When Is Undertaking Allowed?

Undertaking means overtaking a slower vehicle on its left, or kerb side, rather than pulling out to the right. It isn't automatically illegal on UK motorways and dual carriageways, but it can be dangerous and you can be penalised if it's judged to be careless or dangerous driving. So when is undertaking actually allowed, and when will it land you in trouble? This guide explains what the Highway Code says, the specific situations where undertaking is acceptable, and the penalties you could face if you get it wrong.

The Highway Code's general position is clear: you should not overtake on the left, or move into a lane on your left in order to overtake. As a default rule, overtaking happens on the right.

However, the Code recognises that real traffic doesn't always allow for that. In congested conditions, where lanes are moving at similar speeds, the traffic in a left-hand lane can sometimes flow faster than the lane to its right. In that situation the Code says you may keep pace with the traffic in your own lane, even if that means passing vehicles on your right. The key point is that you should stay in your lane rather than weaving in and out to get ahead, weaving between lanes is far more dangerous than simply keeping your position.

So undertaking in slow-moving, lane-disciplined traffic isn't the problem. Deliberately darting to the left to jump a queue is.

Beyond congested traffic, there are a few specific situations where passing on the left is generally considered acceptable and safe.

Average Speed Check Zones

On motorway stretches with average speed limits, often through roadworks, overhead gantries usually instruct drivers to stay in lane. If your lane is moving faster than a vehicle to your right, it's safer to hold your position and pass on the inside than to weave across lanes. Staying in lane is exactly what the signage is asking you to do.

When a Vehicle Ahead Is Turning Right or Making a U-Turn

If a vehicle in the right-hand lane is signalling to turn right or perform a U-turn, you're permitted to continue past it on the left. The important caution here is timing: don't undertake too early. Wait until the vehicle has clearly indicated and actually begun to turn before you pass it on the inside, so there's no doubt about its intentions.

One-Way Streets

On a one-way street (not a dual carriageway), traffic can legally pass on either side, so undertaking rules don't apply in the same way. You can move past slower vehicles on the left or the right, though you should still do so safely and predictably.

If you're ever unsure whether undertaking is acceptable, the safest approach is to assume it isn't, hold your lane, and drive in a calm, predictable way.

Yes. It can be tempting to undertake, especially when faced with a middle-lane hogger, but doing it recklessly carries real consequences.

Undertaking dangerously can earn you a Fixed Penalty Notice for careless driving, or driving without due care and attention. That typically means 3 points on your licence and a £100 fine, though some police forces offer a driver education course as an alternative.

In more serious cases it can be treated as dangerous driving, which carries much harsher penalties: up to 9 points or disqualification, a fine of up to £5,000, and a court summons. Given those stakes, undertaking is never worth the risk to shave a few seconds off a journey.

Undertaking isn't outright illegal in the UK, but the Highway Code discourages it and you can be penalised if it's judged careless or dangerous. It's generally acceptable to pass on the left in slow-moving traffic where your lane is naturally faster, in average speed check zones where you're told to stay in lane, when a vehicle ahead is turning right, and on one-way streets. Outside those situations, the safe rule is simple: hold your lane, overtake on the right, and never weave through traffic to get ahead.