Commuters’ greatest enemy on the State Route 520 bridge used to be gridlock. Then it was tolls. But recently, drivers have encountered a new foe: boats.

Over the last two months, thousands of drivers have pulled onto the bridge expecting to pay a toll in exchange for quick passage over Lake Washington, only to encounter an open drawspan on the bridge for boats to pass through. The new issue comes as part of reconstruction of the bridge. Large boats – those over 45 feet – used to pass through the east navigational channel, but construction barges are fortifying the foundation, blocking the path of vessels, said Dave Becher, construction engineering manager for the Washington State Department of Transportation.

Becher said the bridge has opened 29 times since April 1, with the times boats are allowed to pass a result of negotiations between WSDOT and the U.S. Coast Guard. Boats are not allowed to come through the bridge between 6 and 9 a.m., and 4 and 7 p.m. on weekdays. Otherwise, boaters must alert WSDOT at least an hour advance.

But a few openings have occurred in the late morning, including one at 9 a.m. Thursday, causing grief for the slightly later commuter.

“It’s an unfortunate situation,” Becher said. “We have to balance the impacts to the maritime community and the people using the bridge.”

Drivers will get at least a small reprieve over the holiday weekend, as construction crews will pull out of the east channel Thursday evening, allowing larger vessels to pass through.

The open bridge potentially costs both time and money. For those who know in advance of the opening, they can populate other areas, leading to traffic jams on other routes, as was seen on Interstate 90 Thursday morning. The loss of operation of the bridge also means a temporary loss of revenue from tolling, which is expected to pay for $1 billion of the new $4 billion bridge.

It doesn’t just cost WSDOT, it costs commuters as well.

Mandy Emel is a Capitol Hill resident who uses the bridge every day. She commutes to Redmond for work and is happy to pay $7 a day for peak rates if it means getting places quicker. Emel said she has missed two meetings this week due to early morning closures.

“I found that – surprise – the bridge is closed, is a huge pain and takes 45 minutes,” she said. “Even if WSDOT does have to close the bridge, why on earth would they chose 9 a.m. to do so?”

Emel said she has signed up for alerts to find out in advance when the bridge is closing. WSDOT encouraged anyone who plans to travel SR-520 regularly, to sign up for text or email updates. Drivers can also follow WSDOT on Twitter @wsdot_traffic.