Tolls will increase Sunday on the Highway 520 bridge and the Tacoma Narrows Bridge.

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Only six months after electronic tolls began on the Highway 520 bridge, the rates are going up Sunday, to a peak rate of $3.59 each direction for cars using the Good to Go pass.

The figure represents a 2.5 percent increase for inflation, to be followed by three more annual 2.5 percent increases. The state sold bonds based on rising tolls the first few years, and the finance plan assumes a larger bump of 15 percent in 2016.

Peak drivers who are billed by mail, instead of using a pass, will pay $5.13 instead of an even $5.

Off-peak rates for pass holders also will increase, for instance midday, from 10 a.m. to 2 p.m., from $2.25 to $2.31; or late night, 9 to 11 p.m., from $1.60 to $1.64.

Meanwhile, drivers on the Tacoma Narrows Bridge will notice a much bigger increase starting Sunday, from $2.75 now to $4 round trip, collected when heading to Tacoma. That bridge has fallen short of revenue targets at times because officials held the rates down the first five years.

On Highway 520, the toll revenue is tracking about as expected, at close to $13 million for the quarter that ended in March, according to the state Department of Transportation (DOT).

In May, average weekday traffic was 67,420 vehicles, or 17 percent above estimates, while revenues were $4.9 million, or 9 percent above the finance plan’s target, according to a DOT update released Tuesday. Trip times on 520 have gotten slightly longer in recent weeks, the latest sign some drivers are returning after the initial toll shock, said Jim Bak, spokesman for INRIX, a traffic-data firm in Kirkland. A morning trip that took 7 ½ minutes from I-405 to I-5 in January is now taking just over eight minutes, INRIX found.

Drivers on nearby I-5 and I-405 have spent extra minutes in congestion because of drivers who detour to avoid tolls.

And some travelers have been stuck during drawbridge openings on the 520 bridge. Construction barges and equipment are blocking the east high rise, where columns will be installed for a new six-lane bridge. So boats taller than 44 feet are crossing most days, with one-hour notice, between 9 a.m. and 4 p.m.

The state will not refund or cancel tolls for drivers who get stuck, but advance warning is being given on overhead message signs.

The quick toll increase is driven mainly by delays in startup. When tolls began a year later than expected, on Dec. 29, the DOT chose to stick with its fiscal plan rather than postpone a 2012 toll boost.

Sunday’s increase gives Highway 520 the distinction of being possibly the only U.S. toll road to charge a prime number at rush hour — befitting its fame as the geek road to Microsoft.

However, traditional tolls ending in 5’s and 0’s, based on people digging for cash, eventually will give way to other numbers, says Robert Poole, a road-pricing expert at the Reason Foundation.

Mike Lindblom: 206-515-5631 or

On Twitter @mikelindblom.