This question is referring to law in the Canadian province of Nova Scotia.
Zipper merging is very uncommon here. There are many that simply frown upon it as trying to "get further ahead" in the traffic line, whereas others believe it's the most effective and safe form of merging. Others claim still that it's simply illegal based on our Motor Vehicle Act and actively discouraged by our driver handbook.
Researching this so far, I found various discussions on Reddit (1, 2, 3, 4, 5) as to the legality of zipper merges in Nova Scotia. I've linked the original discussions, but will try to summarize the points here.
Some of the discussions talk about "high speed" zipper merges, i.e. where two lanes are reduced into one (either normally or abnormally), but there is no slow down of traffic. That's a separate case.
To simplify the question, I'm primarily interested in "low speed" zipper merges, where one lane is temporarily blocked, usually due to construction. This can happen on community roads, or our "100-series" highways. This is the most usual case I encounter in day-to-day driving.
The scenario being that I'm driving normally on the highway (near the posted speed-limit of 100 km/h) and by coincidence I notice a backup of traffic in the alternate lane I'm driving in (heading in the same direction, either right or left). That's because the lane I'm driving in eventually comes to a temporary end due to construction, and I'm required to merge into the backed up lane to continue.
What usually happens is the person in the non-blocked lane slows down immediately once they notice they backup, still far from the lane end, and tries to merge to the backed up lane as soon as possible (since that's the one that continues forward).
The less-common alternative is to continue in the non-blocked lane until the lane actually ends, then someone in the blocked lane would allow you to merge in (this is the zipper merge).
Arguments for the Zipper merge being illegal:
The most common cited reasoning for zipper merging being illegal in the Reddit threads is a line of text in the driver handbook that reads:
Merge as soon as safely possible. A common mistake is to approach the merge point at too high a speed in the lane to be closed, then to push into the other lane at the last moment. This causes collisions.
The second oft-cited reasoning is that all construction zones have a "Do Not Pass" proceeding the point of the lane ending, which can even be several kilometers away from the lane end. This is also mentioned on the same page in the driver handbook:
Obey the “Do Not Pass” sign. It prepares traffic for an orderly merge.
The Motor Vehicle Act (amended), section 111A, says that the left-hand lane shall yield to the right-hand lane.
111A (1) Where two lanes of a street or highway merge into one lane, the driver of a vehicle in the left lane shall yield the right of way to a vehicle in the right lane unless the driver of the vehicle in the right lane is directed by a sign to yield to the vehicle in the left lane.
This could be interpreted differently depending on which lane is closing, right or left. If the left lane is closing, drivers on the right would not be allowed to yield to traffic entering on the left (effectively making a zipper merge illegal). However, if the right lane was closing, drivers in the left lane would be required to allow traffic to merge from the right, thereby making a zipper merge legal. The illegality of the former case (left lane closing) does necessitate that it's never permitted for a driver in the right to yield to a driver in the left lane. I don't know if that's actually supported elsewhere in the text of the MVA.
One Redditor even says they were specifically ticketed for attempting a zipper merge, presumably due to the no passing sign.
Arguments for the Zipper merge being legal:
The language around "Merge as soon as safely possible" is unclear. It doesn't mean merge as soon as you notice you need to merge. If you're travelling in the unblocked lane, and there's a sufficient gap in the blocked lane for you to safely "merge early", then by all means do so. But if the traffic in the blocked lane is sufficiently stopped such that you cannot "merge early" there is no harm continuing until the end of the lane. To the contrary, stopping early to try to merge into the blocked lane could be more dangerous, as it gives drivers behind you less reaction time.
The language around the "Do Not Pass" sign isn't clear either. The drivers handbook has a section on passing. In short, it's described as moving to an alternate lane, specifically for the purpose of moving pass another vehicle ("overtaking"), and returning to that lane. But that's not what is happening in the scenario I described. Maintaining your current, unblocked lane is only "passing" the vehicles in the blocked lane in the absolute strictest sense of the word, not the commonly accepted definition of passing. The "Do Not Pass" sign is intended to prevent you from passing normally, assuming neither lane is blocked. The driver handbook is also not necessarily consistent with the actual law.
As mentioned above, the language in the MVA could be interpreted such that left-lane closing zipper merges are illegal, while right-lane closing zipper merges are legal.
The neighbouring province of New Brunswick has recently been trying to educate people on the use of zipper merges (of course, their law and NS laws are different, but the same logic applies).
Conclusion so far
Ergo the common conclusion is that while zipper merges are useful, they cannot legally be used in N.S., and most people here avoid them due to the aforementioned language in the driver handbook and/or MVA.
After several hours of researching, it does not seem to me like zipper merges are explicitly illegal, but I'm by no means certain, and I don't know why someone would be ticketed for it. Hence the question. If anyone has any advice or is able to find specific and clear language in the Motor Vehicle Act that explains it either way, I'd be very interested. Thanks.