If you’re a commuter thinking about dropping your car at the Mercer Island Park-and-Ride and catching a bus from there, you may want to think again; on average, the lot is 100 percent full, according to a Puget Sound Regional Council report released in August.

Even after a 2006-2008 expansion of the lot, which the Mercer Island City Council voted to keep from being larger, every parking space is consistently full, making it one of the most-used park-and-rides in the county.

With the light rail scheduled to begin stopping on the Island around 2020, officials said the city may need to find more stalls somewhere else.

“With the amount of traffic and the amount of people coming from the east side, we always knew it would be a popular lot,” said Sound Transit spokesperson Bruce Gray, of the Mercer Island Park-and-Ride. “Some of our highest volume routes stop there.”

Gray said about a dozen King County Metro bus routes include a stop at the Mercer Island lot, and two of Sound Transit’s most popular lines, the 550 and 554, go through the park-and-ride as well.

Mercer Island City Manager Rich Conrad said the expansion of the park-and-ride came about because Seattle-area voters approved a 1996 ballot measure that added Regional Express buses, including the 550 and 554, to the King County public transit system.

Also, according to Puget Sound Regional Council reports, which began in 2002, the lot on Mercer Island was, on average, up to 103 percent full some quarters before it was expanded. Ashley Harris, who writes park-and-ride reports for the Puget Sound Regional Council, said that meant that cars tended to overfill the lot, parking in undesignated areas like fire lanes.

Conrad said the City Council on Mercer Island did not want the revamped park-and-ride lot to get too big, though.

“Sound Transit came prepared to expand the lot — they had enough money to add hundreds of stalls — but the Council opted for a smaller expansion,” Conrad said.

He said the Mercer Island City Council asked Sound Transit to almost double the size of the lot, which held 257 cars before the expansion, and spend the other money allotted for the project on building more parking space east of Mercer Island. The Council hoped that doing so would reduce the number of cars from off-Island that tended to park in the Mercer Island lot, Conrad said.

According to data collected by Sound Transit before the expansion and published on the City of Mercer Island Web site, slightly over half of the total cars that parked in the original lot belonged to people who did not live on the Island, based on the cars’ license plates.

After working closely with the city on designing the new lot, Gray said Sound Transit settled on a $16.8 million, two-story structure partially sunk into the ground to keep it from being too high. Overall, the expansion bumped the lot’s capacity up to 447 cars, meaning that the 190 added parking stalls cost roughly about $88,000 each.

According to a project report on the City of Mercer Island’s Web site, Sound Transit suggested building a taller lot to cut costs per stall, but the City Council opposed this idea to protect the surrounding neighborhood.

Even with the added space, Harris said the Mercer Island lot was at 100 percent capacity almost immediately after re-opening in January 2008, though Conrad said there was less spill-over from the lot into the surrounding residential areas and the town center than there had been before the expansion. He said the city started requiring permits to park in the nearby area at that time to discourage spill-over parking.

Now, with the light rail scheduled to come to the Island in about a decade, the city has begun to look at other parking options, said Conrad. He said the light rail station, which is to be built on the North end of the Island near 77th and 78th Avenue S.E., is to be served by the Mercer Island Park and Ride lot.

“The city is currently looking at options for some kind of additional parking space to serve commuters,” said Conrad. “It would probably end up being what’s called a public-private partnership.”

Gray said he thought light rail could actually decrease the strain on the Island lot. Light rail will run more frequently than buses and would encourage commuters to catch the train directly from the Eastside rather than driving to Mercer Island first.

Linda Thielke, a spokesperson for King County Metro, also said her organization does not plan to expand parking on Mercer Island.

She said Metro leases parking space at the Mercer Island Presbyterian and Methodist churches and at the South-end QFC. These areas tend to be below capacity, she said, ranging from 20 to 50 percent full on average.

She said Metro has buses that run from the leased lots to the Park and Ride, which is owned by Sound Transit.

According to data, some Park and Rides in King County have worse over-crowding problems than Mercer Island, with one lot in Renton averaging 124 percent capacity all year in 2009 and another in Shoreline averaging 119 percent.

Park-and-ride reports can be downloaded online at: http://psrc.org/data/transportation/parkride.

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