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Is the driver who merges at the last second cutting in line?
When a lane closes in heavy traffic, merging early feels polite and merging late feels like cheating. Traffic engineers recommend the opposite: use both lanes to the end, then take turns.
Merging early wastes a lane. Before the cones, both lanes are free places to wait. When drivers crowd into one lane early, the other lane sits empty and the line stretches twice as far back, past exits and on-ramps it can block. And the empty lane becomes a shortcut: the few drivers who use it really do pass everyone, which is why late merging feels like cheating. When everyone uses both lanes and takes turns at the cone, the line is shorter and no one gets ahead.
What the zipper doesn't do is push many more cars through the single open lane. The line gets shorter and fairer, not much faster. What looks rude from inside one lane can be the rule that works best for everyone.
Try it somewhere else
A cafeteria has two serving lines, but students crowd into one because using the other “feels like cutting.”
Who is really making the wait longer, and what would fix it?
What do the studies show?
Shorter lines, consistently. Minnesota's transportation department reports that the zipper merge cut the length of backups by about 40%. A Virginia study's computer simulations found the queue shrinking from 7,800 ft to 6,000 ft (2,380 m to 1,830 m).
Throughput, mixed. In Minnesota's 2004 tests, the single open lane carried about 1,600 vehicles an hour at every site. Virginia found a significant gain only where three lanes narrowed to one, about 1,285 vehicles an hour instead of 1,154, and no significant change where two lanes became one.
Fewer conflicts. When everyone merges at the same place, there's less weaving, fewer sudden lane changes, and less reason for drivers to block the closing lane, which Minnesota observers saw some drivers, especially truckers, do on purpose.
When should you merge early?
When traffic is light and moving at speed, merge early, as soon as there's a safe gap. The zipper is for heavy, slow traffic, when a line is already forming. Germany writes the rule into law: when a lane ends, drivers must let cars merge in turn “immediately before the start of the narrowing,” a rule Germans call the Reißverschlussverfahren, the zipper procedure.
Related exhibits
Sources: Minnesota Department of Transportation, zipper merge guidance (backups about 40% shorter), as cited in North Carolina DOT, “The Effects of Late Lane Merges on Travel Times” (2015); Minnesota DOT, “Evaluation of 2004 Dynamic Late Merge System” (single-lane throughput of about 1,600 vehicles per hour; blocking behavior). Virginia Transportation Research Council, “Evaluation of the Late Merge Work Zone Traffic Control Strategy” (2005). German Road Traffic Regulations (StVO), §7(4). The diagram shows 20 cars for illustration.