What does Highway Code Rule 123 say?

Rule 123 (Control of the vehicle (rules 117 to 126)) The driver and the environment. It is a legal requirement, it uses “MUST” or “MUST NOT”, so breaking it is a criminal offence that can mean a fine, penalty points, or disqualification.

Is Rule 123 a legal requirement?

Yes. Rule 123 uses “MUST” or “MUST NOT”, which marks it as backed by law. Disobeying it is a criminal offence and can lead to a fine, penalty points on your licence, or disqualification.

How is Rule 123 tested on the driving test?

These foundational rules, mirrors, signals, speed, stopping distances and lighting, are assessed across the whole drive. Strong habits here keep the rest of your test calm and fault-free.

What are the common mistakes related to Rule 123?

The faults examiners most often record around this part of the Code are: skipping or rushing mirror checks before a manoeuvre; carrying an unsuitable speed for the road and conditions; reacting late because hazards were spotted too close. Rehearsing the safe habit during lessons is the most reliable way to avoid them.

Highway Code · Rule 123

Highway Code Rule 123

General rules, techniques and advice for all drivers and riders (103 to 158). A legal requirement (MUST / MUST NOT).

General rules, techniques and advice for all drivers and riders
Legal requirement
OGL v3.0

What the rule says

Law · MUST

Rule 123 (Control of the vehicle (rules 117 to 126)) The driver and the environment. You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road. Generally, if the vehicle is stationary and is likely to remain so for more than a couple of minutes, you should apply the parking brake and switch off the engine to reduce emissions and noise pollution. However it is permissible to leave the engine running if the vehicle is stationary in traffic or for diagnosing faults. Law CUR regs 98 & 107

Rule text reproduced verbatim from the official Highway Code (Crown copyright) under the Open Government Licence v3.0, see the attribution at the foot of this page.

In plain English

Stripped of the formal wording, Rule 123 comes down to one idea: rule 123 (Control of the vehicle (rules 117 to 126)) The driver and the environment. Because it is written with “MUST” or “MUST NOT”, it carries the force of law, ignore it and you are committing an offence, not simply driving badly.

It belongs to the general rules, techniques and advice for all drivers and riders part of the Code, the habits a confident, considerate driver builds until they are automatic. The aim is not to memorise the sentence word for word, but to understand the hazard it protects you from, so you apply it without having to think when it counts.

If you are learning, treat this rule as one piece of a connected set rather than an isolated fact. The related rules below sit in the same section and reinforce each other, reading them together is how the general rules, techniques and advice for all drivers and riders part of the Code starts to feel like common sense rather than a list to revise.

Because this is a legal rule, the consequences of ignoring it reach beyond the test: a “MUST” or “MUST NOT” breach can mean a fixed penalty, points on your licence, or in serious cases prosecution. Either way, the safe move is to build the habit early, while a driving instructor can correct it, rather than relearning it under test pressure. That is exactly what the practice routes and coaching in the DriveRoutes app are designed to help with, turning the rules below into the way you naturally drive.

Why rule 123 matters on the road

These are the foundations every other skill builds on. Solid mirror work, sensible speed and good lighting habits quietly prevent the situations the rest of the Code has to deal with.

Common faults examiners record

In the general rules, techniques and advice for all drivers and riders part of the Code, the faults most often written on the marking sheet tend to be the same handful. Knowing them in advance is the quickest way to drive them out of your own habits:

Skipping or rushing mirror checks before a manoeuvre.
Carrying an unsuitable speed for the road and conditions.
Reacting late because hazards were spotted too close.

On the day

On the day, applying Rule 123 is about doing the safe, deliberate thing slightly earlier than feels necessary: read the situation in good time, observe fully, and act smoothly. The examiner is looking for planned driving, not perfection, and good habits formed in lessons carry you through.

Quick checklist

Read the situation early and plan your response.
Observe fully before you commit to anything.
Keep your speed suitable for the road and conditions.

More from General rules, techniques and advice for all drivers and riders

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Rule 123, your questions

: Rule 123 (Control of the vehicle (rules 117 to 126)) The driver and the environment. It is a legal requirement, it uses “MUST” or “MUST NOT”, so breaking it is a criminal offence that can mean a fine, penalty points, or disqualification.

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