Undertaking on the motorway is generally discouraged due to the heightened risk of accidents, but there are limited exceptions under UK driving rules. Sergeant Owen Messenger from Devon and Cornwall Police’s Roads Policing Unit has explained when passing on the left may be permissible.
What the police say about undertaking
In an educational video, Sergeant Messenger stated: “You can’t undertake on the motorway. The only time you can undertake is when your lane is genuinely moving more slowly than the other lanes. That’s generally when you’re going through roadworks, or it’s heavy traffic, or something like that.”
He added that undertaking is discouraged because “people aren’t expecting you” – and as drivers often move back to the left lane, their “default position on the motorway”, it can lead to serious crashes. He urged motorists: “Please don’t undertake on the motorway, it’s really really dangerous.”
The key exception: slow-moving traffic
The main exception is congested, slow-moving traffic, where lanes move in queues and one lane may creep ahead of another. In such cases, continuing in your lane can mean you pass traffic to your right without actively trying to undertake.
However, it remains important not to weave in and out of lanes to make progress, as that increases the risk of collisions.
What the Highway Code says
The Highway Code says you should normally overtake on the right and not pass on the left in typical free-flowing traffic. Rule 268 explains that if traffic is moving slowly in queues, drivers may pass slower vehicles on either side as the flow of traffic dictates – as long as they don’t weave between lanes.
The same rule reinforces that motorists must keep left unless overtaking. If you appear on someone’s left unexpectedly, they may have less time and space to respond if they drift or change lanes – and at motorway speeds, small errors can have serious consequences. Being patient and driving safely helps keep you and those around you out of harm's way.
