The Dangers of Undertaking: Why Passing on the Left is Illegal and Unsafe
On UK roads, the rule is clear: you overtake on the right. Passing another vehicle on its left-hand side, known as undertaking, is one of the most misunderstood and dangerous manoeuvres a driver can attempt.
While there are very limited exceptions, undertaking is generally prohibited because it undermines the two cornerstones of road safety: predictability and visibility. Let’s explore why it’s illegal, unsafe, and why sticking to overtaking on the right protects everyone on the road.
1. The Peril of the Blind Spot
Every vehicle has blind spots – areas not visible in mirrors. The largest blind spot in UK cars is over the left shoulder (the passenger side).
- Law shaped by danger – Nearside (left-hand) mirrors were not legally required on new cars until 1st June 1978. Before then, most drivers had no visibility of the left-hand side. The road system was built on the rule: all overtaking happens on the right.
- Still risky today – Even with modern mirrors and technology, blind spots remain. Drivers are trained to expect overtakers on the right. An undertaking car can be easily missed, leading to side-swipes or lane-change collisions.
- Classic vehicles – Many pre-1978 cars are still legally on the road without a nearside mirror, making them especially vulnerable to undertaking.
2. Predictability and Road Discipline
Safe driving relies on predictable behaviour. UK drivers operate with the assumption that:
- Faster traffic will always be in the lane to the right.
- The left lane is for slower traffic, exits, or moving left safely.
When a driver undertakes:
- It shatters this convention, creating confusion.
- Other road users may wrongly believe the left-hand lane is clear and safe to move into.
- A simple misjudgement of speed or distance can result in a serious collision.
3. Increased Risk of Collisions
Undertaking combines blind spots with unpredictability — a recipe for accidents. Common scenarios include:
- Motorway exits – A car in the middle lane begins moving left to leave, colliding with a faster car undertaking in the “clear” left lane.
- Avoiding hazards – A driver swerves left to dodge debris or a pothole, not expecting a vehicle to be racing past on the inside.
- Aggressive weaving – Undertaking often forms part of aggressive lane-hopping, leaving little reaction time and heightening the chance of a multi-car pile-up.
4. The Legal Stance: The Highway Code
The prohibition against undertaking is explicit in the UK Highway Code, Rule 268:
“Do not overtake on the left or move to a lane on your left to overtake.”
Breaking this rule is classed as careless driving (driving without due care and attention). Consequences may include:
- 3 to 9 penalty points on your licence
- A significant fine
- In extreme cases, charges of dangerous driving, leading to mandatory disqualification or even imprisonment.
5. When Is Passing on the Left Permitted?
There are a few exceptions where passing on the left is legal and safe:
- Congested traffic – If lanes are moving at different speeds, it’s acceptable for your left-hand lane to move faster (you are not weaving; the traffic is simply flowing).
- When a vehicle ahead is turning right – On a single carriageway, you may carefully pass a right-turning vehicle on its left if there’s safe space to do so.
In all cases, low speeds and caution are key. These situations are the exception, not the rule.
❓ FAQs About Undertaking in the UK
1. Is undertaking illegal in the UK?
Yes, except in slow-moving traffic or when passing a right-turning vehicle. Otherwise, overtaking on the left breaks Highway Code Rule 268.
2. What is the penalty for undertaking?
It can result in 3–9 penalty points, fines, or even prosecution for dangerous driving in serious cases.
3. Why is undertaking dangerous?
It exploits blind spots, goes against driver expectations, and makes collisions far more likely.
4. Can you undertake on motorways?
Only in congestion, when traffic in your lane is moving faster than the lane on your right. Lane weaving at speed is never allowed.
Conclusion: Safety Over Shortcuts
The rule against undertaking isn’t a technicality — it’s a lifesaving principle rooted in the physical limits of vehicles and over a century of driving practice.
By resisting the temptation to undertake, you protect yourself and others. The safest choice is always the same: be patient, follow the rules, and overtake properly on the right.
Driving Offences Hampshire Police
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