Highway 520 bridge provides little insight to CRC officials

Traffic patterns on tolled Seattle span have fluctuated

By Eric Florip, Columbian transportation & environment reporter


Updated: March 12, 2012, 11:05 PM



Weekday traffic volumes on Seattle’s Highway 520 floating bridge:

Feb. 27 through March 2: 66,000 vehicles per day (35 percent below pre-tolling average)

Feb. 20-24: 59,400 (41 percent below average)

Feb. 13-17: 67,700 (33 percent below average)

Feb. 6-10: 66,900 (34 percent below average)

The Interstate 5 Bridge saw an average load of 123,000 vehicles per day in 2010. On Interstate 205, the Glenn Jackson Bridge carried an average of 138,000 vehicles per day.

Source: Washington State Department of Transportation

When tolling begins on the Interstate 5 Bridge, at least some drivers will head for Interstate 205 — opting to tack a few more miles onto their commute rather than pay the extra charge to get to work. That’s a given.

What’s not clear is just how many commuters will make the switch. But local transportation officials may only have to look north for a hint.

Last December, the Washington State Department of Transportation began tolling the Seattle area’s state Highway 520 floating bridge over Lake Washington. As expected, the move sent thousands of drivers looking for alternate routes. Many picked nearby Interstate 90.

Tolling on the I-5 Bridge could start as soon as 2014. Local planners are banking on tolls to help cover the cost of its replacement, the more than $3 billion Columbia River Crossing. That means traffic counts and diversion rates will play a big role in determining how much revenue tolls will generate to pay for the project.

State and local officials say it’s too soon to tell what implications — if any — Highway 520 traffic patterns will have for I-5, but CRC planners are keeping one eye on the Seattle area as new patterns take shape. The CRC will conduct a detailed analysis of its own project area before tolling begins here.

“While we are interested to see what’s happening on (Highway) 520, at this point it’s too early to make direct comparisons,” said CRC spokeswoman Anne Pressentin. “If there are lessons to be learned, we’re certainly going to apply them.”

I-5 and Highway 520 bridges share a few traits in common. Both sit parallel to another major thoroughfare without tolls. Both pass through a large metro area. And Highway 520 uses electronic, variable tolling — the same system planned for I-5. But that doesn’t necessarily mean Highway 520 patterns offer a sneak-peek for Vancouver commuters.

“Every road is different,” said Patty Michaud, a spokeswoman with WSDOT’s tolling division. “All drivers are different.”

After a huge drop initially, traffic volumes on Highway 520 bounced back some in the following weeks. The numbers have recently hovered between 30 and 40 percent below pre-tolling normals. Trends continue to shift as Seattle-area drivers settle into new routines.

On Tuesday, Jan. 3 — the first post-holiday work day for many commuters — the bridge saw barely half of its normal weekday load of 101,000 vehicles. But volume crept up to almost two-thirds of normal by that week’s end, according to WSDOT data.

Other factors such as holidays and school closures can swing traffic counts. During the week of Jan. 16, for example — when a blanket of snow and ice covered much of the state — traffic on Highway 520 plummeted by 76 percent, according to WSDOT. Other nearby highways and roads saw a similar drop.

Since drivers started steering clear of Highway 520, I-90 has seen its traffic volume jump by about 10 percent, Michaud said, pushing travel times and congestion up with it. Highway 520, meanwhile, flows more quickly and more smoothly for drivers willing to pay the toll, she said.

The increase on I-90 doesn’t account for all of the cars that left Highway 520. Some have shifted to state Highway 522, which travels around Lake Washington to the north. Carpooling and public transit may have also absorbed some commuters; both those options are harder to quantify, said Matt Beaulieu, a WSDOT traffic engineer.

A study conducted before tolling began on the Highway 520 bridge predicted that traffic could be cut in half, and volumes may not return to normal for as much as 20 years. Those estimates were conservative by design, Michaud said, to ensure tolling revenue didn’t come up short. CRC officials have said the same about investment-grade analyses.

When Highway 520 bridge tolling started Dec. 29, WSDOT had some concern about extra loads on Highway 522 and I-90, Michaud said. For the most part, she said, patterns have evolved without major problems.

“So far, we’ve been fairly pleased with how smoothly things are going,” Michaud said.

Eric Florip: 360-735-4541; http://twitter.com/col_enviro;eric.florip@columbian.com.