Is Undertaking Illegal in the UK? Motorway Rules, Fines and Highway Code Explained

Many drivers have found themselves stuck behind a slow-moving vehicle in the middle lane of a motorway and wondered whether it is legal to pass on the left.

This is known as undertaking.

Undertaking is not specifically illegal in the UK, but it can still lead to penalties if the manoeuvre is considered careless or dangerous driving. The rules depend on road conditions, traffic flow and how the manoeuvre is carried out.

Understanding when undertaking is allowed can help drivers stay safe and avoid points, fines or disputes after an accident.

What Is Undertaking in Driving?

Undertaking is when a driver passes another vehicle on the left-hand side instead of overtaking on the right.

In the UK, overtaking is normally expected to happen in the right-hand lane. Passing a vehicle on the left is generally discouraged because it can catch other drivers off guard and increase the risk of collisions.

Drivers often refer to this as:

• Undertaking on a motorway

• Undertaking in traffic

• Overtaking on the left

• Passing on the inside lane

The term is most commonly used on:

• Motorways

• Dual carriageways

• Multi-lane A roads

Is Undertaking Illegal in the UK?

No, undertaking is not automatically illegal in the UK.

However, the Highway Code advises drivers not to overtake on the left or move into a left-hand lane specifically to pass another vehicle.

If police believe the manoeuvre was aggressive, unsafe or careless, drivers can still be prosecuted for:

Possible Offence	Potential Penalty
Careless or inconsiderate driving	3 to 9 penalty points, a fine up to £5,000, a discretionary driving ban, or a driver training course
Dangerous driving	Unlimited fines, mandatory disqualification (minimum 12 months with an extended re-test), and court prosecution or imprisonment
Driving without due care and attention	Fixed Penalty Notice (£100 fine and 3 points) or court action

This means undertaking can become illegal depending on how and where it happens.

What Does the Highway Code Say About Undertaking?

The main rule covering undertaking is Highway Code Rule 268.

The Highway Code states:

Do not overtake on the left or move to a lane on your left to overtake.

However, the rule also explains that in congested conditions, where adjacent lanes of traffic are moving at similar speeds, traffic in left-hand lanes may sometimes be moving faster than traffic to the right. In these conditions, you may keep up with the traffic in your lane even if this means passing traffic in the lane to your right.

This is one of the main exceptions that makes undertaking legal in certain situations.

When Can You Undertake on a Motorway?

There are situations where undertaking on a motorway may be considered acceptable.

Slow-Moving or Congested Traffic

If traffic is heavily congested and the left lane is naturally moving faster than the lane on the right, drivers can continue progressing in their lane.

For example:

• Lane one is moving at 25mph

• Lane two is moving at 15mph

• You stay in lane one and gradually pass traffic on the right

This is normally considered acceptable because changing lanes repeatedly would create more risk.

Average Speed Check Areas

Motorways with average speed cameras often display “stay in lane” signs.

In these situations, traffic flows are more controlled and drivers may legally continue in their lane if it moves faster than traffic on the right.

Examples commonly include:

• Smart motorways

• Roadworks

• Temporary motorway restrictions

When a Vehicle Is Turning Right

If a vehicle is positioned to turn right or perform a U-turn, it may be acceptable to pass on the left if it is safe to do so.

Drivers should still proceed carefully and avoid squeezing through small gaps.

When Is Undertaking Dangerous?

Undertaking becomes risky when drivers move unpredictably between lanes or attempt aggressive passes.

Situations that increase danger include:

• High-speed lane switching

• Passing vehicles in blind spots

• Undertaking while weaving through traffic

• Passing close to motorway exits or junctions

• Undertaking drivers who may move left unexpectedly

Many motorway accidents happen because drivers do not expect vehicles to pass on the left.

This is especially important on busy UK motorways where lane discipline is poor.

Can You Undertake a Middle Lane Hogger?

Middle lane hogging is a common cause of frustration for motorway drivers.

Although many drivers feel tempted to undertake slow middle lane traffic, the safest option is usually to overtake correctly on the right whenever possible.

If traffic is flowing normally and you deliberately move into the left lane just to pass a middle lane hogger, police could still view it as careless driving.

However, if you are already travelling in the left lane during congestion and traffic naturally moves faster than the middle lane, continuing in your lane is generally acceptable.

Undertaking vs Overtaking: What Is the Difference?

Undertaking	Overtaking
Passing on the left	Passing on the right
Usually discouraged	Standard motorway manoeuvre
Can lead to penalties if unsafe	Legal when completed safely
Common in congestion	Common in flowing traffic

Most UK driving guidance recommends overtaking on the right wherever possible.

Can You Undertake on a Dual Carriageway?

Yes, similar rules apply on dual carriageways.

Drivers may continue in their lane if traffic conditions mean the left lane is moving faster.

However, deliberately moving left purely to pass another vehicle at speed could still result in a careless driving offence.

Drivers should pay particular attention near:

• Slip roads

• Roundabouts

• Junctions

• Lane merges

Is Undertaking Illegal on Smart Motorways?

Smart motorways follow the same general undertaking rules as other UK motorways.

Because traffic speeds are often controlled through variable speed limits, it is more common for lanes to move at different speeds.

In heavy traffic or controlled motorway sections, staying in your lane while passing slower traffic on the right is normally permitted.

Drivers should still:

• Follow overhead lane signs

• Keep safe distances

• Avoid unnecessary lane changes

• Watch for lane closures

What Are the Penalties for Undertaking?

Drivers caught undertaking dangerously may face penalties under careless or dangerous driving laws.

Careless Driving Penalties

Typical penalties can include:

Penalty Type	Possible Outcome
Fixed penalty notice	£100 fine and 3 penalty points
Penalty points	3 to 9 points (imposed by a court for more severe incidents)
Driver awareness course	Offered in some cases for minor first-time infractions
Insurance impact	Higher premiums

Dangerous Driving Penalties

In serious cases, penalties can become much more severe. Under the current sentencing guidelines, the courts have a strictly recalibrated baseline for immediate driving bans for flagrant safety risks.

Potential outcomes include:

• Unlimited fines

• Mandatory driving bans (typically 12 to 36 months under current judicial benchmarks)

• Mandatory extended re-tests before reclaiming a licence

• Court prosecution and potential imprisonment

• Vehicle seizure in extreme cases

• Substantial increases in insurance costs

Police and courts will normally assess:

• Traffic conditions

• Speed

• Driver behaviour

• Risk created for other road users

Why Is Undertaking Discouraged?

Undertaking is discouraged because many drivers instinctively check mirrors more carefully before moving right than moving left.

This creates extra risk when another vehicle suddenly appears on the inside.

Undertaking also increases the chance of:

• Side-swipe collisions

• Panic braking

• Sudden lane changes

• Motorway confusion

Good lane discipline helps reduce these risks.

Tips for Safe Motorway Driving

Whether you drive a petrol, hybrid or electric vehicle, motorway awareness is essential for safer long-distance driving.

Helpful motorway driving habits include:

• Keep left unless overtaking

• Leave safe braking distances

• Avoid aggressive lane changes

• Check mirrors and blind spots regularly

• Stay patient in congestion

• Follow smart motorway signs carefully

Drivers choosing newer vehicles with motorway driving assistance systems may also benefit from:

• Blind spot monitoring

• Adaptive cruise control

• Lane keeping assist

• 360-degree parking cameras

If you regularly drive long motorway journeys, comparing modern car leasing deals can make it easier to access vehicles with the latest safety technology.

You can also read our related guides:

• Headlight Flashing Meaning

• How Long Do Points Stay on Your Driving Licence

• Is It Illegal to Drive Barefoot

• Do Minimum Speed Limits Exist in the UK

• Speed Camera Signs UK

Does Undertaking Affect Insurance Claims?

It can.

If an accident happens while undertaking, insurers may examine whether the manoeuvre contributed to the collision.

Even if undertaking was technically permitted in slow traffic, insurers could still investigate:

• Driver positioning

• Speed differences

• Lane changes

• Dashcam footage

• Highway Code compliance

This can affect liability decisions after a motorway accident.

Understanding the Rules Around Undertaking

Undertaking remains one of the most misunderstood motorway driving topics in the UK.

While it is not automatically illegal, the legality depends heavily on traffic conditions, driver behaviour and overall safety.

The safest approach is usually to maintain proper lane discipline, avoid aggressive passing and follow Highway Code guidance at all times.

For drivers covering regular motorway miles, newer vehicles with modern safety technology can also make long-distance driving more comfortable and safer.