I couldn't find a better word to explain than dual join but in the UK some slip roads to motorways have two lanes where the right lane immediately joins the motorway as usual whereas the left lane carries on separetly and joins later. One example is M4 eastbound to M25 northbound connection on this link.

Is there anything special, any restrictions, etc. with the left lane in this kind of slip roads or can anyone drive through them normally?

2 Answers 2


Anyone may use either lane, unless signs tell you otherwise.

I can't recall anywhere with signs giving restrictions, but if the slip is on an incline you might find slow lorries restricted to the left lane.

The purpose of staggered merging is safety. Drivers only have to think about one set of merging traffic at a time.


There's no restrictions, unless, as user16259 says, signs say otherwise.

However, note that there's usually a spot where the line between the two lanes goes solid - after that you're not allowed to switch between them. The yellow car in your Google link is breaking that rule!

You'll also find that in some places the left lane doesn't merge at all, but becomes an extra lane on the motorway (as with the junction in your link), and in others, where there is another junction soon after, that lane then splits off to become the off-slip for the next junction (e.g M3 J7&8)

  • I'm missing how the yellow car in the Google link is breaking any rules. Commented Jun 3, 2018 at 14:01
  • 2
    @laconicdroid if you follow the road in streetview, the yellow car goes on to cross the solid white line. That's only allowed in an emergency.
    – user16259
    Commented Jun 3, 2018 at 14:49
  • @LaconicDroid as seen here
    – AakashM
    Commented Jun 20, 2018 at 11:43

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