.

4. A quick mini‑story to remember it

Imagine you’re on a busy motorway at dusk.
Traffic is flowing normally when you suddenly see brake lights and a long queue appear over a rise. You brake firmly, check your mirrors, and tap your hazard button for a few flashes to warn the car behind that the road ahead has suddenly become dangerous.
Once you’re sure they’ve seen the queue and slowed, you turn the hazards off and just use your normal brake lights. Later, your car breaks down and you roll onto the hard shoulder. You stop, put hazards on and get yourself behind the barrier to stay safe. A recovery truck arrives, and when your car is no longer a danger to passing traffic, the hazards go off again.

That’s essentially when you may use hazard warning lights :

  • To warn of a real hazard ,
  • For as short a time as necessary ,
  • And not as a shortcut for parking, thanking, or routine driving.

5. Tiny country-law nuance (for completeness)

Different countries and even different US states vary slightly on when you can use hazards while moving (for example, some allow them in very poor visibility or severe weather).

But the safe, exam‑friendly rule that works almost everywhere:

  • Use hazard lights only when your vehicle is or is approaching a real hazard , especially when stopped or when there’s sudden danger ahead.

TL;DR:
You may use hazard warning lights when your vehicle is a temporary hazard or obstruction , usually when stationary, and briefly on fast roads to warn of a danger ahead — not for normal parking, thanking, or routine driving.

Information gathered from public forums or data available on the internet and portrayed here.