Arena game would add 5 to 10 minutes to commute

If the Sonics return to Seattle, their fans probably would add several minutes of regional traffic delay on game nights, causing typical rush-hour traffic to last until 7 p.m.

But these slowdowns wouldn't be much different from what already happens before other Sodo sports events.

On a typical Mariners game night, drivers on southbound I-5 take five to 10 minutes longer than usual to pass through the city.

Government leaders can either relieve or worsen the traffic, depending on decisions they make about tolling, transit, even sidewalks.

As the debate intensifies over whether to use up to $200 million in public financing to help build a $490 million basketball arena, traffic congestion has become a major sticking point.

So far, arguments have focused mainly on whether sports traffic would clog streets that the Port of Seattle relies upon to move freight through Sodo. But an equally important question — and one that will be discussed before the Metropolitan King County Council on Thursday — is how the regional highway system will function for all drivers.

An arena event in Sodo drawing 20,000 fans would bring about 6,000 more cars, according to a transportation study by Parametrix for arena investor Chris Hansen. That's a credible estimate, based on the Mariners' experience at nearby Safeco Field.

The pro-arena side says most fans will show up just before game time, as the arena might not even open until 6:30 p.m.

By then, travel times on I-5 have started to ease, by about one-third from the 5 p.m. peak. A few thousand more cars would bring the volumes back up to that of a full commute.

Even more than the size of a game-day surge, what people might notice most is that it would happen more often.

A special event won't be so special, if both the Sonics and a pro hockey team arrive, along with concerts. The pro-arena study mentions 92 dates per year, including 17 when the arena is used the same day as a Mariners or Sounders FC game, pushing total attendance above 40,000.

Seattle already ranks fourth in congestion for North American cities, in a quarterly report this week by the Tom Tom traffic data company. That's not Hansen's fault, basketball fans like to say.

Existing conditions

Hampered by a batting slump, the Mariners draw half-capacity crowds of an average 22,000 fans, similar to a sold-out basketball game.

The state Department of Transportation (DOT) examined 14 baseball and soccer nights in May and June, and found that the 11-mile trip from North 145th Street to the I-5/I-90 junction takes an average 36 minutes at 6 p.m., compared with 26 minutes on other Monday through Thursday evenings. The 10-minute delay gradually shrinks to zero by 7 p.m.

Similarly, traffic on westbound I-90 takes 23 minutes to go from I-405 to Rainier Avenue South on the game weekdays, compared with 19 minutes otherwise. The stop-and-go conditions approaching downtown last until 6:20 p.m. on game days, instead of easing by 6 p.m.

"If the Mariners' 20,000 fans cause a five-minute delay, the new arena will cause a five-minute delay," speculates Mark Hallenbeck, director of the Washington State Transportation Center at the University of Washington.

But the counts also show game crowds make no difference on Fridays — the region already is gorged with traffic for several hours, explains Mark Leth, a traffic engineer for the DOT in Shoreline.

And there is no delay for I-5 traffic arriving from the south. One advantage of Sodo, compared with rebuilding an arena at Seattle Center, is there are several ways fans can arrive — Highway 599 to 99, I-5, Fourth Avenue South — says Hallenbeck.

An unstable system

Because of Seattle's position on a narrow isthmus, it doesn't take much to choke the roads and transit. An alarm glitch in the downtown transit tunnel forced thousands of bus and train commuters onto surface streets on Aug. 12, 2010. The tunnel is full enough that light-rail trains are sometimes delayed by buses within. A freeway wreck under the state convention center fouls traffic on I-5 and a floating bridge, then ripples to Highway 99.

State data show that during the two years that ended April 30, there were 371 blocking crashes and stalls on I-5 between Mercer Street and the Albro/Swift exit, in both directions, between 4 p.m. and 8 p.m. In other words, one blockage every two days.

The most sensitive point is the stretch of southbound I-5 from downtown to I-90, says Hallenbeck.

"If anything bad happens there, that whole interchange blows up," he said.

Freight interests say highway delays affect interstate cargo that arrives well into the evening.

"I'm coming from the perspective that there already are problems," said John Persak, spokesman for International Longshoremen's and Warehousemen's Union Local 19. "They're coming from the perspective that things are OK now."

Toll diversion

Arena traffic could be similar to congestion that drivers on local highways found when they tried to avoid the new tolls on Highway 520.

Recently, the state DOT estimated that if the future Highway 99 tunnel is tolled at $3.25 southbound and $2.50 northbound in later afternoon, as many as 9,100 cars might avoid the tunnel from 3 p.m. to 6 p.m. At that rate, the state thought I-5 would become stop-and-go and fail to absorb more traffic, while most cars use the side streets, and possibly get in the way of buses.

Bottom line?

State tolling rates likely would cause greater traffic effects, for good or ill, than a Sonics game.

Hallenbeck points out that if I-90 is tolled, as several state leaders have suggested, then the 520-related diversion downtown goes away, and there would be less cross-lake driving overall.

"You would certainly counterbalance some of the arena traffic," he said.

North-end fans who can afford $55 game tickets, $25 parking and $8 beers might be happy to pay a $3.25 tunnel toll and exit at the new Highway 99 interchange next to the stadium district.

City Councilman Mike O'Brien, who earlier campaigned against the $2 billion tunnel, was explaining to a town-hall crowd Tuesday night that it will help clear out the postgame traffic.

One piece of the pie

A rush of 6,000 cars is only a small piece, about 2 percent, of the average 284,000 cars a day on I-5 passing south downtown. John Perlic of Parametrix points out that only 4,000 of the 6,000 cars would converge near the I-5/I-90 junction. There would be 1,000 to 1,500 cars each way, a bit less than the one-hour capacity of a highway lane.

"There would definitely be some effect," Perlic acknowledges. "It's a little hard to generalize the magnitude."

One key is getting cars to exit earlier, for instance at James Street from the north, or to major surface roads south of Sodo.

And what if more people walk, bike and ride?

"This stadium is in the Stadium District, in the most transit-rich district in the state. So what's the issue here?" said supporter Greg Kulseth at the Tuesday forum.

Two light-rail stations and the King Street Station are two-thirds of a mile from the arena site.

Hansen likely would be required to fund sidewalks and railroad-safety gates on South Holgate Street, to help fans reach transit stops and parked cars, said Bob Chandler, a senior project manager for Seattle DOT. Hansen also has promised a 1,500- to 2,000-space garage to reduce the parking crunch on Sodo streets.

Pro-arena officials, including King County Executive Dow Constantine and Seattle Mayor Mike McGinn, haven't yet proposed a detailed traffic-reduction strategy.

The pro-arena study assumes 81 percent of fans drive, based on the Mariners.

Perlic emphasizes that's an opening-year estimate, not counting those who can ride future light-rail lines being built to Lynnwood, Bellevue and Kent by the early 2020s. He's hopeful the driving share can shrink to 70 percent.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com.

On Twitter @mikelindblom.

By Mike Lindblom Seattle Times transportation reporter

By Mike Lindblom
Seattle Times transportation reporter