Merge (traffic)

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Two lanes merging into one

In traffic engineering, a merge is the point where two streams of traffic travelling in the same direction from multiple roads or in multiple lanes on the same road are required to merge into a single lane.

A merge may be a permanent road feature, for example at the end of a dual carriageway, or a temporary feature, common during roadworks.


Slip road[edit]

Generally speaking, at a slip road onto a controlled-access highway or otherwise, traffic on the highway has priority over traffic joining at the slip road, and therefore the slip road traffic should accelerate to the speed on the major road and merge into a gap in the stream of traffic in lane one. At some slip roads, traffic continues into a new lane (a "lane gain") and therefore does not need to merge.[1]

Early merge[edit]

A variable message sign warning drivers of a lane closure on the M25

The early merge method dictates that one stream of traffic will maintain priority over another at the merge, and therefore traffic in the other lane should merge at the first opportunity. To encourage drivers to merge early, authorities may employ a static or dynamic early merge strategy. Strictly speaking an early merge is different from a conventional merging method, in that the traffic is encouraged to merge well in advance of the merge point.[2]

A static early merge strategy involves the placement of advance notices for a fixed distance in advance of the lane closure. This method may reduce the chance for rear-end collisions by warning drivers of the closure in advance of congestion.[2]

A dynamic merge strategy involves advance notices for a variable distance from the lane closure. For example, in the US state of Indiana, a dynamic no-passing zone is created in advance of the merge, to ensure drivers can move into the open lane before reaching the end of a queue.[2]

Late merge[edit]

A 'Queue in both lanes' sign instructs drivers to use both lanes up to the merge point
Zipper method road signage in Warsaw

The late merge method dictates that both streams of traffic should continue to drive up to the point of closure, and merge at the marked taper. Both streams should create alternating gaps into which vehicles from the other stream can merge at the merge point. In heavy traffic flow, traffic should therefore queue in both lanes in advance of the merge.[2]

The late merge method has not been found to increase throughput (throughput is the number of vehicles that pass through a point in a given period of time). However, it considerably reduces queue ("backup") length (because drivers use the ending lane until its end) and reduces speed differences between the two lanes, increasing safety.[3] The late merge operates at a nearly 20 percent higher than a conventional merge.[2] In the case of Interstate 77 in North Carolina, where signs directed people to use the zipper merge, the length of the backup was reduced from eight miles to two.[4]

A possible dynamic merge method would use variable message signs to instruct drivers to use all available lanes and to merge in turn if queueing is detected. At other times, drivers will use a conventional merging method.[2]

Governments hold campaigns to promote the late merge method because irritation and aggression are common among drivers who are not educated about the benefits of the technique, sometimes including straddling lanes to block late mergers. Often drivers who change lanes too early do not like to see other drivers continue until the end of the drop-away lane, even though this late merging is encouraged by the authorities.[5]

In various jurisdictions[edit]

In most countries, a driver can be penalized for not using the late merge method, but in some countries only where a traffic sign so indicates.[citation needed]

United Kingdom[edit]

Under the Highway Code, drivers should not use the right lane except when overtaking other vehicles, therefore they should move back to the left-hand lane as soon as it is safe to do so (Rule 137). The Code does however recommend merging in turn if it is safe and if vehicles are travelling at a very low speed (Rule 134).[6]

United States[edit]

Most states in the United States require merging traffic to yield to through traffic which already exists in the lane they wish to enter.[citation needed] This further complicates the common understanding of proper merging protocol, as even though zipper merging is widely encouraged, those doing so are still legally required to yield, and those who choose not to let them merge are not doing anything wrong from a legal standpoint. Traffic already in the lane being merged into has the right of way over the merging traffic from the lane that will disappear.

See also[edit]


  1. ^ "Motorways - Joining the motorway (259)". THE HIGHWAY CODE. Retrieved 2021-09-13.
  2. ^ a b c d e f "Dynamic Late Merge Control Concept for Work Zones on Rural Freeways - FHWA Work Zone". Retrieved 2021-09-13.
  3. ^ Johnson, Ken. "Late Merge...the Zipper System" (PDF). Minnesota DOT. Retrieved 30 April 2013.
  4. ^ Boyle, John (7 July 2019). "". Asheville Citizen-Times. Retrieved 7 July 2019. External link in |title= (help)
  5. ^ The "Zipper Merge" May Seem Aggressive, But It's Actually A Highway Congestion Reliever
  6. ^ "Multi-lane carriageways (133 to 143)". THE HIGHWAY CODE. Retrieved 2021-09-13.

Further reading[edit]

External links[edit]