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A simple solution to the problem of weaving.
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The concept of weaving is where traffic in one lane is trying to move into another lane
while the traffic in the other lane is trying to move into the first lane.
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Weaving Traffic
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Weaving is one of the most dangerous features of junction design.
Despite this there are numerous junctions where traffic is caused to weave,
mainly because providing a bridge is costly and in some cases impractical.
To make weaving a safer process the following measures are commonly employed:
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allow the lanes to join over a long distance
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providing additional lanes
Most cases of weaving occur where a slip road joins onto the main carriageway of a motorway or trunk road,
continues alongside it and then leaves at the next junction.
In these circumstances one or both of the measures mentioned above are applicable and weaving is not a major problem.
However, the dangers of weaving become more evident when the system is employed within a single junction,
typically where two slips roads merge.
In these cases space is short so additional lanes and an extended join are not an option.
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Flow of Traffic Weaving Together
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But there is another approach that can help make weaving in these tight junctions a little safer.
This alternative requires just a simple change to the road markings and turns weaving into merging.
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By closing one of the lanes the traffic is forced to merge as opposed to weave.
Drivers in the red lane do not need to switch lanes.
Meanwhile all vehicles in the blue lane are forced to move over,
which means that drivers in the red lane know that all vehicles coming alongside them intend to merge in.
This change in priorities removes all the uncertainty involved in weaving.
The drawback of merging lanes is that we lose some road space.
This means merging two lanes is not a viable option if we expect large amounts of traffic to be using the junction.
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Flow of Traffic Merging Together
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But the merging concept is quite versatile.
Adding just one lane to the system provides extra flow to accommodate large volumes of traffic from one direction (blue route).
In the diagram shown right,
traffic on the blue route not wanting to switch onto the red route can use the lower lane.
This overcomes the main problem with the two-lane system, above-right,
where drivers on the blue route that wish to remain on the blue route are forced to change from the lower lane to the upper lane
and then return to the lower lane again.
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High Volume of Traffic on the Blue Route
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If traffic is expected to be heavy on both routes then a fourth lane can be added.
Although this adds more width to the road, the length of the cross-over point
(which is usually the constrained parameter when it comes to slip roads meeting within a junction)
does not need to be extended.
This is the most ideal situation since the two outside lanes occupy the vehicles that do not want to switch routes.
The added advantage over a four-lane weaving system is that vehicles not wanting to switch routes are encouraged to use the outer lanes
leaving only the vehicles intending to merge in the two centre lanes.
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High Volume of Traffic on Both Routes
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