For the past week, frustration and tempers have risen along the banks of the Mississippi River as drivers have waited 10 to 20 minutes or more to cross the Hwy. 61 bridge in Hastings.

The problem? The so-called zipper merge, which reduces two lanes of traffic in both directions to one lane during the summer-long construction project. Driver frustration at the bridge has led to some fist-shaking, ranting, lane blocking and two "unusual" cases of road rage, officials say.

But most of the problems are a result of a simple misunderstanding, they added.

"The first few days, people just didn't understand the zipper merge," said Hastings Police Chief Mike McMenomy.

The idea behind the zipper merge is for cars to use both lanes of traffic as they approach the bridge. Once drivers reach the bridge, they take turns going across.

But what has happened is this: Most drivers have queued up in the left lane, McMenomy said. Then along comes a driver in the right lane, passing the long line of cars on the left.

Some drivers in that left line may be having flashbacks to elementary school when some kid butted in the lunch line. But the right-lane passing maneuver is legit under the zipper-merge rules of the road.

Drivers crossing the Hastings bridge are starting to get the hang of it now, McMenomy said. "The majority of people are well-behaved,'' he said. "But then you have those circumstances once in a while when someone takes it personally."

Last week, a driver verbally assaulted a state trooper. And on Monday, a driver verbally threatened a flagman who would not let him on the bridge, eventually leading police on an 8-mile chase through town and into the country.

But local officials say both cases are unusual. The Monday incident occurred at about 11 p.m. and involved a man from Florida who was allegedly intoxicated.

During the chase, that driver blew across a set of stop sticks thrown down by the police, escaping with two flat tires. Another set of stop sticks blew out the remaining two tires, sending the driver into a ditch, but not before he hit two squad cars, McMenomy said.

"You're not going to see that every day," Hastings City Council Member Tony Alongi said. "In fact, I don't think we'll see it again. That was an extreme case."

For the most part, commuters over the bridge are grinning and bearing it, he said.

Kent Barnard, a spokesman for the Minnesota Department of Transportation said the zipper method of merging, formally known as "the dynamic lane merge system," is supposed to lessen congestion and create less traffic backup.

Drivers unfamiliar with the zipper merge have been guilty "of playing vigilante and straddling the lanes" on Hwy. 61 to prevent others from getting by, or blocking intersections to prevent traffic from getting through, Barnard said. "That is against the law and drivers can get a ticket if police are there."

As a result of motorists' misbehavior, more police are being dispatched to patrol the area, which will remain down to one lane until mid-July, Barnard said.

Many drivers are finding alternate routes, McMenomy said. And for those who can't, "it's going to be an interesting summer," he said.

Staff writer Tim Harlow contributed to this report. mlsmith@startribune.com • 612-673-4788