I noticed that the subject of Box Junctions was being over-complicated and obfuscated on a web forum recently.
Rule 174 of the Highway Code says:
Box junctions. These have criss-cross yellow lines painted on the road (see ‘Road markings’). You MUST NOT enter the box until your exit road or lane is clear. However, you may enter the box and wait when you want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right.
It seems odd to me that there should suddenly be a problem with this, given that it has been worded pretty much the same way for the last 30 years or more (box junctions have been around since the late 60s).
The bottom line is that – and it has always been clearly understood that this is what it meant – if you are travelling straight on, you will be committing an offence if you (or any part of your vehicle) stop inside the area marked with the yellow lines. If you are moving slowly in traffic, it is up to you to decide if you should enter the boxed area and run the risk of having to stop in it if the traffic comes to a standstill, hence the Highway Code wording about your exit road or lane being clear. If you keep moving and manage to clear it then you have not committed any offence – though you might have taken a bit of a gamble. Technically, your exit road or lane IS clear if you get it right, whether by good judgement or good luck. It doesn’t matter how slowly you are travelling – if you clear the boxed area you have not done anything wrong.
The Highway Code DOES NOT say it is an offence to enter the box junction if someone else is already passing through it. If that was how you had to use them, there’d be gridlock for 12 hours every day as a result of reduced traffic flow, not to mention lots of bumps as people tried to work out the different permutations!
You can stop in a box junction if you are turning right, and the only thing preventing you from completing the manoeuvre is traffic coming towards you or also queuing to turn right. If my memory serves me correctly, I think that once upon a time you weren’t allowed to wait behind another vehicle waiting to turn (i.e. only one car at a time was allowed in). I might be wrong about that, but it is a moot point anyway, since it is what the Highway Code says now that matters. However, this may explain why people think it is wrong to follow other traffic through a junction when going ahead, even though it only applied to turning right.
Box junctions are intended to prevent traffic being gridlocked. They used to work very well, though they are less effective these days as driver arrogance and ignorance has increased. Lorries and buses think nothing of blocking box junctions during rush hour, and it is only in London (where cameras are used to enforce the rules, and where there is a flourishing business in legal firms trying to get people off when they’re caught) where they are taken seriously – though perhaps not for the reasons originally intended.