car broken down on highway

How not to Merge

Continuing with the traffic simulation examples examining here the effect of roads narrowing. Despite the institution of largely uniform national road rules last century government at all levels utterly failed to adequately advertise the change or ensure drivers were properly informed, and due to the continuing lack of understanding from the public it shall be repeated in full here. Merging lanes has only 2 possible circumstances in Australia which are neatly divided by the question; Are you crossing a dividing line?

  • Case 1: not crossing a dividing line, the “zipper” merge, formally known as “Giving way when lines of traffic merge into a single line of traffic
    cars zipper merging without lanes
    Once the dividing line is removed if any part of any other vehicle is ahead of your vehicle, you must give way.
  • Case 2: crossing a dividing line, formally known as “Giving way when moving from one marked lane or line of traffic to another marked lane or line of traffic”
    cars merging across lanes
    Whether there is an obstruction, the lane ends, or it is a slip lane onto a highway, the driver crossing the line must give way to vehicles in the lane they are entering.

Not complicated is it?

That is until you consider the follow variation on the above image.
The original image presented first is published in the road rules and teaching documents but it fails to reinforce the common situation shown here, leading to the sadly commonplace manoeuvre in Melbourne of a zero clearance merge where a driver changing lanes will simply move into the path of other traffic alongside them, hoping that the other driver will value their car (and time) more and avoid the collision.

This exceptionally inconsiderate and dangerous manoeuvre is not modelled within the driver model though the most aggressive of the simulated drivers will ignore the comfortable stopping distances of drivers behind them when merging or changing lanes. For comparison of different merge conditions a long length of divided road was simulated with a single choke point slightly longer than the longest vehicle, and a free flowing road of the same length with single or dual lanes of traffic.

car throughput vs narrowing roads

The difference in throughput per lane of a single lane road compared to the dual lane road only appears as saturation turns into stop-start traffic, the two lane road being able to maintain faster flowing sections with overtaking between the standing waves. Merging with a zipper method where cars must give way to those in front the peak throughput drops below half that simulated for a single lane road, merging and unmerging traffic on this two lane simulation had just one quarter of the throughput of the fully open dual lane road. It gets even worse when the left lane ends requiring drivers to give way when merging into the open lane (such as a broken down car blocking one lane) with less than one sixteenth of the peak flow achieved of the unobstructed road.

car broken down on highway

Varying the percentage of drivers who obeyed the give way rules when entering the closed lane merge provided a proportional effect increasing throughput towards the zipper efficiency, though increasing the throughput running a large risk of collision if another driver were to enter at speed expecting others to be following the rules. This has lead to cautious drivers expecting others to ignore the rules and further encourage the behaviour by making space or being courteous. We have all experienced the exasperation when at an intersection we are giving way yet the other driver wants to be “nice” and let us through, leaving liability on us should anything go wrong, but the zero clearance merge behavioural shift is much more pervasive to the point that now those following the law are the ones most in danger of having an accident.


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