You've got to have your wits about you when driving - especially when two lanes merge into one.

Merging in turn, or 'zip merging' is a traffic flow measure that drivers are supposed to adopt when a motorway lane or dual lane closes leaving only one lane available.

It is intended that drivers merge at the point of the lane closure instead of indicating early and merging as soon as possible.

But it can be frustrating when a car just appears to push in, reports Glasgow Live.

Is it alright to zip-merge?

The practice is rather contentious among the driving population: But the studies show it is actually effective – as the traffic flow measure helps curb congestion, slash the risk of crashes and speeding by making optimal use of road space.

A study in 2008 by an American work zone engineer found that the three primary benefits of zip-merging are:

Over here in the UK, the consensus is the same although the law seems to be a wee bit less clear on the idea.

See rule 134 of the Highway Code :

“You should follow the signs and road markings and get into the lane as directed.

"In congested road conditions do not change lanes unnecessarily.

"Merging in turn is recommended but only if safe and appropriate when vehicles are travelling at a very low speed, e.g. when approaching road works or a road traffic incident. It is not recommended at high speed.”

Why do some people have an issue with zip-merging?

You might think indicating and moving over earlier on approach is the right thing to do, but it simply creates one long, slow line of traffic and a near-empty outside lane. The idea is that two lanes of slow moving traffic are faster than one.

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The AA advise: "Next time you find yourself in slow-moving traffic approaching road works or an incident that's blocking one lane, try to resist your deep-seated urge to queue early, use all available lanes and merge in turn when you reach the lane closure.

"Also, do let in drivers who need to merge. They'll probably thank you for it and leave you in a slightly better mood."