Past lifting restrictions didn't work out too well, as this excerpt from my book Crossing the Canal: An Illustrated History of Duluth's Aerial Bridge, explains:

In 1970 the city installed a new emergency engine for the Aerial Lift Bridge to replace its back-up gasoline engines. Bridge Supervisor Bob Brown explained that the batteries had worn down prematurely due to overuse of the bridge by pleasure craft.

Brown reported that in July 1970, 1,588 vessels passed under the bridge. "Of that number," Brown said, "552 passages were for the excursion boats Flame and Flamingo, which constituted 35 percent of all traffic. Sailboats and other small craft made up another 404 passages or 25 percent of the lifts. The only reason the batteries had lasted as long as they had was because  former Bridge Supervisor Al Hass switched to distilled water in the batteries  ten years earlier. On top of the additional boat traffic, the bridge experienced increased automobile traffic, with 119,000 vehicles crossing the bridge that June alone.

Besides the new batteries and back-up engine, canal traffic had to be limited to extend the bridge's life, Brown argued. One possible solution would be to limit lifts by establishing "bridge hours"—a limit on the hours the bridge would raise for anything but "essential marine traffic." The Coast Guard agreed with him. It sent a notice to mariners that the Guard "will support owners of drawbridges who refuse to open their bridges for vessels capable of sailing beneath the structure." The same would apply to the lift bridge. Boat owners were encouraged to take down antennas, fishing rigs, false smoke stacks, or anything else that could be easily removed to lower a craft's height to clear the road span's bottom. Not only were the pleasure boats wearing out the bridge and causing unnecessary delays, they were delaying larger vessels that needed the canal.

Two years later the problem remained. The Park Point Community Club took up the cause, telling the newspaper that the bridge was being "abused." Residents were sick of unnecessarily being "bridged," a popular Duluth term for being stuck in traffic waiting for the bridge to raise and lower. Many Park Pointers were still thrilled by the sight of a thousand-foot ore boat passing under the bridge, but few cared much for other craft. They had long ago learned to ride out the wait by keeping a book in the car to read, and Park Pointers always have an excuse if late: "I got bridged" is enough for any other Duluthian to understand the delay.

By 1972, Brown's argument hadn't changed much. In 1971 the bridge lifted 5,873 times, 1,700 for the Flame and Flamingo—both of which, according to Brown, sported "needlessly long radio antennas and false smokestacks." At an estimated cost to the city of $23 per lift, those two vessels alone placed a burden of $39,000 on Duluth taxpayers.

Despite the hullabaloo, officials never put "bridge hours" into place. Not that the perception of excursion boats making the bridge raise unnecessarily ever changed. Tom Mackay, longtime Park Point resident and captain of the Vista Fleet, told reporters in 2005 that "People have cussed at me from their cars. I've had people threaten to fire a shot across my bow the next time I cause a delay." 

The Park Point Community Club made another effort to establish "bridge hours" in the mid-1990s. With the Coast Guard's consent, the bridge raised and lowered just once every half hour. But after a year the Coast Guard considered the experiment a failure, and the bridge returned to lifting on an as-needed basis.

Sorry, Claire, but that's another Eric Ringsred issue. Ringsred purchased the horn and organized TOOT to operate it. But that organization neglected it, expected the City  and Coast Guard to maintain it, and then dismantled it in 2006 rather than pay the $15,000 required to rewire it after the Coast Guard found it unreasonable supply the power (it required special three-phase electric power, and no other canal device did, so it made no sense for the Coast Guard to pay for and maintain that feed). I believe TOOT later sold the horn.

(There's a much more complete explanation in "Crossing the Canal," but I won't take up space with another huge comment.)