Managing the snow
Published 4:15 pm Thursday, October 2, 2008
Eastbound travelers on Interstate 90 over Snoqualmie Pass may experience minor delays at the 468th Avenue Southeast interchange (Exit 34) east of North Bend as the state has restricted traffic for the second year.
The Washington State Department of Transportation began implementing traffic flow management tactics (TFM) by blocking lanes during bad weather to restrict traffic flow last year at Denny Creek on I-90. But the location of the block wasn’t ideal. When backups got too great and WSDOT had to close the road, drivers had to backtrack to North Bend or Issaquah, said Allen Minerich, WSDOT assistant superintendent at Snoqualmie Pass. This year, the block was moved from Denny Creek to the 468th Avenue Southeast Interchange at Exit 34 in North Bend, so travelers can take an alternate route if they don’t want to wait, or they can go into town and get coffee, food, gas, or find accommodations for the night.
Sunday, Nov. 26, was the perfect example of how TFM works, said Mike Westbay, WSDOT communications manager. Using TFM, the Pass stayed open all day, except one hour when the Pass had a sudden squall of snow and ice that froze the road and caused several spinouts.
“We were able to get right on it, straighten out the vehicles, get them over the pass and open again within an hour,” Westbay said.
Restricting the number of cars on the interstate creates space between individual vehicles, preventing the chain reaction that occurs when one spinout causes another spinout, making traffic stop.
“If we don’t [restrict cars], then the spinouts from cars and trucks totally closes the Pass down,” Minerich said.
Every time the highway closes, it costs up to $500,000 per hour in lost time to state businesses and residents, he said. TFM does create traffic backups, but the wait is usually only 30 minutes to an hour instead of several hours, Minerich said.
“When a truck has to stop for a car [that can’t make it through the snow], it can’t get going again,” Minerich said. “They have to get out and get chains on. Then before you know it, the eastbound lanes are totally full. Once it plugs up, it takes hours and hours to get everyone moving again.”
A popular route, Snoqualmie Pass gets an average of 27,000 cars per day during the week, which builds to as many as 35,000 cars per day on the weekends, said Westbay. Those numbers are higher around holidays – the Pass reached 47,500 cars on Nov. 22 and 31,400 on Nov. 26 – Minerich said. Normally these numbers don’t cause problems, but combined with brisk winds, snow and ice, it’s a recipe for disaster.
TFM is used to regulate the number of eastbound vehicles attempting to cross Snoqualmie Pass per hour, Minerich said. It is also implemented on westbound lanes, but only occasionally because travelers heading west don’t have as much trouble, said Teresa McCoy, Snoqualmie Pass traffic systems operations supervisor. McCoy didn’t give a specific reason why drivers traveling westbound don’t experience as much trouble.
TFM allows WSDOT snowplows to make their rounds faster so tow trucks and de-icers can get through, Minerich said. It also creates space for emergency vehicles, Minerich said. Information about the number of snow-related accidents on the Pass before and after TFM was not available.
TFM is implemented when WSDOT gets a weather report predicting more than 6 inches of snow in a 12-hour period at Snoqualmie Pass. It is only applied between 6 a.m. and 6 p.m. on Fridays, Saturdays, Sundays, holidays and travel days around the holidays. The on-duty supervisor at Snoqualmie Pass – or a WSDOT authority – initiates TFM, calling for snowplows, operators and incident response vehicles to be transferred to I-90, Minerich said. They also call neighboring “shops” in Cle Elum, Ellensburg, Selah, Pasco and Toppenish to request help on the east side, or Preston and Bellevue on the west, said McCoy.
WSDOT employees travel to I-90’s Exit 34 at the 468th Avenue Southeast interchange to set up signs warning travelers of closing lanes and set out orange barrels to force travelers into the right lanes. Yakima and Hyak Traffic Control Centers and the Seattle DOT also broadcast messages over the radio, warning travelers of delays. Depending on the severity of the conditions and the number of travelers, one or two of the three lanes are blocked on the left side of the highway. Keeping only one lane open allows about 800 cars per hour through, and two lanes allows 1,600 cars per hour, Minerich said. The block only extends for a few hundred feet before travelers can spread into three lanes again, McCoy said.
The idea for TFM was taken from Donner Pass, or Interstate 80, in California, which has an average traffic flow of 29,000 cars per day – similar to the numbers at Snoqualmie Pass, Westbay said.
“[California DOT has] been doing it for over 20 years,” Westbay said. “We tried it last year right around Thanksgiving while we had one lane restricted around Lake [Keechelus] where we had a rock slope that slid. We determined it is an effective management rule to keep the Pass clear.”
TFM is not implemented when Washington State Patrol is enforcing tire-chain requirements at milepost 47 by Denny Creek, Minerich said. In that situation, one lane is already blocked by a patrol car, which slows traffic and creates the same effect as TFM when people stop to put on chains.
As helpful as TFM may be for WSDOT, residents of Snoqualmie Pass hate it, McCoy said.
“Last year especially, when we were trying to work the kinks out of it, it caused some severe backups,” McCoy said. “They can be really inconvenienced by it.”
So far there have been no additional considerations for residents over regular drivers, as some Pass residents had suggested, McCoy said.
Though Pass residents expressed concerns about the tactic, surrounding cities have had few complaints.
“It looks like a good possibility, a good place to start,” said North Bend Mayor Ken Hearing. “From a layman’s point of view, it makes common sense. Otherwise, you end up with people trying to pass snowplows and do things like that.”
“We haven’t had any problems,” said Judy Tokarsyck, executive director of the Cle Elum-Roslyn Chamber of Commerce. “It’s been open as far as communication.”
