If a pilot can land a 300-ton jetliner traveling 170 miles per hour on a strip of pavement at the Twin Cities airport without slipping, why is it that cars weighing 2 tons have been spinning and sliding through metro-area intersections for much of the winter?
The answer is complicated, involving traffic patterns, timing and, of course, money.
“We could provide the same level of service the airport does, but we’d go broke,” said Minneapolis street maintenance supervisor Mike Kennedy.
The airport uses plows, sweepers and blowers when snow falls, as well as an ice-busting chemical that costs about $15,000 for each pass. Kennedy said it would cost $455,000 to coat 1,000 miles of city streets once with the chemical, potassium acetate.
Sodium chloride (aka rock salt) remains the chief de-icing tool for street and highway departments, in part because it’s so cheap. But it can’t be used at the airport because it corrodes airplane bodies and parts. Cars, obviously, don’t qualify for the same protection.
Crews at Minneapolis-St. Paul International Airport are also able to rotate incoming and outgoing planes among four runways during a snowstorm, allowing ground crews to attack pavement when it’s free of traffic — a far different task than clearing a metro freeway during a rush hour. Plows and sweepers and chemical sprayers are followed by specially equipped cars that measure traction on the runway against FAA minimum requirements.
While metro streets and highways have been plagued with packed snow and stubborn ice since December, causing thousands of traffic accidents, the airport has not suspended traffic on its runways once this winter.
On Wednesday, as ice on Twin Cities streets began transforming into spring slush, MSP’s runways were dry and clear, as they had been for most of the time since a 9.9-inch snowfall ended Feb. 21.
MSP has less than 6 miles of runways — though they’re about as wide as 10 highway lanes — compared with 1,100 miles of streets in Minneapolis alone. And the airport has more snow- and ice-removal devices than the entire city of Atlanta, according to assistant MSP operations director Paul Sichko. The airport handles about 1,150 flights per day. Interstate 35W at Diamond Lake Road in Minneapolis alone carries 172,000 cars per day.
Those numbers inspire envy and defensiveness among road people at other public agencies.
“They don’t have the traffic we do,” Minnesota Department of Transportation spokesman Kevin Gutknecht said. “They clear it off much more frequently when we do. They don’t let ice form. Snow doesn’t get compacted.”
Sichko agreed that it’s not a fair fight.
“I know MnDOT has much more pavement to maintain than we do,” he said, adding that local agencies don’t have the luxury of temporarily keeping traffic off roads. Traffic compresses slush and snow, causing ice to form and to bond to pavement.
Potassium acetate, used on the airport runways, can melt ice in conditions down to about 22 degrees, Sichko said, so it’s not much better at melting than sodium chloride.
And like rock salt, it can also be harmful to aquatic life, said Brooke Asleson, metro watershed project manager for the Minnesota Pollution Control Agency.
In laboratory conditions, with controlled concentrations and distribution, rock salt can melt ice at 6 below zero, Kennedy said. But on the streets, it works down to about 15 above, he said.
Minneapolis, St. Paul and MnDOT also use magnesium chloride and calcium chloride, which work differently from rock salt and can — again, in the lab — melt ice to 25 below. In real life, 5 degrees is more like it, Kennedy said. The magnesium chloride is often mixed with molasses to help it stick to pavement better. St. Paul street maintenance engineer Matt Morreim said in rural areas the substance has been blamed for luring wildlife onto the roads.
During heavy snows, airport crews might repeat treatments of potassium acetate every 15 minutes. Rock salt treatment on Minneapolis streets costs about 3 percent of what an equivalent treatment with potassium acetate would cost.
At the same time, the designated routine after a heavy snow in Minneapolis and St. Paul means that side streets don’t get plowed until the day after the snow stops — by which time traffic has also created icy ruts that might resist melting and plowing.
In Wisconsin’s Polk County, the highway department has been treating its roads partly with cheese brine — a salt solution used in making provolone and mozzarellae. Highways chief Steve Warndahl said it hasn’t been tested in a lab, but based on tests in a jar outside his office it didn’t freeze until the temperature reached 21 below, as opposed to 6 below for rock salt solution.
“All I know is we put it in a bottle and it didn’t freeze, so we use a bunch of it,” he said, adding that it doesn’t smell and doesn’t get slimy, as magnesium chloride sometimes does. The county buys it from a local dairy for about 8 cents per gallon, compared with about $1.50 per gallon for magnesium chloride.
The supply from the local dairy has not been entirely consistent, though. MnDOT spokeswoman Christine Krueger said that’s one reason MnDOT does not use it.
“We don’t have a lot of cheese production, like Wisconsin does,” she said.
MnDOT has used soybean oil and corn-based substances for winter road treatment, and numerous agencies use beet juice and other vegetable products, but more for adhesion than de-icing. St. Croix County, Wis., is using beet juice on its roads, and a popular anecdote this winter is that roads there, including I-94 east of Hudson, have been in better driving condition than those in Minnesota.
Chris Ouellette, the Wisconsin Transportation Department’s northwest region spokeswoman, said that I-94 in Wisconsin gets far less traffic than metro roads and is the only four-lane highway St. Croix County has to manage.
At MSP, 58.4 inches of snow has fallen this season, 25 percent more than normal to date.
“Most of the snow that fell in December is still here,” Kennedy said. “It hasn’t left us with a lot of options.”
But there’s some good/bad news, due to slowly rising temperatures.
“We now see some of the washboarded ice starting to break on us” on some streets, Kennedy said. “On residential streets, that 6-inch hardpack is going to fluff up on us. It might start to look like we haven’t even been there.”