On a UK motorway, you may only pass traffic on your left when traffic is slow and moving in queues in all lanes, and you are simply keeping up with the traffic in your lane rather than changing lanes to overtake.

Core rule

  • The normal rule is overtake only on the right on motorways.
  • You must not move to a lane on your left specifically to get past a vehicle (no deliberate “undertaking”).

When passing on the left is allowed

  • In congested conditions, if all lanes are moving slowly at similar speeds, you may stay in your lane even if this means you pass vehicles in the lane to your right.
  • You must not weave in and out of lanes; you just keep a steady course in your lane as it happens to move slightly faster.

Why it matters

  • Drivers mainly expect faster traffic to come from the right, so sudden undertaking can be dangerous and might be treated as careless driving if you do it outside the limited exceptions.
  • The safest and legal approach is to use the right-hand lanes only for overtaking, then return to the left when it is safe, and only “pass” on the left in slow, queued traffic as described above.

Information gathered from public forums or data available on the internet and portrayed here.